Bump to 2.335.1 (#4484)

Co-authored-by: Francesco Renzi <rentziass@github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.devcontainer/devcontainer.json

@@ -4,7 +4,7 @@
   "features": {
     "ghcr.io/devcontainers/features/docker-in-docker:2": {},
     "ghcr.io/devcontainers/features/dotnet": {
-      "version": "8.0.420"
+      "version": "8.0.421"
     },
     "ghcr.io/devcontainers/features/node:1": {
       "version": "20"

.gitignore

@@ -27,4 +27,5 @@ TestResults
 TestLogs
 .DS_Store
 .mono
-**/*.DotSettings.user
+**/*.DotSettings.user
+**/*.lscache

images/Dockerfile

@@ -5,8 +5,8 @@ ARG TARGETOS
 ARG TARGETARCH
 ARG RUNNER_VERSION
 ARG RUNNER_CONTAINER_HOOKS_VERSION=0.7.0
-ARG DOCKER_VERSION=29.4.0
-ARG BUILDX_VERSION=0.33.0
+ARG DOCKER_VERSION=29.5.3
+ARG BUILDX_VERSION=0.34.1
 
 RUN apt update -y && apt install curl unzip -y
 

releaseNote.md

@@ -1,36 +1,40 @@
 ## What's Changed
-* Bump flatted from 3.2.7 to 3.4.2 in /src/Misc/expressionFunc/hashFiles by @dependabot[bot] in https://github.com/actions/runner/pull/4307
-* Add DAP server by @rentziass in https://github.com/actions/runner/pull/4298
-* Bump @typescript-eslint/eslint-plugin from 8.57.1 to 8.57.2 in /src/Misc/expressionFunc/hashFiles by @dependabot[bot] in https://github.com/actions/runner/pull/4310
-* Remove AllowCaseFunction feature flag by @ericsciple in https://github.com/actions/runner/pull/4316
-* chore: update Node versions by @github-actions[bot] in https://github.com/actions/runner/pull/4319
-* Batch and deduplicate action resolution across composite depths by @stefanpenner in https://github.com/actions/runner/pull/4296
-* Add support for Bearer token in action archive downloads by @TingluoHuang in https://github.com/actions/runner/pull/4321
-* Bump brace-expansion in /src/Misc/expressionFunc/hashFiles by @dependabot[bot] in https://github.com/actions/runner/pull/4318
-* Add devtunnel connection for debugger jobs by @rentziass in https://github.com/actions/runner/pull/4317
-* Update Docker to v29.3.1 and Buildx to v0.33.0 by @github-actions[bot] in https://github.com/actions/runner/pull/4324
-* Bump @typescript-eslint/eslint-plugin from 8.57.2 to 8.58.1 in /src/Misc/expressionFunc/hashFiles by @dependabot[bot] in https://github.com/actions/runner/pull/4327
-* Bump actions/github-script from 8 to 9 by @dependabot[bot] in https://github.com/actions/runner/pull/4331
-* Bump typescript from 5.9.3 to 6.0.2 in /src/Misc/expressionFunc/hashFiles by @dependabot[bot] in https://github.com/actions/runner/pull/4329
-* fix: only show changed versions in node upgrade PR description by @salmanmkc in https://github.com/actions/runner/pull/4332
-* Bump System.Formats.Asn1, Cryptography.Pkcs, ProtectedData, ServiceController, CodePages, Threading.Channels, @actions/glob, @typescript-eslint/parser, lint-staged, picomatch by @Copilot in https://github.com/actions/runner/pull/4333
-* feat: add `job.workflow_*` typed accessors to JobContext by @salmanmkc in https://github.com/actions/runner/pull/4335
-* Add WS bridge over DAP TCP server by @rentziass in https://github.com/actions/runner/pull/4328
-* chore: update Node versions by @github-actions[bot] in https://github.com/actions/runner/pull/4355
-* Bump Docker version to 29.4.0 by @Copilot in https://github.com/actions/runner/pull/4352
-* Update dotnet sdk to latest version @8.0.420 by @github-actions[bot] in https://github.com/actions/runner/pull/4356
-* Bump @typescript-eslint/parser from 8.58.1 to 8.59.0 in /src/Misc/expressionFunc/hashFiles by @dependabot[bot] in https://github.com/actions/runner/pull/4360
-* Bump System.Formats.Asn1 and System.Security.Cryptography.Pkcs by @dependabot[bot] in https://github.com/actions/runner/pull/4362
-* Add vulnerability-alerts permission by @salmanmkc in https://github.com/actions/runner/pull/4350
-* Bump @typescript-eslint/eslint-plugin from 8.58.1 to 8.59.0 in /src/Misc/expressionFunc/hashFiles by @dependabot[bot] in https://github.com/actions/runner/pull/4359
-* Bump System.ServiceProcess.ServiceController from 10.0.3 to 10.0.6 by @dependabot[bot] in https://github.com/actions/runner/pull/4358
-* Bump typescript from 6.0.2 to 6.0.3 in /src/Misc/expressionFunc/hashFiles by @dependabot[bot] in https://github.com/actions/runner/pull/4353
-* Bump Microsoft.DevTunnels.Connections from 1.3.16 to 1.3.39 by @dependabot[bot] in https://github.com/actions/runner/pull/4339
+* Bump System.ServiceProcess.ServiceController from 10.0.6 to 10.0.7 by @dependabot[bot] in https://github.com/actions/runner/pull/4370
+* Bump @actions/glob from 0.6.1 to 0.7.0 in /src/Misc/expressionFunc/hashFiles by @dependabot[bot] in https://github.com/actions/runner/pull/4367
+* feat: propagate actions dependencies by @nodeselector in https://github.com/actions/runner/pull/4372
+* Not retry and report action download 403. by @TingluoHuang in https://github.com/actions/runner/pull/4391
+* Update setup job starting logs by @GitPaulo in https://github.com/actions/runner/pull/4383
+* fix: expand commit hash regex to support SHA-256 (64-char) hashes by @yaananth in https://github.com/actions/runner/pull/4347
+* Move dap setup to setup job step by @rentziass in https://github.com/actions/runner/pull/4403
+* Add support for Ubuntu 26.04 (liblttng-ust1t64, libicu77-80) by @dvaldivia in https://github.com/actions/runner/pull/4394
+* Update dotnet sdk to latest version @8.0.421 by @github-actions[bot] in https://github.com/actions/runner/pull/4428
+* Update Docker to v29.5.0 and Buildx to v0.34.0 by @github-actions[bot] in https://github.com/actions/runner/pull/4425
+* Execute debugger REPL commands inside job container by @rentziass in https://github.com/actions/runner/pull/4420
+* Send welcome message in debugger console on connect by @rentziass in https://github.com/actions/runner/pull/4419
+* Update snapshot-if context and functions by @drielenr in https://github.com/actions/runner/pull/4443
+* chore: update Node versions by @github-actions[bot] in https://github.com/actions/runner/pull/4452
+* Allow disable node v8 maglev jit compiler on node24. by @TingluoHuang in https://github.com/actions/runner/pull/4447
+* Update Node 24 default date to June 16th, 2026 by @salmanmkc in https://github.com/actions/runner/pull/4462
+* Populate telemetry for non-action post-job steps by @drielenr in https://github.com/actions/runner/pull/4463
+* Add SDK types and results plumbing for background step control by @lokesh755 in https://github.com/actions/runner/pull/4472
+* Add job execution view model by @rentziass in https://github.com/actions/runner/pull/4470
+* Add thread-safety locks to StepsContext by @lokesh755 in https://github.com/actions/runner/pull/4475
+* Add background step deferral infrastructure and metadata plumbing by @lokesh755 in https://github.com/actions/runner/pull/4479
+* Wire job execution view into DAP by @rentziass in https://github.com/actions/runner/pull/4471
+* Background steps execution engine by @lokesh755 in https://github.com/actions/runner/pull/4476
+* Update Docker to v29.5.2 and Buildx to v0.34.1 by @github-actions[bot] in https://github.com/actions/runner/pull/4451
+* BrokerServer should not retry on 401. by @TingluoHuang in https://github.com/actions/runner/pull/4445
+* Add new env var to allow single-prefix multiline logs on stdout by @nuclearpidgeon in https://github.com/actions/runner/pull/4424
+* Bump Microsoft.DevTunnels.Connections from 1.3.39 to 1.3.48 by @dependabot[bot] in https://github.com/actions/runner/pull/4441
+* Bump System.Formats.Asn1 and System.Security.Cryptography.Pkcs by @dependabot[bot] in https://github.com/actions/runner/pull/4369
 
 ## New Contributors
-* @stefanpenner made their first contribution in https://github.com/actions/runner/pull/4296
+* @GitPaulo made their first contribution in https://github.com/actions/runner/pull/4383
+* @dvaldivia made their first contribution in https://github.com/actions/runner/pull/4394
+* @drielenr made their first contribution in https://github.com/actions/runner/pull/4443
+* @nuclearpidgeon made their first contribution in https://github.com/actions/runner/pull/4424
 
-**Full Changelog**: https://github.com/actions/runner/compare/v2.333.1...v2.334.0
+**Full Changelog**: https://github.com/actions/runner/compare/v2.334.0...v2.335.0
 
 _Note: Actions Runner follows a progressive release policy, so the latest release might not be available to your enterprise, organization, or repository yet.
 To confirm which version of the Actions Runner you should expect, please view the download instructions for your enterprise, organization, or repository.

releaseVersion

@@ -1 +1 @@
-<Update to ./src/runnerversion when creating release>
+2.335.1

src/Misc/externals.sh

@@ -7,7 +7,7 @@ NODE_ALPINE_URL=https://github.com/actions/alpine_nodejs/releases/download
 # When you update Node versions you must also create a new release of alpine_nodejs at that updated version.
 # Follow the instructions here: https://github.com/actions/alpine_nodejs?tab=readme-ov-file#getting-started
 NODE20_VERSION="20.20.2"
-NODE24_VERSION="24.15.0"
+NODE24_VERSION="24.16.0"
 
 get_abs_path() {
   # exploits the fact that pwd will print abs path when no args

src/Runner.Common/BrokerServer.cs

@@ -108,7 +108,7 @@ namespace GitHub.Runner.Common
 
         public bool ShouldRetryException(Exception ex)
         {
-            if (ex is AccessDeniedException || ex is RunnerNotFoundException || ex is HostedRunnerDeprovisionedException)
+            if (ex is AccessDeniedException || ex is VssUnauthorizedException || ex is RunnerNotFoundException || ex is HostedRunnerDeprovisionedException)
             {
                 return false;
             }

src/Runner.Common/Constants.cs

@@ -179,6 +179,7 @@ namespace GitHub.Runner.Common
                 public static readonly string EmitCompositeMarkers = "actions_runner_emit_composite_markers";
                 public static readonly string BatchActionResolution = "actions_batch_action_resolution";
                 public static readonly string UseBearerTokenForCodeload = "actions_use_bearer_token_for_codeload";
+                public static readonly string OverrideDebuggerWelcomeMessage = "actions_runner_override_debugger_welcome_message";
             }
 
             // Node version migration related constants
@@ -205,7 +206,7 @@ namespace GitHub.Runner.Common
                 public static readonly string Node20DeprecationUrl = "https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/";
 
                 // Node 20 migration dates (hardcoded fallbacks, can be overridden via job variables)
-                public static readonly string Node24DefaultDate = "June 2nd, 2026";
+                public static readonly string Node24DefaultDate = "June 16th, 2026";
                 public static readonly string Node20RemovalDate = "September 16th, 2026";
 
                 // Variable keys for server-overridable dates
@@ -307,6 +308,7 @@ namespace GitHub.Runner.Common
                 public static readonly string ForcedInternalNodeVersion = "ACTIONS_RUNNER_FORCED_INTERNAL_NODE_VERSION";
                 public static readonly string ForcedActionsNodeVersion = "ACTIONS_RUNNER_FORCE_ACTIONS_NODE_VERSION";
                 public static readonly string PrintLogToStdout = "ACTIONS_RUNNER_PRINT_LOG_TO_STDOUT";
+                public static readonly string DisableStdoutMultilineLogPrefixing = "ACTIONS_RUNNER_DISABLE_STDOUT_MULTILINE_LOG_PREFIXING";
                 public static readonly string ActionArchiveCacheDirectory = "ACTIONS_RUNNER_ACTION_ARCHIVE_CACHE";
                 public static readonly string SymlinkCachedActions = "ACTIONS_RUNNER_SYMLINK_CACHED_ACTIONS";
                 public static readonly string EmitCompositeMarkers = "ACTIONS_RUNNER_EMIT_COMPOSITE_MARKERS";

src/Runner.Common/JobServerQueue.cs

@@ -837,6 +837,15 @@ namespace GitHub.Runner.Common
                             timelineRecord.Variables[variable.Key] = variable.Value.Clone();
                         }
                     }
+
+                    // Merge background step metadata
+                    if (rec.IsBackground)
+                    {
+                        timelineRecord.IsBackground = rec.IsBackground;
+                    }
+                    timelineRecord.BackgroundControlType = rec.BackgroundControlType ?? timelineRecord.BackgroundControlType;
+                    timelineRecord.BackgroundControlStepIds = rec.BackgroundControlStepIds ?? timelineRecord.BackgroundControlStepIds;
+                    timelineRecord.ParallelGroupId = rec.ParallelGroupId ?? timelineRecord.ParallelGroupId;
                 }
                 else
                 {

src/Runner.Common/StdoutTraceListener.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Diagnostics;
 using System.Globalization;
 using System.IO;
@@ -9,10 +9,12 @@ namespace GitHub.Runner.Common
     public sealed class StdoutTraceListener : ConsoleTraceListener
     {
         private readonly string _hostType;
+        private readonly bool _disablePrefixMultilineLogs = false;
 
         public StdoutTraceListener(string hostType)
         {
             this._hostType = hostType;
+            this._disablePrefixMultilineLogs = StringUtil.ConvertToBoolean(Environment.GetEnvironmentVariable(Constants.Variables.Agent.DisableStdoutMultilineLogPrefixing));
         }
 
         // Copied and modified slightly from .Net Core source code. Modification was required to make it compile.
@@ -26,11 +28,20 @@ namespace GitHub.Runner.Common
 
             if (!string.IsNullOrEmpty(message))
             {
-                var messageLines = message.Split(Environment.NewLine);
-                foreach (var messageLine in messageLines)
+                if (!this._disablePrefixMultilineLogs)
+                {
+                    var messageLines = message.Split(Environment.NewLine);
+                    foreach (var messageLine in messageLines)
+                    {
+                        WriteHeader(source, eventType, id);
+                        WriteLine(messageLine);
+                        WriteFooter(eventCache);
+                    }
+                }
+                else
                 {
                     WriteHeader(source, eventType, id);
-                    WriteLine(messageLine);
+                    WriteLine(message);
                     WriteFooter(eventCache);
                 }
             }

src/Runner.Worker/ActionCommandManager.cs

@@ -282,8 +282,15 @@ namespace GitHub.Runner.Worker
                 }
             }
 
-            context.Global.EnvironmentVariables[envName] = command.Data;
-            context.SetEnvContext(envName, command.Data);
+            if (context.DeferredEnvironmentVariables != null)
+            {
+                context.DeferredEnvironmentVariables[envName] = command.Data;
+            }
+            else
+            {
+                context.Global.EnvironmentVariables[envName] = command.Data;
+                context.SetEnvContext(envName, command.Data);
+            }
             context.Debug($"{envName}='{command.Data}'");
         }
 
@@ -334,8 +341,15 @@ namespace GitHub.Runner.Worker
                 throw new Exception("Required field 'name' is missing in ##[set-output] command.");
             }
 
-            context.SetOutput(outputName, command.Data, out var reference);
-            context.Debug($"{reference}='{command.Data}'");
+            if (context.DeferredOutputs != null)
+            {
+                context.DeferredOutputs[outputName] = command.Data;
+            }
+            else
+            {
+                context.SetOutput(outputName, command.Data, out var reference);
+                context.Debug($"{reference}='{command.Data}'");
+            }
         }
 
         private static class SetOutputCommandProperties
@@ -465,8 +479,16 @@ namespace GitHub.Runner.Worker
             }
 
             ArgUtil.NotNullOrEmpty(command.Data, "path");
-            context.Global.PrependPath.RemoveAll(x => string.Equals(x, command.Data, StringComparison.CurrentCulture));
-            context.Global.PrependPath.Add(command.Data);
+            if (context.DeferredPrependPath != null)
+            {
+                context.DeferredPrependPath.RemoveAll(x => string.Equals(x, command.Data, StringComparison.CurrentCulture));
+                context.DeferredPrependPath.Add(command.Data);
+            }
+            else
+            {
+                context.Global.PrependPath.RemoveAll(x => string.Equals(x, command.Data, StringComparison.CurrentCulture));
+                context.Global.PrependPath.Add(command.Data);
+            }
         }
     }
 

src/Runner.Worker/BackgroundStepControlFlowData.cs

@@ -0,0 +1,21 @@
+using System;
+
+namespace GitHub.Runner.Worker
+{
+    /// <summary>
+    /// Pure data for control-flow steps (wait, wait-all, cancel).
+    /// Type uses Pipelines.BackgroundControlTypes string constants.
+    /// </summary>
+    public sealed class BackgroundStepControlFlowData
+    {
+        public string Type { get; set; }
+        public Guid StepId { get; set; }
+        public string StepName { get; set; }
+
+        // Target step IDs (for wait: steps to wait for; for cancel: steps to cancel)
+        public string[] StepIds { get; set; }
+
+        // Parallel group ID for grouping steps in the UI
+        public string ParallelGroupId { get; set; }
+    }
+}

src/Runner.Worker/BackgroundStepCoordinator.cs

@@ -0,0 +1,394 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using GitHub.DistributedTask.WebApi;
+using GitHub.Runner.Common;
+using GitHub.Runner.Common.Util;
+using GitHub.Runner.Sdk;
+using Pipelines = GitHub.DistributedTask.Pipelines;
+
+namespace GitHub.Runner.Worker
+{
+    [ServiceLocator(Default = typeof(BackgroundStepCoordinator))]
+    public interface IBackgroundStepCoordinator : IRunnerService
+    {
+        void InitializeCoordinator(int maxConcurrent);
+        void StartBackgroundStep(IStep step, CancellationToken jobCancellationToken);
+        Task<TaskResult> WaitForUnwaitedStepsAsync(CancellationToken cancellationToken);
+        Task RunControlFlowAsync(IExecutionContext stepContext, object data);
+    }
+
+    /// <summary>
+    /// Coordinates background step execution, waiting, cancellation, and deferred state.
+    /// Extracted from StepsRunner so the main step loop stays clean.
+    /// </summary>
+    public sealed class BackgroundStepCoordinator : RunnerService, IBackgroundStepCoordinator
+    {
+        private const int DefaultMaxBackgroundSteps = 10;
+        private readonly Dictionary<string, (IStep Step, Task Task, CancellationTokenSource Cts)> _backgroundSteps = new();
+
+        // IDs of background steps that have already been completed (waited on or canceled).
+        // Used to avoid waiting on or flushing the same step more than once.
+        private readonly HashSet<string> _completedStepIds = new();
+
+        // IDs of background steps that were explicitly canceled via a `cancel` control step.
+        // These steps are expected to be canceled, so their (Canceled) result must not be
+        // merged into the overall job result.
+        private readonly HashSet<string> _explicitlyCanceledStepIds = new();
+        private SemaphoreSlim _backgroundSlotSemaphore = new SemaphoreSlim(DefaultMaxBackgroundSteps);
+
+        /// <summary>
+        /// Reset per-job state. Call at the start of each job.
+        /// </summary>
+        public void InitializeCoordinator(int maxConcurrent)
+        {
+            _backgroundSteps.Clear();
+            _completedStepIds.Clear();
+            _explicitlyCanceledStepIds.Clear();
+            var max = maxConcurrent > 0 ? maxConcurrent : DefaultMaxBackgroundSteps;
+            _backgroundSlotSemaphore = new SemaphoreSlim(max);
+        }
+
+        // -----------------------------------------------------------------
+        // Starting background steps
+        // -----------------------------------------------------------------
+
+        /// <summary>
+        /// Prepare and launch a background step. Does not block the caller.
+        /// </summary>
+        public void StartBackgroundStep(IStep step, CancellationToken jobCancellationToken)
+        {
+            var stepId = step.ExecutionContext?.ContextName ?? step.DisplayName;
+
+            // Isolate GitHubContext so concurrent steps don't overwrite each other's GITHUB_OUTPUT paths
+            if (step.ExecutionContext.ExpressionValues.TryGetValue("github", out var ghCtx) && ghCtx is GitHubContext sharedGitHub)
+            {
+                step.ExecutionContext.ExpressionValues["github"] = sharedGitHub.ShallowCopy();
+            }
+
+            var bgCts = CancellationTokenSource.CreateLinkedTokenSource(jobCancellationToken);
+
+            // Evaluate timeout on the main thread (needs expression context)
+            var timeoutMinutes = 0;
+            try
+            {
+                var templateEvaluator = step.ExecutionContext.ToPipelineTemplateEvaluator();
+                timeoutMinutes = templateEvaluator.EvaluateStepTimeout(step.Timeout, step.ExecutionContext.ExpressionValues, step.ExecutionContext.ExpressionFunctions);
+            }
+            catch (Exception ex)
+            {
+                Trace.Info($"Error determining timeout for background step '{stepId}': {ex.Message}");
+            }
+
+            var task = ExecuteBackgroundStepCoreAsync(step, bgCts, stepId, timeoutMinutes);
+            _backgroundSteps[stepId] = (step, task, bgCts);
+            Trace.Info($"Background step '{stepId}' queued (slot will be acquired asynchronously).");
+        }
+
+        // -----------------------------------------------------------------
+        // Safety net
+        // -----------------------------------------------------------------
+
+        // Drain any background steps that weren't already waited on by an explicit wait/cancel
+        // control step, then merge the final results of all background steps into a single result
+        // for the caller to fold into the job result.
+        public async Task<TaskResult> WaitForUnwaitedStepsAsync(CancellationToken cancellationToken)
+        {
+            var unwaitedIds = _backgroundSteps.Keys.Where(id => !_completedStepIds.Contains(id)).ToList();
+            if (unwaitedIds.Count > 0)
+            {
+                Trace.Info($"Safety net: {unwaitedIds.Count} unwaited background step(s) at post-job boundary: {string.Join(", ", unwaitedIds)}");
+                await WaitForStepTasksAsync(unwaitedIds, cancellationToken);
+                CompleteWaitedSteps(unwaitedIds);
+            }
+
+            var result = TaskResult.Succeeded;
+            foreach (var (stepId, (step, _, _)) in _backgroundSteps)
+            {
+                // A step that succeeded does not set a Result by default, so a missing
+                // value means the step succeeded and there is nothing to merge.
+                if (!step.ExecutionContext.Result.HasValue)
+                {
+                    continue;
+                }
+
+                // A step explicitly canceled via a `cancel` control step is expected to be canceled,
+                // so a Canceled result must not influence the overall job result. However, if the step
+                // failed (e.g. before the cancellation took effect), that failure should still count.
+                if (_explicitlyCanceledStepIds.Contains(stepId) &&
+                    step.ExecutionContext.Result.Value == TaskResult.Canceled)
+                {
+                    continue;
+                }
+
+                result = TaskResultUtil.MergeTaskResults(result, step.ExecutionContext.Result.Value);
+            }
+
+            if (result != TaskResult.Succeeded)
+            {
+                Trace.Info($"Background steps reported result '{result}' to caller.");
+            }
+
+            return result;
+        }
+
+        // -----------------------------------------------------------------
+        // Control-flow step dispatch
+        // -----------------------------------------------------------------
+
+        /// <summary>
+        /// Execute a control-flow step (wait, wait-all, cancel) and propagate results.
+        /// </summary>
+        public async Task RunControlFlowAsync(IExecutionContext stepContext, object data)
+        {
+            var controlFlow = data as BackgroundStepControlFlowData;
+            switch (controlFlow.Type)
+            {
+                case Pipelines.BackgroundControlTypes.Wait:
+                {
+                    var ids = controlFlow.StepIds ?? Array.Empty<string>();
+                    stepContext.Output($"Waiting for background step(s) to complete: {DescribeSteps(ids)}");
+                    await WaitForStepTasksAsync(ids, stepContext.CancellationToken);
+                    stepContext.Result = CompleteWaitedSteps(ids);
+                    ReportCompletedSteps(stepContext, "Finished waiting for background step(s).", ids);
+                    break;
+                }
+
+                case Pipelines.BackgroundControlTypes.WaitAll:
+                {
+                    var remaining = _backgroundSteps.Keys.Where(id => !_completedStepIds.Contains(id)).ToList();
+                    stepContext.Output(remaining.Count > 0
+                        ? $"Waiting for all background step(s) to complete: {DescribeSteps(remaining)}"
+                        : "No background steps remaining to wait for.");
+                    await WaitForStepTasksAsync(remaining, stepContext.CancellationToken);
+                    stepContext.Result = CompleteWaitedSteps(remaining);
+                    ReportCompletedSteps(stepContext, "Finished waiting for all background step(s).", remaining);
+                    break;
+                }
+
+                case Pipelines.BackgroundControlTypes.Cancel:
+                {
+                    var cancelIds = controlFlow.StepIds ?? Array.Empty<string>();
+                    stepContext.Output($"Cancelling background step(s): {DescribeSteps(cancelIds)}");
+                    await CancelStepsAsync(controlFlow.StepIds);
+                    stepContext.Result = TaskResult.Succeeded;
+                    ReportCompletedSteps(stepContext, "Finished cancelling background step(s).", cancelIds);
+                    break;
+                }
+
+                default:
+                    throw new ArgumentException($"Unknown background step control type '{controlFlow.Type}'.");
+            }
+        }
+
+        // -----------------------------------------------------------------
+        // Private helpers
+        // -----------------------------------------------------------------
+
+        // Resolve background step IDs to their display names for customer-facing output.
+        private string DescribeSteps(IEnumerable<string> stepIds)
+        {
+            var names = stepIds
+                .Select(id => _backgroundSteps.TryGetValue(id, out var entry) ? entry.Step.DisplayName : id)
+                .ToList();
+            return names.Count > 0 ? string.Join(", ", names) : "(none)";
+        }
+
+        // Emit a completion summary plus the final result of each affected background step.
+        private void ReportCompletedSteps(IExecutionContext stepContext, string summary, IEnumerable<string> stepIds)
+        {
+            stepContext.Output(summary);
+            foreach (var id in stepIds)
+            {
+                if (_backgroundSteps.TryGetValue(id, out var entry))
+                {
+                    var result = entry.Step.ExecutionContext.Result?.ToString() ?? "Unknown";
+                    stepContext.Output($"  {entry.Step.DisplayName}: {result}");
+                }
+            }
+        }
+
+        private async Task ExecuteBackgroundStepCoreAsync(
+            IStep step, CancellationTokenSource bgCts,
+            string stepId, int timeoutMinutes)
+        {
+            Trace.Info($"Background step '{stepId}' waiting for slot.");
+            await _backgroundSlotSemaphore.WaitAsync(bgCts.Token);
+            Trace.Info($"Background step '{stepId}' acquired slot.");
+
+            step.ExecutionContext.Start();
+
+            if (timeoutMinutes > 0)
+            {
+                step.ExecutionContext.SetTimeout(TimeSpan.FromMinutes(timeoutMinutes));
+            }
+
+            using var cancelReg = bgCts.Token.Register(() =>
+            {
+                Trace.Info($"Background step '{stepId}': cancellation signalled, sending CancelToken to process.");
+                step.ExecutionContext.CancelToken();
+            });
+
+            TaskResult? result = null;
+            try
+            {
+                await step.RunAsync();
+                result = step.ExecutionContext.Result ?? TaskResult.Succeeded;
+            }
+            catch (OperationCanceledException) when (bgCts.Token.IsCancellationRequested)
+            {
+                result = TaskResult.Canceled;
+            }
+            catch (OperationCanceledException) when (step.ExecutionContext.CancellationToken.IsCancellationRequested)
+            {
+                Trace.Info($"Background step '{stepId}' timed out after {timeoutMinutes} minutes.");
+                step.ExecutionContext.Error($"The background step '{step.DisplayName}' has timed out after {timeoutMinutes} minutes.");
+                result = TaskResult.Failed;
+            }
+            catch (Exception ex)
+            {
+                Trace.Info($"Background step '{stepId}' failed: {ex.Message}");
+                step.ExecutionContext.Error(ex);
+                result = TaskResult.Failed;
+            }
+            finally
+            {
+                _backgroundSlotSemaphore.Release();
+
+                if (step.ExecutionContext.CommandResult != null)
+                {
+                    result = TaskResultUtil.MergeTaskResults(result, step.ExecutionContext.CommandResult.Value);
+                }
+
+                step.ExecutionContext.Result = result;
+                step.ExecutionContext.ApplyContinueOnError(step.ContinueOnError);
+
+                step.ExecutionContext.Complete(step.ExecutionContext.Result);
+                Trace.Info($"Background step '{stepId}' completed with result: {step.ExecutionContext.Result}");
+            }
+        }
+
+        private async Task CancelStepsAsync(string[] cancelStepIds)
+        {
+            if (cancelStepIds == null || cancelStepIds.Length == 0)
+            {
+                return;
+            }
+
+            // Mark these steps as expected-to-be-canceled so their result does not
+            // affect the overall job result.
+            foreach (var id in cancelStepIds)
+            {
+                _explicitlyCanceledStepIds.Add(id);
+            }
+
+            var idsToCancel = cancelStepIds
+                .Where(id => _backgroundSteps.ContainsKey(id) && !_backgroundSteps[id].Task.IsCompleted)
+                .ToArray();
+
+            if (idsToCancel.Length > 0)
+            {
+                Trace.Info($"Cancelling {idsToCancel.Length} background step(s): {string.Join(", ", idsToCancel)}");
+                await CancelWithGracePeriodAsync(idsToCancel);
+            }
+
+            // Flush deferred state and mark canceled steps as completed.
+            CompleteWaitedSteps(cancelStepIds);
+        }
+
+        private async Task WaitForStepTasksAsync(IEnumerable<string> stepIds, CancellationToken cancellationToken)
+        {
+            var ids = stepIds.ToList();
+            var tasks = new List<Task>();
+
+            foreach (var stepId in ids)
+            {
+                if (_backgroundSteps.TryGetValue(stepId, out var entry) && !entry.Task.IsCompleted)
+                {
+                    tasks.Add(entry.Task);
+                }
+                else if (!_backgroundSteps.ContainsKey(stepId))
+                {
+                    Trace.Info($"Wait references unknown background step: {stepId}");
+                }
+            }
+
+            if (tasks.Count > 0)
+            {
+                Trace.Info($"Waiting for {tasks.Count} background step(s)...");
+                try
+                {
+                    await Task.WhenAll(tasks).WaitAsync(cancellationToken);
+                }
+                catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
+                {
+                    Trace.Info("Wait interrupted by job cancellation — cancelling background steps.");
+                    await CancelWithGracePeriodAsync(ids);
+                }
+            }
+        }
+
+        private async Task CancelWithGracePeriodAsync(IEnumerable<string> stepIds, double graceSeconds = 7.5)
+        {
+            var cancelledSteps = new List<(string StepId, Task Task, IStep Step)>();
+            foreach (var stepId in stepIds)
+            {
+                if (_backgroundSteps.TryGetValue(stepId, out var entry) && !entry.Task.IsCompleted)
+                {
+                    entry.Step.ExecutionContext.CancelToken();
+                    entry.Cts.Cancel();
+                    cancelledSteps.Add((stepId, entry.Task, entry.Step));
+                }
+            }
+
+            if (cancelledSteps.Count > 0)
+            {
+                try
+                {
+                    await Task.WhenAll(cancelledSteps.Select(s => s.Task)).WaitAsync(TimeSpan.FromSeconds(graceSeconds));
+                }
+                catch (TimeoutException)
+                {
+                    Trace.Info($"Some background steps did not terminate within {graceSeconds}s grace period.");
+
+                    // The step tasks above never completed, so their finally block never ran and
+                    // their result was never set. Force-mark them as canceled so the abandoned
+                    // steps still report a terminal result.
+                    foreach (var (stepId, task, step) in cancelledSteps)
+                    {
+                        if (!task.IsCompleted && !step.ExecutionContext.Result.HasValue)
+                        {
+                            step.ExecutionContext.Result = TaskResult.Canceled;
+                            Trace.Info($"Background step '{stepId}' did not terminate within grace period; marking as canceled.");
+                        }
+                    }
+                }
+            }
+        }
+
+        private TaskResult CompleteWaitedSteps(IEnumerable<string> stepIds)
+        {
+            var result = TaskResult.Succeeded;
+            foreach (var id in stepIds)
+            {
+                _completedStepIds.Add(id);
+                if (_backgroundSteps.TryGetValue(id, out var entry))
+                {
+                    // Flush deferred state for the completed step.
+                    entry.Step.ExecutionContext.FlushDeferredOutputs();
+                    entry.Step.ExecutionContext.FlushDeferredEnvironment();
+                    entry.Step.ExecutionContext.FlushDeferredOutcomeConclusion();
+                    Trace.Info($"Flushed deferred state for background step '{id}'.");
+
+                    if (entry.Step.ExecutionContext.Result.HasValue)
+                    {
+                        result = TaskResultUtil.MergeTaskResults(result, entry.Step.ExecutionContext.Result.Value);
+                    }
+                }
+            }
+            return result;
+        }
+    }
+}

src/Runner.Worker/Dap/DapDebugger.cs

@@ -16,6 +16,7 @@ using Microsoft.DevTunnels.Connections;
 using Microsoft.DevTunnels.Contracts;
 using Microsoft.DevTunnels.Management;
 using Newtonsoft.Json;
+using Pipelines = GitHub.DistributedTask.Pipelines;
 
 namespace GitHub.Runner.Worker.Dap
 {
@@ -27,6 +28,7 @@ namespace GitHub.Runner.Worker.Dap
         public string DisplayName { get; set; }
         public TaskResult? Result { get; set; }
         public int FrameId { get; set; }
+        public int? SourceLine { get; set; }
     }
 
     /// <summary>
@@ -54,6 +56,9 @@ namespace GitHub.Runner.Worker.Dap
         // Frame IDs for completed steps start at 1000
         private const int _completedFrameIdBase = 1000;
 
+        // Stable session-scoped source reference for the synthesized job step list.
+        private const int _jobStepsSourceReference = 1;
+
         private TcpListener _listener;
         private TcpClient _client;
         private NetworkStream _stream;
@@ -63,6 +68,7 @@ namespace GitHub.Runner.Worker.Dap
         private volatile DapSessionState _state = DapSessionState.NotStarted;
         private CancellationTokenRegistration? _cancellationRegistration;
         private bool _isFirstStep = true;
+        private bool _welcomeMessageSent;
 
         // Dev Tunnel relay host for remote debugging
         private TunnelRelayTunnelHost _tunnelRelayHost;
@@ -97,6 +103,8 @@ namespace GitHub.Runner.Worker.Dap
         // Track completed steps for stack trace
         private readonly List<CompletedStepInfo> _completedSteps = new List<CompletedStepInfo>();
         private int _nextCompletedFrameId = _completedFrameIdBase;
+        private JobExecutionView _jobStepsSource;
+        private bool _jobCompleted;
 
         // Client connection tracking for reconnection support
         private volatile bool _isClientConnected;
@@ -239,6 +247,179 @@ namespace GitHub.Runner.Worker.Dap
             }
         }
 
+        public Task OnJobStepsInitializedAsync(IEnumerable<IStep> steps, IEnumerable<IStep> initialPostSteps)
+        {
+            if (!IsActive)
+            {
+                return Task.CompletedTask;
+            }
+
+            try
+            {
+                IExecutionContext jobContext;
+                lock (_stateLock)
+                {
+                    if (_state != DapSessionState.Ready &&
+                        _state != DapSessionState.Paused &&
+                        _state != DapSessionState.Running)
+                    {
+                        return Task.CompletedTask;
+                    }
+
+                    jobContext = _jobContext;
+                }
+
+                var stepList = steps?.Where(step => step != null).ToList() ?? new List<IStep>();
+                var initialPostStepList = initialPostSteps?.Where(step => step != null).ToList() ?? new List<IStep>();
+                var jobId = jobContext?.GetGitHubContext("job");
+                var snapshot = new JobExecutionView(
+                    jobId,
+                    stepList,
+                    initialPostStepList,
+                    PredictPostSteps(jobContext, stepList, initialPostStepList));
+
+                lock (_stateLock)
+                {
+                    _jobStepsSource = snapshot;
+                    _jobCompleted = false;
+                }
+                Trace.Info("DAP job steps source initialized");
+            }
+            catch (Exception ex)
+            {
+                Trace.Warning("DAP OnJobStepsInitialized error.");
+                Trace.Error(ex);
+            }
+
+            return Task.CompletedTask;
+        }
+
+        public void OnPostStepRegistered(IStep step)
+        {
+            try
+            {
+                if (step is IActionRunner postRunner && postRunner.Action != null)
+                {
+                    JobExecutionView snapshot;
+                    lock (_stateLock)
+                    {
+                        snapshot = _jobStepsSource;
+                    }
+
+                    var line = snapshot?.TryClaimPredictedStep(MatchKeyFor(postRunner.Action.Id), step);
+                    if (line.HasValue)
+                    {
+                        Trace.Info($"DAP job steps source claimed predicted post step '{step.DisplayName}' at line {line.Value}.");
+                    }
+                    else
+                    {
+                        Trace.Info($"DAP job steps source had no predicted line for post step '{step.DisplayName}'.");
+                    }
+                }
+            }
+            catch (Exception ex)
+            {
+                Trace.Warning("DAP OnPostStepRegistered error.");
+                Trace.Error(ex);
+            }
+        }
+
+        private IReadOnlyList<JobExecutionView.PredictedPostStep> PredictPostSteps(
+            IExecutionContext jobContext,
+            IReadOnlyList<IStep> steps,
+            IReadOnlyList<IStep> initialPostSteps)
+        {
+            if (jobContext == null || steps == null || steps.Count == 0)
+            {
+                return Array.Empty<JobExecutionView.PredictedPostStep>();
+            }
+
+            IActionManager actionManager;
+            try
+            {
+                actionManager = HostContext.GetService<IActionManager>();
+            }
+            catch (Exception ex)
+            {
+                Trace.Info($"DAP post-step predictor skipped because IActionManager is unavailable ({ex.Message}).");
+                return Array.Empty<JobExecutionView.PredictedPostStep>();
+            }
+
+            var predictions = new List<JobExecutionView.PredictedPostStep>();
+            var seenActionIds = new HashSet<Guid>();
+            if (initialPostSteps != null)
+            {
+                foreach (var postStep in initialPostSteps)
+                {
+                    if (postStep is IActionRunner postRunner && postRunner.Action != null)
+                    {
+                        seenActionIds.Add(postRunner.Action.Id);
+                    }
+                }
+            }
+
+            foreach (var step in steps)
+            {
+                if (step is not IActionRunner runner ||
+                    runner.Stage == ActionRunStage.Post ||
+                    runner.Action == null)
+                {
+                    continue;
+                }
+
+                var action = runner.Action;
+                if (action.Reference is not Pipelines.RepositoryPathReference repoRef)
+                {
+                    continue;
+                }
+
+                if (!seenActionIds.Add(action.Id))
+                {
+                    continue;
+                }
+
+                Definition definition;
+                try
+                {
+                    definition = actionManager.LoadAction(jobContext, action);
+                }
+                catch (Exception ex)
+                {
+                    Trace.Info($"DAP post-step predictor could not load action '{repoRef.Name}' ({ex.Message}).");
+                    continue;
+                }
+
+                if (definition?.Data?.Execution?.HasPost != true)
+                {
+                    continue;
+                }
+
+                predictions.Add(new JobExecutionView.PredictedPostStep(
+                    GetPostDisplayName(runner),
+                    MatchKeyFor(action.Id)));
+            }
+
+            predictions.Reverse();
+            return predictions;
+        }
+
+        private static string GetPostDisplayName(IActionRunner runner)
+        {
+            var displayName = string.IsNullOrEmpty(runner.DisplayName) ? "step" : runner.DisplayName;
+            if (runner.Stage == ActionRunStage.Pre &&
+                displayName.StartsWith("Pre ", StringComparison.OrdinalIgnoreCase))
+            {
+                displayName = displayName.Substring("Pre ".Length);
+            }
+
+            return $"Post {displayName}";
+        }
+
+        private static string MatchKeyFor(Guid actionId)
+        {
+            return $"post:{actionId:N}";
+        }
+
         public async Task OnJobCompletedAsync()
         {
             if (_state != DapSessionState.NotStarted)
@@ -252,6 +433,11 @@ namespace GitHub.Runner.Worker.Dap
                         if (_jobContext != null)
                         {
                             Trace.Info("Job completed — pausing for inspection");
+                            lock (_stateLock)
+                            {
+                                _jobCompleted = true;
+                            }
+
                             SendStoppedEvent("completed", "Job completed — inspect variables before the session ends.");
 
                             await WaitForCommandAsync(_jobContext.CancellationToken);
@@ -358,6 +544,7 @@ namespace GitHub.Runner.Worker.Dap
                 {
                     _state = DapSessionState.Terminated;
                 }
+                _jobStepsSource = null;
             }
 
             _isClientConnected = false;
@@ -416,7 +603,8 @@ namespace GitHub.Runner.Worker.Dap
                     {
                         DisplayName = step.DisplayName,
                         Result = result,
-                        FrameId = _nextCompletedFrameId++
+                        FrameId = _nextCompletedFrameId++,
+                        SourceLine = _jobStepsSource?.TryGetLineForStep(step)
                     });
                 }
             }
@@ -467,6 +655,7 @@ namespace GitHub.Runner.Worker.Dap
                         "next" => HandleNext(request),
                         "setBreakpoints" => HandleSetBreakpoints(request),
                         "setExceptionBreakpoints" => HandleSetExceptionBreakpoints(request),
+                        "source" => HandleSource(request),
                         "completions" => HandleCompletions(request),
                         "stepIn" => CreateResponse(request, false, "Step In is not supported. Actions jobs debug at the step level - use 'next' to advance to the next step.", body: null),
                         "stepOut" => CreateResponse(request, false, "Step Out is not supported. Actions jobs debug at the step level - use 'continue' to resume.", body: null),
@@ -490,6 +679,11 @@ namespace GitHub.Runner.Worker.Dap
                     });
                     Trace.Info("Sent initialized event");
                 }
+
+                if (request.Command == "configurationDone")
+                {
+                    SendWelcomeMessage();
+                }
             }
             catch (Exception ex)
             {
@@ -508,6 +702,7 @@ namespace GitHub.Runner.Worker.Dap
         internal void HandleClientConnected()
         {
             _isClientConnected = true;
+            _welcomeMessageSent = false;
             Trace.Info("Client connected to debug session");
 
             // If we're paused, re-send the stopped event so the new client
@@ -818,10 +1013,39 @@ namespace GitHub.Runner.Worker.Dap
             });
         }
 
+        internal void SendWelcomeMessage()
+        {
+            if (_welcomeMessageSent)
+            {
+                return;
+            }
+            _welcomeMessageSent = true;
+
+            var debuggerConfig = _jobContext?.Global?.Debugger;
+            if (debuggerConfig?.OverrideWelcomeMessage == true)
+            {
+                if (!string.IsNullOrEmpty(debuggerConfig.WelcomeMessage))
+                {
+                    SendOutput("console", debuggerConfig.WelcomeMessage);
+                    Trace.Info("Sent custom welcome message");
+                }
+                else
+                {
+                    Trace.Info("Welcome message suppressed by override");
+                }
+            }
+            else
+            {
+                SendOutput("console", DapReplParser.GetGeneralHelp());
+                Trace.Info("Sent default welcome message");
+            }
+        }
+
         internal async Task OnStepStartingAsync(IStep step, bool isFirstStep)
         {
             bool pauseOnNextStep;
             CancellationToken cancellationToken;
+
             lock (_stateLock)
             {
                 if (_state != DapSessionState.Ready &&
@@ -833,6 +1057,7 @@ namespace GitHub.Runner.Worker.Dap
 
                 _currentStep = step;
                 _currentStepIndex = _completedSteps.Count;
+                _jobCompleted = false;
                 pauseOnNextStep = _pauseOnNextStep;
                 cancellationToken = _jobContext?.CancellationToken ?? CancellationToken.None;
             }
@@ -860,6 +1085,9 @@ namespace GitHub.Runner.Worker.Dap
             // Send stopped event to debugger (only if client is connected)
             SendStoppedEvent(reason, description);
 
+            // Emit a banner so the user knows where REPL commands will execute
+            SendExecutionContextBanner();
+
             // Wait for debugger command
             await WaitForCommandAsync(cancellationToken);
         }
@@ -1012,29 +1240,46 @@ namespace GitHub.Runner.Worker.Dap
         private Response HandleStackTrace(Request request)
         {
             IStep currentStep;
-            int currentStepIndex;
             CompletedStepInfo[] completedSteps;
+            JobExecutionView jobStepsSource;
+            bool jobCompleted;
             lock (_stateLock)
             {
                 currentStep = _currentStep;
-                currentStepIndex = _currentStepIndex;
                 completedSteps = _completedSteps.ToArray();
+                jobStepsSource = _jobStepsSource;
+                jobCompleted = _jobCompleted;
             }
 
             var frames = new List<StackFrame>();
+            var source = jobStepsSource != null ? BuildJobStepsSource(jobStepsSource) : null;
 
             // Add current step as the top frame
-            if (currentStep != null)
+            if (jobCompleted && jobStepsSource != null)
+            {
+                frames.Add(new StackFrame
+                {
+                    Id = _currentFrameId,
+                    Name = "Complete job [completed]",
+                    Source = source,
+                    Line = jobStepsSource.CompleteJobLine,
+                    Column = 1,
+                    PresentationHint = "normal"
+                });
+            }
+            else if (currentStep != null)
             {
                 var resultIndicator = currentStep.ExecutionContext?.Result != null
                     ? $" [{currentStep.ExecutionContext.Result}]"
                     : " [running]";
+                var currentSourceLine = jobStepsSource?.TryGetLineForStep(currentStep);
 
                 frames.Add(new StackFrame
                 {
                     Id = _currentFrameId,
                     Name = MaskUserVisibleText($"{currentStep.DisplayName ?? "Current Step"}{resultIndicator}"),
-                    Line = currentStepIndex + 1,
+                    Source = currentSourceLine.HasValue ? source : null,
+                    Line = currentSourceLine ?? 0,
                     Column = 1,
                     PresentationHint = "normal"
                 });
@@ -1060,7 +1305,8 @@ namespace GitHub.Runner.Worker.Dap
                 {
                     Id = completedStep.FrameId,
                     Name = MaskUserVisibleText($"{completedStep.DisplayName}{resultStr}"),
-                    Line = 1,
+                    Source = completedStep.SourceLine.HasValue ? source : null,
+                    Line = completedStep.SourceLine ?? 0,
                     Column = 1,
                     PresentationHint = "subtle"
                 });
@@ -1075,6 +1321,76 @@ namespace GitHub.Runner.Worker.Dap
             return CreateResponse(request, true, body: body);
         }
 
+        private Source BuildJobStepsSource(JobExecutionView snapshot)
+        {
+            return new Source
+            {
+                Name = MaskUserVisibleText(snapshot.SourceFileName),
+                Path = MaskUserVisibleText($"{SanitizeSourcePathSegment(snapshot.JobId)}/{snapshot.SourceFileName}"),
+                SourceReference = _jobStepsSourceReference,
+                PresentationHint = "normal"
+            };
+        }
+
+        private static string SanitizeSourcePathSegment(string value)
+        {
+            if (string.IsNullOrWhiteSpace(value))
+            {
+                return "job";
+            }
+
+            var builder = new StringBuilder(value.Length);
+            foreach (var character in value)
+            {
+                builder.Append(char.IsControl(character) || character == '/' || character == '\\'
+                    ? '_'
+                    : character);
+            }
+
+            return builder.Length == 0 ? "job" : builder.ToString();
+        }
+
+        internal Response HandleSource(Request request)
+        {
+            SourceArguments args;
+            try
+            {
+                args = request.Arguments?.ToObject<SourceArguments>();
+            }
+            catch (Exception ex)
+            {
+                Trace.Warning($"Failed to parse source arguments: {ex.GetType().Name}");
+                return CreateResponse(request, false, "Invalid source arguments.", body: null);
+            }
+
+            var sourceReference = args?.Source?.SourceReference ?? args?.SourceReference;
+            if (!sourceReference.HasValue)
+            {
+                return CreateResponse(request, false, "Missing source reference.", body: null);
+            }
+
+            JobExecutionView snapshot;
+            lock (_stateLock)
+            {
+                snapshot = _jobStepsSource;
+            }
+
+            if (snapshot == null)
+            {
+                return CreateResponse(request, false, "Job steps source not yet available.", body: null);
+            }
+
+            if (sourceReference.Value != _jobStepsSourceReference)
+            {
+                return CreateResponse(request, false, $"Unknown source reference: {sourceReference.Value}.", body: null);
+            }
+
+            return CreateResponse(request, true, body: new SourceResponseBody
+            {
+                Content = MaskUserVisibleText(snapshot.Content)
+            });
+        }
+
         private Response HandleScopes(Request request)
         {
             var args = request.Arguments?.ToObject<ScopesArguments>();
@@ -1195,7 +1511,12 @@ namespace GitHub.Runner.Worker.Dap
 
                 case RunCommand run:
                     var context = GetExecutionContextForFrame(frameId);
-                    return await _replExecutor.ExecuteRunCommandAsync(run, context, cancellationToken);
+                    bool isActionStep;
+                    lock (_stateLock)
+                    {
+                        isActionStep = _currentStep is IActionRunner;
+                    }
+                    return await _replExecutor.ExecuteRunCommandAsync(run, context, isActionStep, cancellationToken);
 
                 default:
                     return new EvaluateResponseBody
@@ -1407,6 +1728,40 @@ namespace GitHub.Runner.Worker.Dap
             });
         }
 
+        /// <summary>
+        /// Emits a console output banner telling the user whether REPL
+        /// commands will execute on the host or inside the job container.
+        /// </summary>
+        private void SendExecutionContextBanner()
+        {
+            if (!_isClientConnected)
+            {
+                return;
+            }
+
+            bool isActionStep = _currentStep is IActionRunner;
+            var container = _jobContext?.Global?.Container;
+
+            string target;
+            if (isActionStep && container != null &&
+                (!string.IsNullOrEmpty(container.ContainerId) ||
+                 FeatureManager.IsContainerHooksEnabled(_jobContext?.Global?.Variables)))
+            {
+                var image = container.ContainerImage ?? "container";
+                var shortId = !string.IsNullOrEmpty(container.ContainerId) && container.ContainerId.Length >= 12
+                    ? container.ContainerId.Substring(0, 12)
+                    : container.ContainerId ?? "";
+                var idSuffix = !string.IsNullOrEmpty(shortId) ? $" ({shortId})" : "";
+                target = $"job container: {image}{idSuffix}";
+            }
+            else
+            {
+                target = "runner host";
+            }
+
+            SendOutput("console", $"\nCommands will run on {target}\n");
+        }
+
         private string MaskUserVisibleText(string value)
         {
             if (string.IsNullOrEmpty(value))

src/Runner.Worker/Dap/DapMessages.cs

@@ -537,6 +537,46 @@ namespace GitHub.Runner.Worker.Dap
 
     #endregion
 
+    #region Source Request/Response
+
+    /// <summary>
+    /// Arguments for 'source' request.
+    /// </summary>
+    public class SourceArguments
+    {
+        /// <summary>
+        /// Source descriptor. Some clients send sourceReference only here.
+        /// </summary>
+        [JsonProperty("source", NullValueHandling = NullValueHandling.Ignore)]
+        public Source Source { get; set; }
+
+        /// <summary>
+        /// The reference to the source.
+        /// </summary>
+        [JsonProperty("sourceReference", NullValueHandling = NullValueHandling.Ignore)]
+        public int? SourceReference { get; set; }
+    }
+
+    /// <summary>
+    /// Response body for 'source' request.
+    /// </summary>
+    public class SourceResponseBody
+    {
+        /// <summary>
+        /// Content of the source as a string.
+        /// </summary>
+        [JsonProperty("content")]
+        public string Content { get; set; }
+
+        /// <summary>
+        /// Optional content type / mime type of the source.
+        /// </summary>
+        [JsonProperty("mimeType", NullValueHandling = NullValueHandling.Ignore)]
+        public string MimeType { get; set; }
+    }
+
+    #endregion
+
     #region Scopes Request/Response
 
     /// <summary>

src/Runner.Worker/Dap/DapReplExecutor.cs

@@ -9,6 +9,7 @@ using GitHub.DistributedTask.Pipelines.ContextData;
 using GitHub.Runner.Common;
 using GitHub.Runner.Common.Util;
 using GitHub.Runner.Sdk;
+using GitHub.Runner.Worker.Container;
 using GitHub.Runner.Worker.Handlers;
 
 namespace GitHub.Runner.Worker.Dap
@@ -43,6 +44,7 @@ namespace GitHub.Runner.Worker.Dap
         public async Task<EvaluateResponseBody> ExecuteRunCommandAsync(
             RunCommand command,
             IExecutionContext context,
+            bool isActionStep,
             CancellationToken cancellationToken)
         {
             if (context == null)
@@ -52,7 +54,7 @@ namespace GitHub.Runner.Worker.Dap
 
             try
             {
-                return await ExecuteScriptAsync(command, context, cancellationToken);
+                return await ExecuteScriptAsync(command, context, isActionStep, cancellationToken);
             }
             catch (Exception ex)
             {
@@ -65,9 +67,17 @@ namespace GitHub.Runner.Worker.Dap
         private async Task<EvaluateResponseBody> ExecuteScriptAsync(
             RunCommand command,
             IExecutionContext context,
+            bool isActionStep,
             CancellationToken cancellationToken)
         {
-            // 1. Resolve shell — same logic as ScriptHandler
+            // 1. Resolve step host — container or host, same as ActionRunner.
+            //    Only action steps (user-defined run:/uses:) execute inside the
+            //    container.  Infrastructure steps (Set up job, Initialize
+            //    containers, Complete job, etc.) always run on the host.
+            var stepHost = CreateStepHost(context, isActionStep);
+            var isContainerStepHost = stepHost is IContainerStepHost;
+
+            // 2. Resolve shell — same logic as ScriptHandler
             string shellCommand;
             string argFormat;
 
@@ -87,9 +97,9 @@ namespace GitHub.Runner.Worker.Dap
                 argFormat = ScriptHandlerHelpers.GetScriptArgumentsFormat(shellCommand);
             }
 
-            _trace.Info("Resolved REPL shell");
+            _trace.Info($"Resolved REPL shell (container={isContainerStepHost})");
 
-            // 2. Expand ${{ }} expressions in the script body, just like
+            // 3. Expand ${{ }} expressions in the script body, just like
             //    ActionRunner evaluates step inputs before ScriptHandler sees them
             var contents = ExpandExpressions(command.Script, context);
             contents = ScriptHandlerHelpers.FixUpScriptContents(shellCommand, contents);
@@ -111,25 +121,47 @@ namespace GitHub.Runner.Worker.Dap
 
             try
             {
-                // 3. Format arguments with script path
-                var resolvedPath = scriptFilePath.Replace("\"", "\\\"");
+                // 4. Resolve script path — translate for container if needed
+                var resolvedPath = stepHost.ResolvePathForStepHost(context, scriptFilePath).Replace("\"", "\\\"");
                 if (string.IsNullOrEmpty(argFormat) || !argFormat.Contains("{0}"))
                 {
                     return ErrorResult($"Invalid shell option '{shellCommand}'. Shell must be a valid built-in (bash, sh, cmd, powershell, pwsh) or a format string containing '{{0}}'");
                 }
                 var arguments = string.Format(argFormat, resolvedPath);
 
-                // 4. Resolve shell command path
+                // 5. Resolve shell command path — for containers, use the shell
+                //    name directly (it will be resolved inside the container);
+                //    for host execution, resolve the full path on the host.
                 string prependPath = string.Join(
                     Path.PathSeparator.ToString(),
                     Enumerable.Reverse(context.Global.PrependPath));
-                var commandPath = WhichUtil.Which(shellCommand, false, _trace, prependPath)
-                    ?? shellCommand;
+                var fileName = isContainerStepHost
+                    ? shellCommand
+                    : WhichUtil.Which(shellCommand, false, _trace, prependPath) ?? shellCommand;
 
-                // 5. Build environment — merge from execution context like a real step
+                // 6. Build environment — merge from execution context like a real step
                 var environment = BuildEnvironment(context, command.Env);
 
-                // 6. Resolve working directory
+                // 7. Handle PrependPath — mirrors Handler.AddPrependPathToEnvironment
+                if (context.Global.PrependPath.Count > 0)
+                {
+                    if (stepHost is IContainerStepHost containerHost)
+                    {
+                        containerHost.PrependPath = prependPath;
+                    }
+                    else
+                    {
+                        string taskEnvPATH;
+                        environment.TryGetValue(Constants.PathVariable, out taskEnvPATH);
+                        string originalPath = context.Global.Variables?.Get(Constants.PathVariable) ?? // Prefer a job variable.
+                            taskEnvPATH ?? // Then a task-environment variable.
+                            System.Environment.GetEnvironmentVariable(Constants.PathVariable) ?? // Then an environment variable.
+                            string.Empty;
+                        environment[Constants.PathVariable] = PathUtil.PrependPath(prependPath, originalPath);
+                    }
+                }
+
+                // 8. Resolve working directory — translate for container
                 var workingDirectory = command.WorkingDirectory;
                 if (string.IsNullOrEmpty(workingDirectory))
                 {
@@ -141,48 +173,60 @@ namespace GitHub.Runner.Worker.Dap
                         : null;
                     workingDirectory = workspace ?? _hostContext.GetDirectory(WellKnownDirectory.Work);
                 }
+                workingDirectory = stepHost.ResolvePathForStepHost(context, workingDirectory);
 
                 _trace.Info("Executing REPL command");
 
                 // Stream execution info to debugger
                 SendOutput("console", $"$ {shellCommand} {command.Script.Substring(0, Math.Min(command.Script.Length, 80))}{(command.Script.Length > 80 ? "..." : "")}\n");
 
-                // 7. Execute via IProcessInvoker (same as DefaultStepHost)
-                int exitCode;
-                using (var processInvoker = _hostContext.CreateService<IProcessInvoker>())
+                // NOTE: When container hooks are enabled, ContainerStepHost routes
+                // execution through IContainerHookManager which does not raise
+                // OutputDataReceived/ErrorDataReceived events. Output will not be
+                // streamed to the debug console in that mode.
+                if (isContainerStepHost && FeatureManager.IsContainerHooksEnabled(context.Global?.Variables))
                 {
-                    processInvoker.OutputDataReceived += (sender, args) =>
-                    {
-                        if (!string.IsNullOrEmpty(args.Data))
-                        {
-                            var masked = _hostContext.SecretMasker.MaskSecrets(args.Data);
-                            SendOutput("stdout", masked + "\n");
-                        }
-                    };
-
-                    processInvoker.ErrorDataReceived += (sender, args) =>
-                    {
-                        if (!string.IsNullOrEmpty(args.Data))
-                        {
-                            var masked = _hostContext.SecretMasker.MaskSecrets(args.Data);
-                            SendOutput("stderr", masked + "\n");
-                        }
-                    };
-
-                    exitCode = await processInvoker.ExecuteAsync(
-                        workingDirectory: workingDirectory,
-                        fileName: commandPath,
-                        arguments: arguments,
-                        environment: environment,
-                        requireExitCodeZero: false,
-                        outputEncoding: null,
-                        killProcessOnCancel: true,
-                        cancellationToken: cancellationToken);
+                    const string hookWarning = "Container hooks are enabled. REPL output will not be streamed to the debug console for this command.";
+                    _trace.Warning(hookWarning);
+                    SendOutput("stderr", hookWarning + "\n");
                 }
 
+                // 9. Execute via IStepHost — handles docker exec for containers,
+                //    direct process execution for host, and container hooks
+                stepHost.OutputDataReceived += (sender, args) =>
+                {
+                    if (!string.IsNullOrEmpty(args.Data))
+                    {
+                        var masked = _hostContext.SecretMasker.MaskSecrets(args.Data);
+                        SendOutput("stdout", masked + "\n");
+                    }
+                };
+
+                stepHost.ErrorDataReceived += (sender, args) =>
+                {
+                    if (!string.IsNullOrEmpty(args.Data))
+                    {
+                        var masked = _hostContext.SecretMasker.MaskSecrets(args.Data);
+                        SendOutput("stderr", masked + "\n");
+                    }
+                };
+
+                int exitCode = await stepHost.ExecuteAsync(
+                    context: context,
+                    workingDirectory: workingDirectory,
+                    fileName: fileName,
+                    arguments: arguments,
+                    environment: environment,
+                    requireExitCodeZero: false,
+                    outputEncoding: null,
+                    killProcessOnCancel: true,
+                    inheritConsoleHandler: false,
+                    standardInInput: null,
+                    cancellationToken: cancellationToken);
+
                 _trace.Info($"REPL command exited with code {exitCode}");
 
-                // 8. Return only the exit code summary (output was already streamed)
+                // 10. Return only the exit code summary (output was already streamed)
                 return new EvaluateResponseBody
                 {
                     Result = exitCode == 0 ? $"(exit code: {exitCode})" : $"Process completed with exit code {exitCode}.",
@@ -198,6 +242,43 @@ namespace GitHub.Runner.Worker.Dap
             }
         }
 
+        /// <summary>
+        /// Creates the appropriate <see cref="IStepHost"/> for the current
+        /// execution context, mirroring how <see cref="ActionRunner"/> decides
+        /// between host and container execution.
+        ///
+        /// Only action steps (user-defined run:/uses: steps) run inside the
+        /// job container.  Infrastructure steps like "Set up job", "Initialize
+        /// containers", "Stop containers", and "Complete job" always execute
+        /// on the host regardless of whether a container is configured.
+        /// </summary>
+        internal IStepHost CreateStepHost(IExecutionContext context, bool isActionStep)
+        {
+            if (!isActionStep)
+            {
+                _trace.Info("Creating DefaultStepHost for REPL execution (infrastructure step)");
+                return _hostContext.CreateService<IDefaultStepHost>();
+            }
+
+            var container = context?.Global?.Container;
+            if (container != null)
+            {
+                // Container hooks don't always set ContainerId, but the container
+                // step host handles that internally
+                var hooksEnabled = FeatureManager.IsContainerHooksEnabled(context.Global?.Variables);
+                if (hooksEnabled || !string.IsNullOrEmpty(container.ContainerId))
+                {
+                    _trace.Info("Creating ContainerStepHost for REPL execution");
+                    var containerStepHost = _hostContext.CreateService<IContainerStepHost>();
+                    containerStepHost.Container = container;
+                    return containerStepHost;
+                }
+            }
+
+            _trace.Info("Creating DefaultStepHost for REPL execution");
+            return _hostContext.CreateService<IDefaultStepHost>();
+        }
+
         /// <summary>
         /// Expands <c>${{ }}</c> expressions in the input string using the
         /// runner's template evaluator — the same evaluation path that processes

src/Runner.Worker/Dap/DebuggerConfig.cs

@@ -1,4 +1,4 @@
-using GitHub.DistributedTask.Pipelines;
+using GitHub.DistributedTask.Pipelines;
 
 namespace GitHub.Runner.Worker.Dap
 {
@@ -8,10 +8,12 @@ namespace GitHub.Runner.Worker.Dap
     /// </summary>
     public sealed class DebuggerConfig
     {
-        public DebuggerConfig(bool enabled, DebuggerTunnelInfo tunnel)
+        public DebuggerConfig(bool enabled, DebuggerTunnelInfo tunnel, bool overrideWelcomeMessage = false, string welcomeMessage = null)
         {
             Enabled = enabled;
             Tunnel = tunnel;
+            OverrideWelcomeMessage = overrideWelcomeMessage;
+            WelcomeMessage = welcomeMessage;
         }
 
         /// <summary>Whether the debugger is enabled for this job.</summary>
@@ -23,6 +25,19 @@ namespace GitHub.Runner.Worker.Dap
         /// </summary>
         public DebuggerTunnelInfo Tunnel { get; }
 
+        /// <summary>
+        /// When true, the runner overrides the default welcome message with
+        /// <see cref="WelcomeMessage"/>. A null or empty <see cref="WelcomeMessage"/>
+        /// suppresses the message entirely. When false, the default help text is shown.
+        /// </summary>
+        public bool OverrideWelcomeMessage { get; }
+
+        /// <summary>
+        /// Optional welcome message content for the debugger console. Only used when
+        /// <see cref="OverrideWelcomeMessage"/> is true.
+        /// </summary>
+        public string WelcomeMessage { get; }
+
         /// <summary>Whether the tunnel configuration is complete and valid.</summary>
         public bool HasValidTunnel => Tunnel != null
             && !string.IsNullOrEmpty(Tunnel.TunnelId)

src/Runner.Worker/Dap/IDapDebugger.cs

@@ -1,4 +1,5 @@
-using System.Threading.Tasks;
+using System.Collections.Generic;
+using System.Threading.Tasks;
 using GitHub.Runner.Common;
 
 namespace GitHub.Runner.Worker.Dap
@@ -19,6 +20,8 @@ namespace GitHub.Runner.Worker.Dap
     {
         Task StartAsync(IExecutionContext jobContext);
         Task WaitUntilReadyAsync();
+        Task OnJobStepsInitializedAsync(IEnumerable<IStep> steps, IEnumerable<IStep> initialPostSteps);
+        void OnPostStepRegistered(IStep step);
         Task OnStepStartingAsync(IStep step);
         void OnStepCompleted(IStep step);
         Task OnJobCompletedAsync();

src/Runner.Worker/Dap/JobExecutionView.cs

@@ -1,111 +1,99 @@
 using System;
 using System.Collections.Generic;
-using GitHub.Runner.Sdk;
+using System.Globalization;
+using System.Text;
 
 namespace GitHub.Runner.Worker.Dap
 {
-    /// <summary>
-    /// Stateful, append-only container that wraps <see cref="JobExecutionViewRenderer"/>
-    /// for runtime use. Maintains a mutable list of entries, caches the rendered YAML,
-    /// and provides O(1) lookup from <see cref="IStep"/> identity to the current line
-    /// in the rendered YAML where that step's <c>- step:</c> key appears.
-    ///
-    /// Each <see cref="Append"/> can register the entry in one of three modes:
-    ///   - With a non-null <c>stepIdentity</c>: registers the IStep→line mapping
-    ///     immediately. Used for entries whose real <see cref="IStep"/> is already
-    ///     known at append time.
-    ///   - With a non-null <c>matchKey</c>: registers an unclaimed placeholder
-    ///     that a later <see cref="TryClaim"/> binds to a real <see cref="IStep"/>.
-    ///     Used for entries whose <see cref="IStep"/> is materialized later. A
-    ///     placeholder that is never claimed simply stays in the view and is never
-    ///     paused on — the IStep→line mapping is only populated on claim.
-    ///   - With neither: a static entry that needs no line lookup.
-    ///
-    /// <see cref="Append"/> and <see cref="AppendRange"/> never remove or reorder
-    /// existing entries. <see cref="TryClaim"/> does not re-render. The IStep→line
-    /// mapping is rebuilt on every render, so lookups stay accurate even if a later
-    /// Append happens to shift previously-emitted entries.
-    /// </summary>
     internal sealed class JobExecutionView
     {
-        private readonly object _lock = new();
-        private readonly string _jobId;
-        private readonly List<JobExecutionViewEntry> _entries = new();
-        private readonly List<IStep> _stepIdentities = new();
-        private readonly Dictionary<IStep, int> _lineByStep =
-            new(ReferenceEqualityComparer.Instance);
-        // Map matchKey -> entry index for placeholders awaiting a future
-        // TryClaim. Removed when claimed.
-        private readonly Dictionary<string, int> _unclaimedByKey =
-            new(StringComparer.Ordinal);
-        private string _yaml;
-        private IReadOnlyList<int> _entryStartLines = Array.Empty<int>();
+        private const string _sourceFileName = "execution.yml";
 
-        public JobExecutionView(string jobId)
+        private readonly object _lock = new object();
+        private readonly List<SourceEntry> _preEntries = new List<SourceEntry>();
+        private readonly List<SourceEntry> _mainEntries = new List<SourceEntry>();
+        private readonly List<SourceEntry> _postEntries = new List<SourceEntry>();
+        private readonly List<StepLine> _lineByStep = new List<StepLine>();
+        private string _content;
+        private int _completeJobLine;
+
+        public JobExecutionView(
+            string jobId,
+            IEnumerable<IStep> steps,
+            IEnumerable<IStep> initialPostSteps,
+            IEnumerable<PredictedPostStep> predictedPostSteps = null)
         {
-            if (string.IsNullOrWhiteSpace(jobId))
-            {
-                throw new ArgumentException("jobId must not be null or whitespace.", nameof(jobId));
-            }
+            JobId = string.IsNullOrWhiteSpace(jobId) ? "job" : jobId;
 
-            _jobId = jobId;
+            _preEntries.Add(new SourceEntry("Set up job"));
+            AddSteps(steps);
+            AddPredictedPostSteps(predictedPostSteps);
+            AddSteps(initialPostSteps);
+            _postEntries.Add(SourceEntry.CreateSyntheticCompleteJob());
             Render();
         }
 
-        public string JobId
-        {
-            get { return _jobId; }
-        }
+        public string JobId { get; }
+        public string SourceFileName => _sourceFileName;
 
-        /// <summary>
-        /// Currently rendered YAML. Always reflects all entries appended so far,
-        /// plus the synthetic Setup header and Cleanup footer emitted by the renderer.
-        /// </summary>
-        public string Yaml
+        public string Content
         {
             get
             {
                 lock (_lock)
                 {
-                    return _yaml;
+                    return _content;
                 }
             }
         }
 
-        /// <summary>Number of entries (excludes synthetic Setup/Cleanup boundaries).</summary>
-        public int EntryCount
+        public int CompleteJobLine
         {
             get
             {
                 lock (_lock)
                 {
-                    return _entries.Count;
+                    return _completeJobLine;
                 }
             }
         }
 
-        /// <summary>
-        /// 1-based line where entry <paramref name="entryIndex"/>'s <c>- step:</c> key
-        /// currently appears in <see cref="Yaml"/>.
-        /// </summary>
-        public int GetLine(int entryIndex)
+        public int? TryClaimPredictedStep(string matchKey, IStep step)
         {
+            if (string.IsNullOrEmpty(matchKey) || step == null)
+            {
+                return null;
+            }
+
             lock (_lock)
             {
-                if (entryIndex < 0 || entryIndex >= _entries.Count)
+                var existingLine = TryGetLineForStepNoLock(step);
+                if (existingLine.HasValue)
                 {
-                    throw new ArgumentOutOfRangeException(nameof(entryIndex));
+                    return existingLine;
                 }
 
-                return _entryStartLines[entryIndex];
+                foreach (var entry in _postEntries)
+                {
+                    if (!string.Equals(entry.MatchKey, matchKey, StringComparison.Ordinal))
+                    {
+                        continue;
+                    }
+
+                    if (entry.Step != null && !ReferenceEquals(entry.Step, step))
+                    {
+                        return null;
+                    }
+
+                    entry.Step = step;
+                    Render();
+                    return TryGetLineForStepNoLock(step);
+                }
+
+                return null;
             }
         }
 
-        /// <summary>
-        /// 1-based line for the entry whose <see cref="IStep"/> reference identity
-        /// matches <paramref name="step"/>. Returns null if <paramref name="step"/>
-        /// is null or has not been registered.
-        /// </summary>
         public int? TryGetLineForStep(IStep step)
         {
             if (step == null)
@@ -115,162 +103,256 @@ namespace GitHub.Runner.Worker.Dap
 
             lock (_lock)
             {
-                if (_lineByStep.TryGetValue(step, out var line))
-                {
-                    return line;
-                }
-
-                return null;
+                return TryGetLineForStepNoLock(step);
             }
         }
 
-        /// <summary>
-        /// Append a new entry. Exactly one of <paramref name="stepIdentity"/>
-        /// or <paramref name="matchKey"/> may be non-null (or both may be
-        /// null for a static entry that needs no line lookup):
-        ///   - <paramref name="stepIdentity"/> non-null: registers the
-        ///     IStep→line mapping immediately. Use when the real
-        ///     <see cref="IStep"/> is known at append time.
-        ///   - <paramref name="matchKey"/> non-null: registers an unclaimed
-        ///     placeholder that a later <see cref="TryClaim"/> binds to a
-        ///     real <see cref="IStep"/>.
-        /// Re-renders the YAML and updates the start-line table.
-        /// </summary>
-        /// <returns>1-based line number of the newly-appended entry's <c>- step:</c> key.</returns>
-        public int Append(JobExecutionViewEntry entry, IStep stepIdentity = null, string matchKey = null)
+        private int? TryGetLineForStepNoLock(IStep step)
         {
-            ArgUtil.NotNull(entry, nameof(entry));
-            if (stepIdentity != null && matchKey != null)
+            foreach (var stepLine in _lineByStep)
             {
-                throw new ArgumentException(
-                    "Append cannot register both a step identity and a placeholder match key on the same entry; pass at most one.");
+                if (ReferenceEquals(stepLine.Step, step))
+                {
+                    return stepLine.Line;
+                }
             }
 
-            lock (_lock)
-            {
-                if (stepIdentity != null && _lineByStep.ContainsKey(stepIdentity))
-                {
-                    throw new InvalidOperationException("step already registered in execution view");
-                }
-                if (matchKey != null && _unclaimedByKey.ContainsKey(matchKey))
-                {
-                    throw new InvalidOperationException($"matchKey already registered: {matchKey}");
-                }
-
-                _entries.Add(entry);
-                _stepIdentities.Add(stepIdentity);
-                Render();
-
-                int index = _entries.Count - 1;
-                if (matchKey != null)
-                {
-                    _unclaimedByKey[matchKey] = index;
-                }
-                return _entryStartLines[index];
-            }
+            return null;
         }
 
-        /// <summary>
-        /// Bind a previously-appended placeholder entry (registered via
-        /// <see cref="Append(JobExecutionViewEntry, IStep, string)"/> with
-        /// a non-null <c>matchKey</c>) to a real <see cref="IStep"/>.
-        /// Returns the 1-based line of the now-claimed entry on success.
-        /// Returns null when no unclaimed placeholder exists for
-        /// <paramref name="matchKey"/>, OR when <paramref name="stepIdentity"/>
-        /// is already registered for a different entry (defensive).
-        /// Does not re-render: claim only updates the IStep -> line index.
-        /// </summary>
-        public int? TryClaim(string matchKey, IStep stepIdentity)
+        private void AddSteps(IEnumerable<IStep> steps)
         {
-            if (matchKey == null)
+            if (steps == null)
             {
-                throw new ArgumentNullException(nameof(matchKey));
-            }
-            if (stepIdentity == null)
-            {
-                throw new ArgumentNullException(nameof(stepIdentity));
+                return;
             }
 
-            lock (_lock)
+            foreach (var step in steps)
             {
-                if (!_unclaimedByKey.TryGetValue(matchKey, out int index))
+                if (step == null)
                 {
-                    return null;
-                }
-                if (_lineByStep.ContainsKey(stepIdentity))
-                {
-                    // Bail rather than double-register the step.
-                    return null;
+                    continue;
                 }
 
-                _unclaimedByKey.Remove(matchKey);
-                _stepIdentities[index] = stepIdentity;
-                _lineByStep[stepIdentity] = _entryStartLines[index];
-                return _entryStartLines[index];
+                GetEntries(GetSection(step)).Add(new SourceEntry(step));
             }
         }
 
-        /// <summary>
-        /// Bulk-append for the initial population. Equivalent to calling
-        /// <see cref="Append"/> once per pair, but renders only once at the end.
-        /// State is left unchanged if any input is invalid.
-        /// </summary>
-        public void AppendRange(IEnumerable<(JobExecutionViewEntry entry, IStep stepIdentity)> items)
+        private void AddPredictedPostSteps(IEnumerable<PredictedPostStep> steps)
         {
-            ArgUtil.NotNull(items, nameof(items));
-
-            // Materialize first so we don't enumerate twice.
-            var materialized = new List<(JobExecutionViewEntry entry, IStep stepIdentity)>(items);
-            for (int i = 0; i < materialized.Count; i++)
+            if (steps == null)
             {
-                if (materialized[i].entry == null)
-                {
-                    throw new ArgumentException($"items[{i}].entry is null.", nameof(items));
-                }
+                return;
             }
 
-            lock (_lock)
+            foreach (var step in steps)
             {
-                // Validate no duplicates within the input or with existing identities,
-                // before mutating state.
-                var seen = new HashSet<IStep>(ReferenceEqualityComparer.Instance);
-                foreach (var (_, stepIdentity) in materialized)
+                if (step == null)
                 {
-                    if (stepIdentity == null)
-                    {
-                        continue;
-                    }
-                    if (_lineByStep.ContainsKey(stepIdentity) || !seen.Add(stepIdentity))
-                    {
-                        throw new InvalidOperationException("step already registered in execution view");
-                    }
+                    continue;
                 }
 
-                foreach (var (entry, stepIdentity) in materialized)
-                {
-                    _entries.Add(entry);
-                    _stepIdentities.Add(stepIdentity);
-                }
-                Render();
+                _postEntries.Add(new SourceEntry(step.DisplayName, step.MatchKey));
+            }
+        }
+
+        private List<SourceEntry> GetEntries(SourceSection section)
+        {
+            switch (section)
+            {
+                case SourceSection.Pre:
+                    return _preEntries;
+                case SourceSection.Post:
+                    return _postEntries;
+                default:
+                    return _mainEntries;
+            }
+        }
+
+        private static SourceSection GetSection(IStep step)
+        {
+            if (step is IActionRunner actionRunner)
+            {
+                return GetSection(actionRunner.Stage);
+            }
+
+            if (step.ExecutionContext != null)
+            {
+                return GetSection(step.ExecutionContext.Stage);
+            }
+
+            return SourceSection.Main;
+        }
+
+        private static SourceSection GetSection(ActionRunStage stage)
+        {
+            switch (stage)
+            {
+                case ActionRunStage.Pre:
+                    return SourceSection.Pre;
+                case ActionRunStage.Post:
+                    return SourceSection.Post;
+                default:
+                    return SourceSection.Main;
             }
         }
 
-        // Caller MUST hold _lock (constructor's call is safe — no concurrent access yet).
         private void Render()
         {
-            var result = JobExecutionViewRenderer.Render(_jobId, _entries.AsReadOnly());
-            _yaml = result.Yaml;
-            _entryStartLines = result.EntryStartLines;
-
             _lineByStep.Clear();
-            for (int i = 0; i < _stepIdentities.Count; i++)
+            _completeJobLine = 0;
+
+            var sb = new StringBuilder();
+            var line = 1;
+
+            AppendSection(sb, "pre", _preEntries, ref line, appendSeparatorLine: true);
+            AppendSection(sb, "main", _mainEntries, ref line, appendSeparatorLine: true);
+            AppendSection(sb, "post", _postEntries, ref line, appendSeparatorLine: false);
+
+            _content = sb.ToString();
+        }
+
+        private void AppendSection(
+            StringBuilder sb,
+            string sectionName,
+            IReadOnlyList<SourceEntry> entries,
+            ref int line,
+            bool appendSeparatorLine)
+        {
+            sb.Append(sectionName).Append(":\n");
+            line++;
+
+            foreach (var entry in entries)
             {
-                var step = _stepIdentities[i];
-                if (step != null)
+                if (entry.Step != null && TryGetLineForStepNoLock(entry.Step) == null)
                 {
-                    _lineByStep[step] = _entryStartLines[i];
+                    _lineByStep.Add(new StepLine(entry.Step, line));
+                }
+
+                sb.Append("  - step: ");
+                sb.Append(FormatYamlString(entry.DisplayName));
+                sb.Append('\n');
+                if (entry.IsSyntheticCompleteJob)
+                {
+                    _completeJobLine = line;
+                }
+
+                line++;
+            }
+
+            if (appendSeparatorLine)
+            {
+                sb.Append('\n');
+                line++;
+            }
+        }
+
+        private static string FormatYamlString(string value)
+        {
+            var sb = new StringBuilder();
+            sb.Append('"');
+            foreach (var c in value)
+            {
+                switch (c)
+                {
+                    case '\\':
+                        sb.Append(@"\\");
+                        break;
+                    case '"':
+                        sb.Append("\\\"");
+                        break;
+                    case '\r':
+                        sb.Append(@"\r");
+                        break;
+                    case '\n':
+                        sb.Append(@"\n");
+                        break;
+                    case '\t':
+                        sb.Append(@"\t");
+                        break;
+                    default:
+                        if (char.IsControl(c))
+                        {
+                            sb.Append(@"\u");
+                            sb.Append(((int)c).ToString("x4", CultureInfo.InvariantCulture));
+                        }
+                        else
+                        {
+                            sb.Append(c);
+                        }
+                        break;
                 }
             }
+
+            sb.Append('"');
+            return sb.ToString();
+        }
+
+        internal sealed class PredictedPostStep
+        {
+            public PredictedPostStep(string displayName, string matchKey)
+            {
+                DisplayName = string.IsNullOrEmpty(displayName) ? "step" : displayName;
+                MatchKey = matchKey;
+            }
+
+            public string DisplayName { get; }
+            public string MatchKey { get; }
+        }
+
+        private sealed class StepLine
+        {
+            public StepLine(IStep step, int line)
+            {
+                Step = step;
+                Line = line;
+            }
+
+            public IStep Step { get; }
+            public int Line { get; }
+        }
+
+        private sealed class SourceEntry
+        {
+            public SourceEntry(string displayName)
+            {
+                DisplayName = string.IsNullOrEmpty(displayName) ? "step" : displayName;
+            }
+
+            public SourceEntry(string displayName, string matchKey)
+                : this(displayName)
+            {
+                MatchKey = matchKey;
+            }
+
+            public SourceEntry(IStep step)
+            {
+                Step = step;
+                DisplayName = string.IsNullOrEmpty(step.DisplayName) ? "step" : step.DisplayName;
+            }
+
+            private SourceEntry(string displayName, bool isSyntheticCompleteJob)
+                : this(displayName)
+            {
+                IsSyntheticCompleteJob = isSyntheticCompleteJob;
+            }
+
+            public static SourceEntry CreateSyntheticCompleteJob()
+            {
+                return new SourceEntry("Complete job", isSyntheticCompleteJob: true);
+            }
+
+            public IStep Step { get; set; }
+            public string DisplayName { get; }
+            public string MatchKey { get; }
+            public bool IsSyntheticCompleteJob { get; }
+        }
+
+        private enum SourceSection
+        {
+            Pre,
+            Main,
+            Post
         }
     }
 }

src/Runner.Worker/Dap/JobExecutionViewRenderer.cs

@@ -1,389 +0,0 @@
-using System;
-using System.Collections.Generic;
-using System.Globalization;
-using System.IO;
-using System.Text;
-using GitHub.Runner.Sdk;
-using YamlDotNet.Core;
-using YamlDotNet.Core.Events;
-
-namespace GitHub.Runner.Worker.Dap
-{
-    /// <summary>
-    /// Phase a step occupies in the runner's flat execution sequence.
-    /// Setup and Cleanup are NOT modeled here — they are synthetic
-    /// boundaries hard-coded by <see cref="JobExecutionViewRenderer"/>
-    /// and cannot be constructed by callers.
-    /// </summary>
-    internal enum JobExecutionPhase
-    {
-        Pre,
-        Main,
-        Post,
-    }
-
-    /// <summary>
-    /// One step in the rendered execution view. Pure data; no link to
-    /// any worker type. Phase 2 will translate runner step objects
-    /// into instances of this record.
-    /// </summary>
-    internal sealed class JobExecutionViewEntry
-    {
-        public JobExecutionViewEntry(
-            JobExecutionPhase phase,
-            string displayName,
-            string uses = null,
-            string run = null,
-            string sourcePath = null,
-            int sourceLine = 0,
-            string id = null,
-            string @if = null,
-            string continueOnError = null,
-            string timeoutMinutes = null,
-            string envYaml = null,
-            string withYaml = null,
-            string shell = null,
-            string workingDirectory = null)
-        {
-            if (string.IsNullOrWhiteSpace(displayName))
-            {
-                throw new ArgumentException("displayName must not be null or whitespace.", nameof(displayName));
-            }
-            if (sourcePath != null && sourceLine < 1)
-            {
-                throw new ArgumentException(
-                    "sourceLine must be >= 1 when sourcePath is provided.",
-                    nameof(sourceLine));
-            }
-
-            Phase = phase;
-            DisplayName = displayName;
-            Uses = uses;
-            Run = run;
-            SourcePath = sourcePath;
-            SourceLine = sourceLine;
-            Id = id;
-            If = @if;
-            ContinueOnError = continueOnError;
-            TimeoutMinutes = timeoutMinutes;
-            EnvYaml = envYaml;
-            WithYaml = withYaml;
-            Shell = shell;
-            WorkingDirectory = workingDirectory;
-        }
-
-        public JobExecutionPhase Phase { get; }
-        public string DisplayName { get; }
-        public string Uses { get; }
-        public string Run { get; }
-        public string SourcePath { get; }
-        public int SourceLine { get; }
-        public string Id { get; }
-        public string If { get; }
-        public string ContinueOnError { get; }
-        public string TimeoutMinutes { get; }
-        // Pre-serialized YAML fragment, already indented for embedding
-        // under the entry's `env:` key (6-space child indent).
-        public string EnvYaml { get; }
-        public string WithYaml { get; }
-        public string Shell { get; }
-        public string WorkingDirectory { get; }
-    }
-
-    /// <summary>
-    /// Output of <see cref="JobExecutionViewRenderer.Render"/>: the YAML
-    /// document plus a parallel array of 1-based line numbers, one per
-    /// input entry, where each entry's <c>- step:</c> key appears.
-    /// Synthetic Setup/Cleanup boundaries are not tracked here.
-    /// </summary>
-    internal readonly struct RenderResult
-    {
-        public RenderResult(string yaml, IReadOnlyList<int> entryStartLines)
-        {
-            Yaml = yaml;
-            EntryStartLines = entryStartLines;
-        }
-
-        public string Yaml { get; }
-        public IReadOnlyList<int> EntryStartLines { get; }
-    }
-
-    /// <summary>
-    /// Renders a job's execution-view YAML. Pure function; no I/O,
-    /// no logging, no static state. Output format and Setup/Cleanup
-    /// boundaries are fixed; callers cannot influence them.
-    ///
-    /// Output is structured as phase-keyed top-level sections:
-    /// <c>setup:</c>, <c>pre:</c>, <c>main:</c>, <c>post:</c>, <c>cleanup:</c>.
-    /// <c>setup:</c> and <c>cleanup:</c> always render; <c>pre:</c>,
-    /// <c>main:</c>, <c>post:</c> only render when they contain at least
-    /// one entry.
-    /// </summary>
-    internal static class JobExecutionViewRenderer
-    {
-        public static RenderResult Render(string jobId, IReadOnlyList<JobExecutionViewEntry> entries)
-        {
-            if (string.IsNullOrWhiteSpace(jobId))
-            {
-                throw new ArgumentException("jobId must not be null or whitespace.", nameof(jobId));
-            }
-            ArgUtil.NotNull(entries, nameof(entries));
-
-            // Pre-validate non-null entries before any output, so partial
-            // state is never observed by callers.
-            for (int i = 0; i < entries.Count; i++)
-            {
-                if (entries[i] == null)
-                {
-                    throw new ArgumentException($"entries[{i}] is null.", nameof(entries));
-                }
-            }
-
-            var sb = new StringBuilder();
-            var startLines = new int[entries.Count];
-            int newlinesEmitted = 0;
-
-            // Header (3 lines).
-            sb.Append("# Job: ").Append(FormatScalar(jobId)).Append('\n');
-            sb.Append("# Runner execution plan — read-only.\n");
-            sb.Append('\n');
-            newlinesEmitted += 3;
-
-            // setup: section — always present.
-            sb.Append("setup:\n");
-            sb.Append("  - step: Setup job\n");
-            newlinesEmitted += 2;
-
-            // Render phase sections in fixed order. Each emits a leading
-            // blank line separator before its header.
-            EmitPhaseSection(sb, "pre", JobExecutionPhase.Pre, entries, startLines, ref newlinesEmitted);
-            EmitPhaseSection(sb, "main", JobExecutionPhase.Main, entries, startLines, ref newlinesEmitted);
-            EmitPhaseSection(sb, "post", JobExecutionPhase.Post, entries, startLines, ref newlinesEmitted);
-
-            // cleanup: section — always present, preceded by a blank line.
-            sb.Append('\n');
-            sb.Append("cleanup:\n");
-            sb.Append("  - step: Complete job\n");
-
-            return new RenderResult(sb.ToString(), Array.AsReadOnly(startLines));
-        }
-
-        private static void EmitPhaseSection(
-            StringBuilder sb,
-            string sectionName,
-            JobExecutionPhase phase,
-            IReadOnlyList<JobExecutionViewEntry> entries,
-            int[] startLines,
-            ref int newlinesEmitted)
-        {
-            // Skip the section entirely if no entries belong to this phase.
-            bool any = false;
-            for (int i = 0; i < entries.Count; i++)
-            {
-                if (entries[i].Phase == phase) { any = true; break; }
-            }
-            if (!any)
-            {
-                return;
-            }
-
-            // Blank line separator + section header.
-            sb.Append('\n');
-            sb.Append(sectionName).Append(":\n");
-            newlinesEmitted += 2;
-
-            for (int i = 0; i < entries.Count; i++)
-            {
-                var entry = entries[i];
-                if (entry.Phase != phase)
-                {
-                    continue;
-                }
-
-                // 1-based line of the `- step:` key for this entry.
-                startLines[i] = newlinesEmitted + 1;
-
-                sb.Append("  - step: ").Append(FormatScalar(entry.DisplayName));
-                sb.Append('\n');
-                newlinesEmitted++;
-
-                switch (phase)
-                {
-                    case JobExecutionPhase.Pre:
-                    case JobExecutionPhase.Post:
-                        if (!string.IsNullOrEmpty(entry.Uses))
-                        {
-                            sb.Append("    action: ").Append(FormatScalar(entry.Uses)).Append('\n');
-                            newlinesEmitted++;
-                        }
-                        // No source: annotation for pre/post.
-                        break;
-
-                    case JobExecutionPhase.Main:
-                        if (!string.IsNullOrEmpty(entry.Id))
-                        {
-                            sb.Append("    id: ").Append(FormatScalar(entry.Id)).Append('\n');
-                            newlinesEmitted++;
-                        }
-                        if (!string.IsNullOrEmpty(entry.Uses))
-                        {
-                            sb.Append("    uses: ").Append(FormatScalar(entry.Uses)).Append('\n');
-                            newlinesEmitted++;
-                        }
-                        if (!string.IsNullOrEmpty(entry.Run))
-                        {
-                            if (entry.Run.IndexOf('\n') < 0)
-                            {
-                                sb.Append("    run: ").Append(FormatScalar(entry.Run)).Append('\n');
-                                newlinesEmitted++;
-                            }
-                            else
-                            {
-                                sb.Append("    run: |\n");
-                                newlinesEmitted++;
-                                newlinesEmitted += AppendIndentedBlock(sb, entry.Run, "      ");
-                            }
-                        }
-                        if (!string.IsNullOrEmpty(entry.If))
-                        {
-                            sb.Append("    if: ").Append(FormatScalar(entry.If)).Append('\n');
-                            newlinesEmitted++;
-                        }
-                        if (!string.IsNullOrEmpty(entry.ContinueOnError))
-                        {
-                            sb.Append("    continue-on-error: ").Append(entry.ContinueOnError).Append('\n');
-                            newlinesEmitted++;
-                        }
-                        if (!string.IsNullOrEmpty(entry.TimeoutMinutes))
-                        {
-                            sb.Append("    timeout-minutes: ").Append(entry.TimeoutMinutes).Append('\n');
-                            newlinesEmitted++;
-                        }
-                        if (!string.IsNullOrEmpty(entry.EnvYaml))
-                        {
-                            sb.Append("    env:\n");
-                            newlinesEmitted++;
-                            sb.Append(entry.EnvYaml).Append('\n');
-                            newlinesEmitted += CountChar(entry.EnvYaml, '\n') + 1;
-                        }
-                        if (!string.IsNullOrEmpty(entry.WithYaml))
-                        {
-                            sb.Append("    with:\n");
-                            newlinesEmitted++;
-                            sb.Append(entry.WithYaml).Append('\n');
-                            newlinesEmitted += CountChar(entry.WithYaml, '\n') + 1;
-                        }
-                        if (!string.IsNullOrEmpty(entry.Shell))
-                        {
-                            sb.Append("    shell: ").Append(FormatScalar(entry.Shell)).Append('\n');
-                            newlinesEmitted++;
-                        }
-                        if (!string.IsNullOrEmpty(entry.WorkingDirectory))
-                        {
-                            sb.Append("    working-directory: ").Append(FormatScalar(entry.WorkingDirectory)).Append('\n');
-                            newlinesEmitted++;
-                        }
-                        if (entry.SourcePath != null)
-                        {
-                            sb.Append("    source: ")
-                              .Append(entry.SourcePath)
-                              .Append(':')
-                              .Append(entry.SourceLine.ToString(CultureInfo.InvariantCulture))
-                              .Append('\n');
-                            newlinesEmitted++;
-                        }
-                        break;
-                }
-            }
-        }
-
-        private static int AppendIndentedBlock(StringBuilder sb, string text, string indent)
-        {
-            int newlines = 0;
-            int i = 0;
-            while (i < text.Length)
-            {
-                int end = text.IndexOf('\n', i);
-                int lineEnd = end < 0 ? text.Length : end;
-                int trimEnd = lineEnd;
-                if (trimEnd > i && text[trimEnd - 1] == '\r')
-                {
-                    trimEnd--;
-                }
-                if (trimEnd > i)
-                {
-                    sb.Append(indent);
-                    sb.Append(text, i, trimEnd - i);
-                }
-                sb.Append('\n');
-                newlines++;
-                if (end < 0)
-                {
-                    break;
-                }
-                i = end + 1;
-            }
-            return newlines;
-        }
-
-        private static int CountChar(string s, char c)
-        {
-            int n = 0;
-            for (int i = 0; i < s.Length; i++)
-            {
-                if (s[i] == c) n++;
-            }
-            return n;
-        }
-
-        /// <summary>
-        /// Formats a single string as a YAML 1.x flow scalar, delegating
-        /// quoting/escaping decisions to YamlDotNet. This avoids maintaining
-        /// our own escape table for every YAML-significant character: we
-        /// just emit the value through the YAML library and use whichever
-        /// scalar style (plain, single-quoted, double-quoted) it picks.
-        /// A new <see cref="Emitter"/> is created per call, so the helper
-        /// is safe to invoke concurrently.
-        /// </summary>
-        internal static string FormatScalar(string value)
-        {
-            if (value == null)
-            {
-                throw new ArgumentNullException(nameof(value));
-            }
-
-            using var sw = new StringWriter(CultureInfo.InvariantCulture);
-            // Force LF line breaks; YamlDotNet's Emitter calls WriteLine,
-            // which would otherwise produce CRLF on Windows and break
-            // both our document-end stripping below and downstream
-            // consumers that assume a single line-break convention.
-            sw.NewLine = "\n";
-            var emitter = new Emitter(sw);
-            emitter.Emit(new StreamStart());
-            emitter.Emit(new DocumentStart(null, null, true));
-            emitter.Emit(new Scalar(null, null, value, ScalarStyle.Any, true, true));
-            emitter.Emit(new DocumentEnd(true));
-            emitter.Emit(new StreamEnd());
-
-            string raw = sw.ToString();
-            // Strip YAML document markers. Emitter elides these for most
-            // scalars but emits "--- " (with space) for some edge cases
-            // (e.g. empty strings). Defensively handle "---\n" too.
-            if (raw.StartsWith("--- ", StringComparison.Ordinal))
-            {
-                raw = raw.Substring(4);
-            }
-            else if (raw.StartsWith("---\n", StringComparison.Ordinal))
-            {
-                raw = raw.Substring(4);
-            }
-            raw = raw.TrimEnd('\n');
-            const string DocEndMarker = "\n...";
-            if (raw.EndsWith(DocEndMarker, StringComparison.Ordinal))
-            {
-                raw = raw.Substring(0, raw.Length - DocEndMarker.Length);
-            }
-            return raw.TrimEnd('\n');
-        }
-    }
-}

src/Runner.Worker/Dap/StepEntryTranslator.cs

@@ -1,240 +0,0 @@
-using System;
-using System.Collections.Generic;
-using GitHub.DistributedTask.ObjectTemplating.Tokens;
-using GitHub.DistributedTask.Pipelines;
-using GitHub.Runner.Sdk;
-
-namespace GitHub.Runner.Worker.Dap
-{
-    /// <summary>
-    /// Translates runner <see cref="IStep"/> instances into pure-data
-    /// <see cref="JobExecutionViewEntry"/> records used by the DAP debugger
-    /// execution view. Filters out runner-internal steps (e.g.
-    /// <see cref="JobExtensionRunner"/>) so the rendered view only shows
-    /// user-visible workflow steps.
-    /// </summary>
-    internal static class StepEntryTranslator
-    {
-        // Run-step internals carried on ActionStep.Inputs that are NOT
-        // user-authored `with:` entries. The runner stores these under
-        // the keys defined in PipelineConstants.ScriptStepInputs, NOT
-        // their kebab-case workflow-YAML spellings.
-        private static readonly HashSet<string> RunStepInternalKeys = new(StringComparer.Ordinal)
-        {
-            PipelineConstants.ScriptStepInputs.Script,
-            PipelineConstants.ScriptStepInputs.Shell,
-            PipelineConstants.ScriptStepInputs.WorkingDirectory,
-        };
-
-        /// <summary>
-        /// Translate an IStep into a JobExecutionViewEntry.
-        /// </summary>
-        /// <param name="step">The IStep to translate. Must not be null.</param>
-        /// <returns>
-        /// A JobExecutionViewEntry, or null if the step is not user-visible
-        /// (JobExtensionRunner and any other non-IActionRunner IStep impls).
-        /// </returns>
-        public static JobExecutionViewEntry TryTranslate(IStep step)
-        {
-            ArgUtil.NotNull(step, nameof(step));
-
-            if (step is JobExtensionRunner)
-            {
-                return null;
-            }
-
-            if (step is not IActionRunner actionRunner)
-            {
-                return null;
-            }
-
-            var phase = actionRunner.Stage switch
-            {
-                ActionRunStage.Pre => JobExecutionPhase.Pre,
-                ActionRunStage.Post => JobExecutionPhase.Post,
-                _ => JobExecutionPhase.Main,
-            };
-
-            string displayName = actionRunner.DisplayName;
-            if (string.IsNullOrWhiteSpace(displayName))
-            {
-                displayName = "run";
-            }
-
-            string uses = null;
-            string run = null;
-            string id = null;
-            string ifCond = null;
-            string continueOnError = null;
-            string timeoutMinutes = null;
-            string envYaml = null;
-            string withYaml = null;
-            string shell = null;
-            string workingDirectory = null;
-
-            var action = actionRunner.Action;
-            var reference = action?.Reference;
-            bool isScript = reference?.Type == ActionSourceType.Script;
-
-            if (reference != null && !isScript)
-            {
-                uses = FormatActionReference(reference);
-            }
-
-            // Only the user-visible Main entry surfaces authored params.
-            // Pre/Post stay minimal (step + action) — they reference the
-            // same Action as the Main entry, and duplicating params adds
-            // noise without information.
-            if (phase == JobExecutionPhase.Main && action != null)
-            {
-                id = FilterAuthoredId(action.ContextName);
-
-                if (!string.IsNullOrEmpty(action.Condition))
-                {
-                    ifCond = action.Condition;
-                }
-
-                if (action.ContinueOnError != null)
-                {
-                    continueOnError = TemplateTokenYamlAdapter.Serialize(action.ContinueOnError, indentSpaces: 0);
-                }
-                if (action.TimeoutInMinutes != null)
-                {
-                    timeoutMinutes = TemplateTokenYamlAdapter.Serialize(action.TimeoutInMinutes, indentSpaces: 0);
-                }
-
-                if (action.Environment is MappingToken envMap && envMap.Count > 0)
-                {
-                    envYaml = TemplateTokenYamlAdapter.Serialize(envMap, indentSpaces: 6);
-                }
-                else if (action.Environment != null && !(action.Environment is MappingToken))
-                {
-                    // Unusual but possible: env: ${{ ... }} expression form.
-                    envYaml = TemplateTokenYamlAdapter.Serialize(action.Environment, indentSpaces: 6);
-                }
-
-                if (isScript)
-                {
-                    var inputs = action.Inputs as MappingToken;
-                    if (inputs != null)
-                    {
-                        if (TryGetMapValue(inputs, PipelineConstants.ScriptStepInputs.Script, out var scriptTok) && scriptTok != null)
-                        {
-                            run = scriptTok.ToString();
-                        }
-                        if (TryGetMapValue(inputs, PipelineConstants.ScriptStepInputs.Shell, out var shellTok) && shellTok != null)
-                        {
-                            string shellText = shellTok.ToString();
-                            if (!string.IsNullOrEmpty(shellText))
-                            {
-                                shell = shellText;
-                            }
-                        }
-                        if (TryGetMapValue(inputs, PipelineConstants.ScriptStepInputs.WorkingDirectory, out var wdTok) && wdTok != null)
-                        {
-                            string wdText = wdTok.ToString();
-                            if (!string.IsNullOrEmpty(wdText))
-                            {
-                                workingDirectory = wdText;
-                            }
-                        }
-                    }
-                }
-                else
-                {
-                    // Action step: surface `with:` entries, filtering any
-                    // run-step internal keys defensively.
-                    if (action.Inputs is MappingToken withMap && withMap.Count > 0)
-                    {
-                        var filtered = FilterMapping(withMap, RunStepInternalKeys);
-                        if (filtered != null && filtered.Count > 0)
-                        {
-                            withYaml = TemplateTokenYamlAdapter.Serialize(filtered, indentSpaces: 6);
-                        }
-                    }
-                }
-            }
-
-            // Source annotation (SourcePath/SourceLine) requires a public
-            // seam onto TemplateToken position info — not wired yet.
-            return new JobExecutionViewEntry(
-                phase: phase,
-                displayName: displayName,
-                uses: uses,
-                run: run,
-                sourcePath: null,
-                sourceLine: 0,
-                id: id,
-                @if: ifCond,
-                continueOnError: continueOnError,
-                timeoutMinutes: timeoutMinutes,
-                envYaml: envYaml,
-                withYaml: withYaml,
-                shell: shell,
-                workingDirectory: workingDirectory);
-        }
-
-        /// <summary>
-        /// Auto-generated step IDs are noise in the view: filter them out.
-        /// The runner's convention (see ExecutionContext) is that auto-
-        /// generated context names start with <c>__</c>. Only user-authored
-        /// IDs survive the filter.
-        /// </summary>
-        internal static string FilterAuthoredId(string contextName)
-        {
-            if (string.IsNullOrWhiteSpace(contextName))
-            {
-                return null;
-            }
-            if (contextName.StartsWith("__", StringComparison.Ordinal))
-            {
-                return null;
-            }
-            return contextName;
-        }
-
-        private static bool TryGetMapValue(MappingToken map, string key, out TemplateToken value)
-        {
-            foreach (var pair in map)
-            {
-                if (pair.Key is StringToken s && string.Equals(s.Value, key, StringComparison.Ordinal))
-                {
-                    value = pair.Value;
-                    return true;
-                }
-            }
-            value = null;
-            return false;
-        }
-
-        private static MappingToken FilterMapping(MappingToken source, HashSet<string> excludeKeys)
-        {
-            var copy = new MappingToken(source.FileId, source.Line, source.Column);
-            foreach (var pair in source)
-            {
-                if (pair.Key is StringToken sk && excludeKeys.Contains(sk.Value))
-                {
-                    continue;
-                }
-                copy.Add(pair);
-            }
-            return copy;
-        }
-
-        internal static string FormatActionReference(ActionStepDefinitionReference reference)
-        {
-            switch (reference)
-            {
-                case RepositoryPathReference repo:
-                    var path = string.IsNullOrEmpty(repo.Path) ? string.Empty : $"/{repo.Path}";
-                    return string.IsNullOrEmpty(repo.Ref)
-                        ? $"{repo.Name}{path}"
-                        : $"{repo.Name}{path}@{repo.Ref}";
-                case ContainerRegistryReference container:
-                    return container.Image;
-                default:
-                    return reference.ToString();
-            }
-        }
-    }
-}

src/Runner.Worker/Dap/TemplateTokenYamlAdapter.cs

@@ -1,223 +0,0 @@
-using System;
-using System.Globalization;
-using System.IO;
-using GitHub.DistributedTask.ObjectTemplating;
-using GitHub.DistributedTask.ObjectTemplating.Tokens;
-using GitHub.Runner.Sdk;
-using YamlDotNet.Core;
-using YamlDotNet.Core.Events;
-
-namespace GitHub.Runner.Worker.Dap
-{
-    /// <summary>
-    /// Adapts a YamlDotNet <see cref="IEmitter"/> as a DT
-    /// <see cref="IObjectWriter"/> so a <see cref="TemplateToken"/> DOM
-    /// can be serialized back to YAML preserving its pre-evaluation form
-    /// (basic <c>${{ }}</c> expressions are written through verbatim).
-    ///
-    /// Used by the DAP execution view to surface user-authored step
-    /// parameters (<c>env:</c>, <c>with:</c>, <c>run:</c>, ...) without
-    /// any expression substitution.
-    /// </summary>
-    internal sealed class TemplateTokenYamlAdapter : IObjectWriter
-    {
-        private readonly IEmitter _emitter;
-
-        public TemplateTokenYamlAdapter(IEmitter emitter)
-        {
-            ArgUtil.NotNull(emitter, nameof(emitter));
-            _emitter = emitter;
-        }
-
-        public void WriteStart()
-        {
-            _emitter.Emit(new StreamStart());
-            _emitter.Emit(new DocumentStart(null, null, true));
-        }
-
-        public void WriteEnd()
-        {
-            _emitter.Emit(new DocumentEnd(true));
-            _emitter.Emit(new StreamEnd());
-        }
-
-        public void WriteNull() =>
-            _emitter.Emit(new Scalar(null, null, "null", ScalarStyle.Plain, true, false));
-
-        public void WriteBoolean(bool value) =>
-            _emitter.Emit(new Scalar(null, null, value ? "true" : "false", ScalarStyle.Plain, true, false));
-
-        public void WriteNumber(double value) =>
-            _emitter.Emit(new Scalar(null, null, value.ToString("R", CultureInfo.InvariantCulture), ScalarStyle.Plain, true, false));
-
-        public void WriteString(string value)
-        {
-            if (value == null)
-            {
-                WriteNull();
-                return;
-            }
-            // Multi-line strings render as block literal so embedded
-            // newlines survive the YAML round trip.
-            var style = value.IndexOf('\n') >= 0 ? ScalarStyle.Literal : ScalarStyle.Any;
-            _emitter.Emit(new Scalar(null, null, value, style, true, true));
-        }
-
-        public void WriteSequenceStart() =>
-            _emitter.Emit(new SequenceStart(null, null, true, SequenceStyle.Any));
-
-        public void WriteSequenceEnd() =>
-            _emitter.Emit(new SequenceEnd());
-
-        public void WriteMappingStart() =>
-            _emitter.Emit(new MappingStart(null, null, true, MappingStyle.Any));
-
-        public void WriteMappingEnd() =>
-            _emitter.Emit(new MappingEnd());
-
-        /// <summary>
-        /// Serialize a TemplateToken to a YAML fragment ready to embed
-        /// under a parent key. Each non-empty line is prefixed by
-        /// <paramref name="indentSpaces"/> spaces. Trailing newlines and
-        /// the YAML stream start/document markers are stripped, so the
-        /// caller controls line breaks.
-        /// </summary>
-        /// <remarks>
-        /// Empty mappings render as <c>{}</c> and empty sequences as
-        /// <c>[]</c> via YamlDotNet's flow style fallback for empty
-        /// collections.
-        /// </remarks>
-        internal static string Serialize(TemplateToken token, int indentSpaces)
-        {
-            if (indentSpaces < 0)
-            {
-                throw new ArgumentOutOfRangeException(nameof(indentSpaces));
-            }
-
-            using var sw = new StringWriter(CultureInfo.InvariantCulture);
-            // Force LF line breaks; YamlDotNet's Emitter calls WriteLine,
-            // which would otherwise produce CRLF on Windows and corrupt
-            // both the document-end stripping below and the per-line
-            // indentation pass that follows.
-            sw.NewLine = "\n";
-            var emitter = new Emitter(sw);
-            var adapter = new TemplateTokenYamlAdapter(emitter);
-            adapter.WriteStart();
-            WriteToken(adapter, token);
-            adapter.WriteEnd();
-
-            string raw = sw.ToString();
-            // Strip YAML document markers. The Emitter most commonly elides
-            // these for our use (DocumentStart isImplicit=true), but emits
-            // them for some scalar edge cases (e.g. empty strings) and may
-            // emit them on their own line for collection roots under some
-            // settings. Strip both shapes defensively so callers never see
-            // a leaked marker leak into the embedded fragment.
-            if (raw.StartsWith("--- ", StringComparison.Ordinal))
-            {
-                raw = raw.Substring(4);
-            }
-            else if (raw.StartsWith("---\n", StringComparison.Ordinal))
-            {
-                raw = raw.Substring(4);
-            }
-            const string DocEndMarker = "\n...";
-            if (raw.EndsWith(DocEndMarker + "\n", StringComparison.Ordinal))
-            {
-                raw = raw.Substring(0, raw.Length - DocEndMarker.Length - 1);
-            }
-            else if (raw.EndsWith(DocEndMarker, StringComparison.Ordinal))
-            {
-                raw = raw.Substring(0, raw.Length - DocEndMarker.Length);
-            }
-            raw = raw.TrimEnd('\n');
-
-            if (indentSpaces == 0)
-            {
-                return raw;
-            }
-
-            // Re-indent every non-empty line. Empty lines remain empty
-            // so YAML block-literal blank lines stay valid.
-            var pad = new string(' ', indentSpaces);
-            var sb = new System.Text.StringBuilder(raw.Length + indentSpaces * 4);
-            int i = 0;
-            while (i < raw.Length)
-            {
-                int end = raw.IndexOf('\n', i);
-                int lineEnd = end < 0 ? raw.Length : end;
-                if (lineEnd > i)
-                {
-                    sb.Append(pad);
-                    sb.Append(raw, i, lineEnd - i);
-                }
-                if (end < 0)
-                {
-                    break;
-                }
-                sb.Append('\n');
-                i = end + 1;
-            }
-            return sb.ToString();
-        }
-
-        /// <summary>
-        /// Mirrors <see cref="TemplateWriter"/>'s recursive walk, with one
-        /// behavioural change: <see cref="BasicExpressionToken"/> is emitted
-        /// via <c>ToDisplayString()</c> instead of <c>ToString()</c>.
-        /// </summary>
-        /// <remarks>
-        /// The workflow parser tokenizes a mixed scalar like
-        /// <c>${{ runner.os }}-primes</c> as a single
-        /// <see cref="BasicExpressionToken"/> whose internal expression is
-        /// <c>format('{0}-primes', runner.os)</c>. <c>ToString()</c> emits
-        /// the normalized form verbatim; <c>ToDisplayString()</c> reverses
-        /// the <c>format(...)</c> rewrite so the user sees the original
-        /// authored form. Other token kinds delegate to the same writer
-        /// calls <see cref="TemplateWriter"/> would make.
-        /// </remarks>
-        private static void WriteToken(IObjectWriter writer, TemplateToken token)
-        {
-            switch (token?.Type ?? TokenType.Null)
-            {
-                case TokenType.Null:
-                    writer.WriteNull();
-                    break;
-                case TokenType.Boolean:
-                    writer.WriteBoolean(((BooleanToken)token).Value);
-                    break;
-                case TokenType.Number:
-                    writer.WriteNumber(((NumberToken)token).Value);
-                    break;
-                case TokenType.String:
-                    writer.WriteString(token.ToString());
-                    break;
-                case TokenType.BasicExpression:
-                    writer.WriteString(((BasicExpressionToken)token).ToDisplayString());
-                    break;
-                case TokenType.InsertExpression:
-                    writer.WriteString(token.ToString());
-                    break;
-                case TokenType.Mapping:
-                    writer.WriteMappingStart();
-                    foreach (var pair in (MappingToken)token)
-                    {
-                        WriteToken(writer, pair.Key);
-                        WriteToken(writer, pair.Value);
-                    }
-                    writer.WriteMappingEnd();
-                    break;
-                case TokenType.Sequence:
-                    writer.WriteSequenceStart();
-                    foreach (var item in (SequenceToken)token)
-                    {
-                        WriteToken(writer, item);
-                    }
-                    writer.WriteSequenceEnd();
-                    break;
-                default:
-                    throw new NotSupportedException($"Unexpected token type '{token.GetType()}'.");
-            }
-        }
-    }
-}

src/Runner.Worker/ExecutionContext.cs

@@ -77,14 +77,23 @@ namespace GitHub.Runner.Worker
 
         List<string> StepEnvironmentOverrides { get; }
 
+        bool IsBackground { get; }
+
         IExecutionContext Root { get; }
 
         // Initialize
         void InitializeJob(Pipelines.AgentJobRequestMessage message, CancellationToken token);
         void CancelToken();
-        IExecutionContext CreateChild(Guid recordId, string displayName, string refName, string scopeName, string contextName, ActionRunStage stage, Dictionary<string, string> intraActionState = null, int? recordOrder = null, IPagingLogger logger = null, bool isEmbedded = false, List<Issue> embeddedIssueCollector = null, CancellationTokenSource cancellationTokenSource = null, Guid embeddedId = default(Guid), string siblingScopeName = null, TimeSpan? timeout = null);
+        IExecutionContext CreateChild(Guid recordId, string displayName, string refName, string scopeName, string contextName, ActionRunStage stage, Dictionary<string, string> intraActionState = null, int? recordOrder = null, IPagingLogger logger = null, bool isEmbedded = false, List<Issue> embeddedIssueCollector = null, CancellationTokenSource cancellationTokenSource = null, Guid embeddedId = default(Guid), string siblingScopeName = null, TimeSpan? timeout = null, bool isBackground = false, string backgroundControlType = null, string[] backgroundControlStepIds = null, string parallelGroupId = null);
         IExecutionContext CreateEmbeddedChild(string scopeName, string contextName, Guid embeddedId, ActionRunStage stage, Dictionary<string, string> intraActionState = null, string siblingScopeName = null);
 
+
+        // Background step deferral properties
+        Dictionary<string, string> DeferredOutputs { get; set; }
+        Dictionary<string, string> DeferredEnvironmentVariables { get; set; }
+        List<string> DeferredPrependPath { get; set; }
+        bool DeferOutcomeConclusion { get; set; }
+
         // logging
         long Write(string tag, string message);
         void QueueAttachFile(string type, string name, string filePath);
@@ -100,6 +109,12 @@ namespace GitHub.Runner.Worker
         void SetGitHubContext(string name, string value);
         void SetOutput(string name, string value, out string reference);
         void SetTimeout(TimeSpan? timeout);
+
+        // Background step deferral flush methods
+        void FlushDeferredOutputs();
+        void FlushDeferredEnvironment();
+        void FlushDeferredOutcomeConclusion();
+
         void AddIssue(Issue issue, ExecutionContextLogOptions logOptions);
         void Progress(int percentage, string currentOperation = null);
         void UpdateDetailTimelineRecord(TimelineRecord record);
@@ -216,6 +231,9 @@ namespace GitHub.Runner.Worker
 
         public bool EchoOnActionCommand { get; set; }
 
+        // Whether this step runs in the background
+        public bool IsBackground => _record.IsBackground;
+
         // An embedded execution context shares the same record ID, record name, and logger
         // as its enclosing execution context.
         public bool IsEmbedded { get; private init; }
@@ -279,6 +297,12 @@ namespace GitHub.Runner.Worker
 
         public List<string> StepEnvironmentOverrides { get; } = new List<string>();
 
+        // Background step deferral properties
+        public Dictionary<string, string> DeferredOutputs { get; set; }
+        public Dictionary<string, string> DeferredEnvironmentVariables { get; set; }
+        public List<string> DeferredPrependPath { get; set; }
+        public bool DeferOutcomeConclusion { get; set; }
+
         public override void Initialize(IHostContext hostContext)
         {
             base.Initialize(hostContext);
@@ -337,7 +361,25 @@ namespace GitHub.Runner.Worker
             }
 
             step.ExecutionContext = Root.CreatePostChild(step.DisplayName, IntraActionState, siblingScopeName);
+            if (step is JobExtensionRunner)
+            {
+                step.ExecutionContext.StepTelemetry.Type = "runner";
+                step.ExecutionContext.StepTelemetry.Action = step.DisplayName.ToLowerInvariant().Replace(' ', '_');
+            }
             Root.PostJobSteps.Push(step);
+
+            if (Root.Global.Debugger?.Enabled == true)
+            {
+                try
+                {
+                    HostContext.GetService<Dap.IDapDebugger>().OnPostStepRegistered(step);
+                }
+                catch (Exception ex)
+                {
+                    Trace.Warning("Failed to notify DAP debugger about registered post job step.");
+                    Trace.Error(ex);
+                }
+            }
         }
 
         public IExecutionContext CreateChild(
@@ -355,7 +397,11 @@ namespace GitHub.Runner.Worker
             CancellationTokenSource cancellationTokenSource = null,
             Guid embeddedId = default(Guid),
             string siblingScopeName = null,
-            TimeSpan? timeout = null)
+            TimeSpan? timeout = null,
+            bool isBackground = false,
+            string backgroundControlType = null,
+            string[] backgroundControlStepIds = null,
+            string parallelGroupId = null)
         {
             Trace.Entering();
 
@@ -396,6 +442,24 @@ namespace GitHub.Runner.Worker
 
             child.EchoOnActionCommand = EchoOnActionCommand;
 
+            // Set background step metadata before InitializeTimelineRecord so it's included in the first update
+            if (isBackground || backgroundControlType != null || parallelGroupId != null)
+            {
+                child._record.IsBackground = isBackground;
+                child._record.BackgroundControlType = backgroundControlType;
+                child._record.BackgroundControlStepIds = backgroundControlStepIds;
+                child._record.ParallelGroupId = parallelGroupId;
+
+                // Initialize deferred state for background steps — flushed at wait/wait-all
+                if (isBackground)
+                {
+                    child.DeferredOutputs = new Dictionary<string, string>();
+                    child.DeferredEnvironmentVariables = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
+                    child.DeferredPrependPath = new List<string>();
+                    child.DeferOutcomeConclusion = true;
+                }
+            }
+
             if (recordOrder != null)
             {
                 child.InitializeTimelineRecord(_mainTimelineId, recordId, _record.Id, ExecutionContextType.Task, displayName, refName, recordOrder, embedded: isEmbedded);
@@ -508,7 +572,11 @@ namespace GitHub.Runner.Worker
                     Type = StepTelemetry?.Type,
                     StartedAt = _record.StartTime,
                     CompletedAt = _record.FinishTime,
-                    Annotations = new List<Annotation>()
+                    Annotations = new List<Annotation>(),
+                    // Populate background step metadata from timeline record fields
+                    IsBackground = _record.IsBackground,
+                    BackgroundControlType = _record.BackgroundControlType,
+                    BackgroundControlStepIds = _record.BackgroundControlStepIds
                 };
 
                 _record.Issues?.ForEach(issue =>
@@ -554,11 +622,22 @@ namespace GitHub.Runner.Worker
 
             _logger.End();
 
-            UpdateGlobalStepsContext();
+            if (!DeferOutcomeConclusion)
+            {
+                UpdateGlobalStepsContext();
+            }
 
             return Result.Value;
         }
 
+        public void FlushDeferredOutcomeConclusion()
+        {
+            if (DeferOutcomeConclusion)
+            {
+                UpdateGlobalStepsContext();
+            }
+        }
+
         public void UpdateGlobalStepsContext()
         {
             // Skip if generated context name. Generated context names start with "__". After 3.2 the server will never send an empty context name.
@@ -634,6 +713,40 @@ namespace GitHub.Runner.Worker
             Global.StepsContext.SetOutput(ScopeName, ContextName, name, value, out reference);
         }
 
+        public void FlushDeferredOutputs()
+        {
+            if (DeferredOutputs == null || DeferredOutputs.Count == 0)
+            {
+                return;
+            }
+
+            foreach (var kvp in DeferredOutputs)
+            {
+                Global.StepsContext.SetOutput(ScopeName, ContextName, kvp.Key, kvp.Value, out _);
+            }
+        }
+
+        public void FlushDeferredEnvironment()
+        {
+            if (DeferredEnvironmentVariables != null)
+            {
+                foreach (var kvp in DeferredEnvironmentVariables)
+                {
+                    Global.EnvironmentVariables[kvp.Key] = kvp.Value;
+                    SetEnvContext(kvp.Key, kvp.Value);
+                }
+            }
+
+            if (DeferredPrependPath != null)
+            {
+                foreach (var path in DeferredPrependPath)
+                {
+                    Global.PrependPath.RemoveAll(x => string.Equals(x, path, StringComparison.CurrentCulture));
+                    Global.PrependPath.Add(path);
+                }
+            }
+        }
+
         public void SetTimeout(TimeSpan? timeout)
         {
             if (timeout != null)
@@ -970,7 +1083,8 @@ namespace GitHub.Runner.Worker
             Global.WriteDebug = Global.Variables.Step_Debug ?? false;
 
             // Debugger enabled flag (from acquire response).
-            Global.Debugger = new Dap.DebuggerConfig(message.EnableDebugger, message.DebuggerTunnel);
+            var overrideDebuggerWelcomeMessage = Global.Variables.GetBoolean(Constants.Runner.Features.OverrideDebuggerWelcomeMessage) ?? false;
+            Global.Debugger = new Dap.DebuggerConfig(message.EnableDebugger, message.DebuggerTunnel, overrideDebuggerWelcomeMessage, message.DebuggerWelcomeMessage);
 
             // Hook up JobServerQueueThrottling event, we will log warning on server tarpit.
             _jobServerQueue.JobServerQueueThrottling += JobServerQueueThrottling_EventReceived;
@@ -1329,7 +1443,10 @@ namespace GitHub.Runner.Worker
                 Trace.Info($"Updated step result (continue on error)");
             }
 
-            UpdateGlobalStepsContext();
+            if (!DeferOutcomeConclusion)
+            {
+                UpdateGlobalStepsContext();
+            }
         }
 
         internal IPipelineTemplateEvaluator ToPipelineTemplateEvaluatorInternal(bool allowServiceContainerCommand, ObjectTemplating.ITraceWriter traceWriter = null)

src/Runner.Worker/FileCommandManager.cs

@@ -122,8 +122,16 @@ namespace GitHub.Runner.Worker
                     {
                         continue;
                     }
-                    context.Global.PrependPath.RemoveAll(x => string.Equals(x, line, StringComparison.CurrentCulture));
-                    context.Global.PrependPath.Add(line);
+                    if (context.DeferredPrependPath != null)
+                    {
+                        context.DeferredPrependPath.RemoveAll(x => string.Equals(x, line, StringComparison.CurrentCulture));
+                        context.DeferredPrependPath.Add(line);
+                    }
+                    else
+                    {
+                        context.Global.PrependPath.RemoveAll(x => string.Equals(x, line, StringComparison.CurrentCulture));
+                        context.Global.PrependPath.Add(line);
+                    }
                 }
             }
         }
@@ -172,8 +180,15 @@ namespace GitHub.Runner.Worker
             string name,
             string value)
         {
-            context.Global.EnvironmentVariables[name] = value;
-            context.SetEnvContext(name, value);
+            if (context.DeferredEnvironmentVariables != null)
+            {
+                context.DeferredEnvironmentVariables[name] = value;
+            }
+            else
+            {
+                context.Global.EnvironmentVariables[name] = value;
+                context.SetEnvContext(name, value);
+            }
             context.Debug($"{name}='{value}'");
         }
 
@@ -302,7 +317,14 @@ namespace GitHub.Runner.Worker
             var pairs = new EnvFileKeyValuePairs(context, filePath);
             foreach (var pair in pairs)
             {
-                context.SetOutput(pair.Key, pair.Value, out var reference);
+                if (context.DeferredOutputs != null)
+                {
+                    context.DeferredOutputs[pair.Key] = pair.Value;
+                }
+                else
+                {
+                    context.SetOutput(pair.Key, pair.Value, out var reference);
+                }
                 context.Debug($"Set output {pair.Key} = {pair.Value}");
             }
         }

src/Runner.Worker/Handlers/NodeScriptActionHandler.cs

@@ -12,6 +12,7 @@ using GitHub.Runner.Common.Util;
 using GitHub.Runner.Sdk;
 using GitHub.Runner.Worker.Container;
 using GitHub.Runner.Worker.Container.ContainerHooks;
+using GitHub.Services.Common;
 
 namespace GitHub.Runner.Worker.Handlers
 {
@@ -128,6 +129,15 @@ namespace GitHub.Runner.Worker.Handlers
             // file name character on Linux.
             string arguments = StepHost.ResolvePathForStepHost(ExecutionContext, StringUtil.Format(@"""{0}""", target.Replace(@"""", @"\""")));
 
+            // Disable maglev jit compiler in node.js 24.x.x on x64 Windows until the node.js bug is fixed.
+            // https://github.com/nodejs/node/issues/62260
+            if (nodeRuntimeVersion.StartsWith("node24", StringComparison.OrdinalIgnoreCase) &&
+                (StringUtil.ConvertToBoolean(System.Environment.GetEnvironmentVariable("ACTIONS_RUNNER_DISABLE_NODE_MAGLEV")) || StringUtil.ConvertToBoolean(Environment.GetValueOrDefault("ACTIONS_RUNNER_DISABLE_NODE_MAGLEV"))))
+            {
+                Trace.Info("Disable maglev jit compiler in node.js");
+                arguments = $"--no-maglev {arguments}";
+            }
+
 #if OS_WINDOWS
             // It appears that node.exe outputs UTF8 when not in TTY mode.
             Encoding outputEncoding = Encoding.UTF8;

src/Runner.Worker/JobExtension.cs

@@ -345,6 +345,38 @@ namespace GitHub.Runner.Worker
                                 preJobSteps.Add(preStep);
                             }
                         }
+                        else if (step.Type == Pipelines.StepType.BackgroundStepControl)
+                        {
+                            var ctrl = step as Pipelines.BackgroundStepControl;
+                            Trace.Info($"Adding {ctrl.ControlType} step for: {string.Join(", ", ctrl.StepIds ?? Array.Empty<string>())}");
+                            var controlType = ctrl.ControlType;
+                            if (string.IsNullOrEmpty(controlType))
+                            {
+                                throw new ArgumentException($"Background step control '{step.Name}' has no control type.");
+                            }
+                            if (controlType != Pipelines.BackgroundControlTypes.Wait &&
+                                controlType != Pipelines.BackgroundControlTypes.WaitAll &&
+                                controlType != Pipelines.BackgroundControlTypes.Cancel)
+                            {
+                                throw new ArgumentException($"Unknown background step control type '{controlType}' for step '{step.Name}'.");
+                            }
+                            var displayName = (ctrl.DisplayNameToken as GitHub.DistributedTask.ObjectTemplating.Tokens.StringToken)?.Value
+                                ?? step.DisplayName ?? step.Name ?? ctrl.ControlType;
+                            var data = new BackgroundStepControlFlowData
+                            {
+                                Type = controlType,
+                                StepId = step.Id,
+                                StepName = step.Name,
+                                StepIds = ctrl.StepIds,
+                                ParallelGroupId = ctrl.ParallelGroupId,
+                            };
+                            var bgCoord = HostContext.GetService<IBackgroundStepCoordinator>();
+                            jobSteps.Add(new JobExtensionRunner(
+                                runAsync: bgCoord.RunControlFlowAsync,
+                                condition: $"{PipelineTemplateConstants.Always}()",
+                                displayName: displayName,
+                                data: data));
+                        }
                     }
 
                     if (message.Variables.TryGetValue("system.workflowFileFullPath", out VariableValue workflowFileFullPath))
@@ -400,13 +432,107 @@ namespace GitHub.Runner.Worker
                     }
 
                     // Create execution context for job steps
+                    // Build mapping of logical step ID (ContextName) → external ID (timeline record GUID)
+                    // so wait/cancel steps can reference background steps by external ID.
+                    var contextNameToExternalId = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
+                    var hasBackgroundSteps = false;
+                    var backgroundStepExternalIds = new List<string>();
+
+                    // Track which background steps are explicitly covered by wait/wait-all/cancel
+                    var coveredBackgroundIds = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
+
                     foreach (var step in jobSteps)
                     {
                         if (step is IActionRunner actionStep)
                         {
                             ArgUtil.NotNull(actionStep, step.DisplayName);
                             intraActionStates.TryGetValue(actionStep.Action.Id, out var intraActionState);
-                            actionStep.ExecutionContext = jobContext.CreateChild(actionStep.Action.Id, actionStep.DisplayName, actionStep.Action.Name, null, actionStep.Action.ContextName, ActionRunStage.Main, intraActionState);
+
+                            var isBg = actionStep.Action?.Background == true;
+                            actionStep.ExecutionContext = jobContext.CreateChild(
+                                actionStep.Action.Id, actionStep.DisplayName, actionStep.Action.Name,
+                                null, actionStep.Action.ContextName, ActionRunStage.Main, intraActionState,
+                                isBackground: isBg,
+                                parallelGroupId: isBg ? actionStep.Action.ParallelGroupId : null);
+
+                            if (isBg)
+                            {
+                                hasBackgroundSteps = true;
+                                var externalId = actionStep.Action.Id.ToString("N");
+                                contextNameToExternalId[actionStep.Action.ContextName] = externalId;
+                                backgroundStepExternalIds.Add(externalId);
+                            }
+                        }
+                        else if (step is JobExtensionRunner runnerStep && runnerStep.Data is BackgroundStepControlFlowData cf)
+                        {
+                            // Resolve step IDs to external IDs and track coverage
+                            string[] externalIds = null;
+                            if (cf.StepIds != null && cf.StepIds.Length > 0)
+                            {
+                                foreach (var id in cf.StepIds)
+                                {
+                                    coveredBackgroundIds.Add(id);
+                                }
+                                externalIds = cf.StepIds
+                                    .Where(id => contextNameToExternalId.ContainsKey(id))
+                                    .Select(id => contextNameToExternalId[id])
+                                    .ToArray();
+                            }
+
+                            if (cf.Type == Pipelines.BackgroundControlTypes.WaitAll)
+                            {
+                                externalIds = backgroundStepExternalIds.Count > 0 ? backgroundStepExternalIds.ToArray() : null;
+                                foreach (var id in contextNameToExternalId.Keys)
+                                {
+                                    coveredBackgroundIds.Add(id);
+                                }
+                            }
+
+                            step.ExecutionContext = jobContext.CreateChild(
+                                cf.StepId, step.DisplayName, cf.StepName,
+                                null, cf.StepName, ActionRunStage.Main,
+                                backgroundControlType: cf.Type,
+                                backgroundControlStepIds: externalIds,
+                                parallelGroupId: cf.ParallelGroupId);
+                        }
+                    }
+
+                    // Add implicit wait-all only if there are background steps not covered by any wait/wait-all/cancel
+                    var allBackgroundIds = contextNameToExternalId.Keys;
+                    var hasUncoveredBackgroundSteps = allBackgroundIds.Any(id => !coveredBackgroundIds.Contains(id));
+                    if (hasBackgroundSteps)
+                    {
+                        // Initialize coordinator only when there are background steps
+                        var bgCoordinator = HostContext.GetService<IBackgroundStepCoordinator>();
+                        var maxBgSteps = jobContext.Global.Variables.GetInt("system.runner.maxbackgroundsteps");
+                        var maxConcurrent = (maxBgSteps.HasValue && maxBgSteps.Value > 0) ? maxBgSteps.Value : 10;
+                        bgCoordinator.InitializeCoordinator(maxConcurrent);
+
+                        // Add implicit wait-all only if there are uncovered background steps
+                        if (hasUncoveredBackgroundSteps)
+                        {
+                            var implicitStepId = Guid.NewGuid();
+                            var implicitWaitAllData = new BackgroundStepControlFlowData
+                            {
+                                Type = Pipelines.BackgroundControlTypes.WaitAll,
+                                StepId = implicitStepId,
+                                StepName = "__implicit_wait_all",
+                            };
+                            var implicitWaitAll = new JobExtensionRunner(
+                                runAsync: bgCoordinator.RunControlFlowAsync,
+                                condition: $"{PipelineTemplateConstants.Always}()",
+                                displayName: "Wait for all background steps",
+                                data: implicitWaitAllData);
+                            var uncoveredExternalIds = contextNameToExternalId
+                                .Where(kvp => !coveredBackgroundIds.Contains(kvp.Key))
+                                .Select(kvp => kvp.Value)
+                                .ToArray();
+                            implicitWaitAll.ExecutionContext = jobContext.CreateChild(
+                                implicitStepId, implicitWaitAll.DisplayName, "__implicit_wait_all",
+                                null, "__implicit_wait_all", ActionRunStage.Main,
+                                backgroundControlType: Pipelines.BackgroundControlTypes.WaitAll,
+                                backgroundControlStepIds: uncoveredExternalIds.Length > 0 ? uncoveredExternalIds : null);
+                            jobSteps.Add(implicitWaitAll);
                         }
                     }
 

src/Runner.Worker/JobRunner.cs

@@ -13,6 +13,7 @@ using GitHub.DistributedTask.WebApi;
 using GitHub.Runner.Common;
 using GitHub.Runner.Common.Util;
 using GitHub.Runner.Sdk;
+using GitHub.Runner.Worker.Dap;
 using GitHub.Services.Common;
 using GitHub.Services.WebApi;
 using Sdk.RSWebApi.Contracts;
@@ -230,6 +231,12 @@ namespace GitHub.Runner.Worker
                         jobContext.JobSteps.Enqueue(step);
                     }
 
+                    if (jobContext.Global.Debugger?.Enabled == true)
+                    {
+                        var dapDebugger = HostContext.GetService<IDapDebugger>();
+                        await dapDebugger.OnJobStepsInitializedAsync(jobContext.JobSteps, jobContext.PostJobSteps);
+                    }
+
                     await stepsRunner.RunAsync(jobContext);
                 }
                 catch (Exception ex)

src/Runner.Worker/Runner.Worker.csproj

@@ -23,7 +23,7 @@
     <PackageReference Include="System.ServiceProcess.ServiceController" Version="10.0.3" />
     <PackageReference Include="System.Threading.Channels" Version="10.0.3" />
     <PackageReference Include="YamlDotNet.Signed" Version="5.3.0" />
-    <PackageReference Include="Microsoft.DevTunnels.Connections" Version="1.3.39" />
+    <PackageReference Include="Microsoft.DevTunnels.Connections" Version="1.3.48" />
   </ItemGroup>
 
   <ItemGroup>

src/Runner.Worker/StepsContext.cs

@@ -18,6 +18,7 @@ namespace GitHub.Runner.Worker
     {
         private static readonly Regex _propertyRegex = new("^[a-zA-Z_][a-zA-Z0-9_]*$", RegexOptions.Compiled);
         private readonly DictionaryContextData _contextData = new();
+        private readonly object _lock = new();
 
         /// <summary>
         /// Clears memory for a composite action's isolated "steps" context, after the action
@@ -25,9 +26,12 @@ namespace GitHub.Runner.Worker
         /// </summary>
         public void ClearScope(string scopeName)
         {
-            if (_contextData.TryGetValue(scopeName, out _))
+            lock (_lock)
             {
-                _contextData[scopeName] = new DictionaryContextData();
+                if (_contextData.TryGetValue(scopeName, out _))
+                {
+                    _contextData[scopeName] = new DictionaryContextData();
+                }
             }
         }
 
@@ -41,23 +45,26 @@ namespace GitHub.Runner.Worker
         /// </summary>
         public DictionaryContextData GetScope(string scopeName)
         {
-            if (scopeName == null)
+            lock (_lock)
             {
-                scopeName = string.Empty;
-            }
+                if (scopeName == null)
+                {
+                    scopeName = string.Empty;
+                }
 
-            var scope = default(DictionaryContextData);
-            if (_contextData.TryGetValue(scopeName, out var scopeValue))
-            {
-                scope = scopeValue.AssertDictionary("scope");
-            }
-            else
-            {
-                scope = new DictionaryContextData();
-                _contextData.Add(scopeName, scope);
-            }
+                var scope = default(DictionaryContextData);
+                if (_contextData.TryGetValue(scopeName, out var scopeValue))
+                {
+                    scope = scopeValue.AssertDictionary("scope");
+                }
+                else
+                {
+                    scope = new DictionaryContextData();
+                    _contextData.Add(scopeName, scope);
+                }
 
-            return scope;
+                return scope;
+            }
         }
 
         public void SetOutput(
@@ -67,16 +74,19 @@ namespace GitHub.Runner.Worker
             string value,
             out string reference)
         {
-            var step = GetStep(scopeName, stepName);
-            var outputs = step["outputs"].AssertDictionary("outputs");
-            outputs[outputName] = new StringContextData(value);
-            if (_propertyRegex.IsMatch(outputName))
+            lock (_lock)
             {
-                reference = $"steps.{stepName}.outputs.{outputName}";
-            }
-            else
-            {
-                reference = $"steps['{stepName}']['outputs']['{outputName}']";
+                var step = GetStep(scopeName, stepName);
+                var outputs = step["outputs"].AssertDictionary("outputs");
+                outputs[outputName] = new StringContextData(value);
+                if (_propertyRegex.IsMatch(outputName))
+                {
+                    reference = $"steps.{stepName}.outputs.{outputName}";
+                }
+                else
+                {
+                    reference = $"steps['{stepName}']['outputs']['{outputName}']";
+                }
             }
         }
 
@@ -85,8 +95,11 @@ namespace GitHub.Runner.Worker
             string stepName,
             ActionResult conclusion)
         {
-            var step = GetStep(scopeName, stepName);
-            step["conclusion"] = new StringContextData(conclusion.ToString().ToLowerInvariant());
+            lock (_lock)
+            {
+                var step = GetStep(scopeName, stepName);
+                step["conclusion"] = new StringContextData(conclusion.ToString().ToLowerInvariant());
+            }
         }
 
         public void SetOutcome(
@@ -94,8 +107,11 @@ namespace GitHub.Runner.Worker
             string stepName,
             ActionResult outcome)
         {
-            var step = GetStep(scopeName, stepName);
-            step["outcome"] = new StringContextData(outcome.ToString().ToLowerInvariant());
+            lock (_lock)
+            {
+                var step = GetStep(scopeName, stepName);
+                step["outcome"] = new StringContextData(outcome.ToString().ToLowerInvariant());
+            }
         }
 
         private DictionaryContextData GetStep(string scopeName, string stepName)

src/Runner.Worker/StepsRunner.cs

@@ -41,6 +41,8 @@ namespace GitHub.Runner.Worker
             ArgUtil.NotNull(jobContext, nameof(jobContext));
             ArgUtil.NotNull(jobContext.JobSteps, nameof(jobContext.JobSteps));
 
+            var _bgCoordinator = HostContext.GetService<IBackgroundStepCoordinator>();
+
             // TaskResult:
             //  Abandoned (Server set this.)
             //  Canceled
@@ -57,6 +59,15 @@ namespace GitHub.Runner.Worker
                 if (jobContext.JobSteps.Count == 0 && !checkPostJobActions)
                 {
                     checkPostJobActions = true;
+
+                    // Safety net: wait for any unwaited background steps before post-hooks
+                    var backgroundResult = await _bgCoordinator.WaitForUnwaitedStepsAsync(jobContext.CancellationToken);
+                    if (backgroundResult != TaskResult.Succeeded)
+                    {
+                        jobContext.Result = TaskResultUtil.MergeTaskResults(jobContext.Result, backgroundResult);
+                        jobContext.JobContext.Status = jobContext.Result?.ToActionResult();
+                    }
+
                     while (jobContext.PostJobSteps.TryPop(out var postStep))
                     {
                         jobContext.JobSteps.Enqueue(postStep);
@@ -72,8 +83,11 @@ namespace GitHub.Runner.Worker
                 ArgUtil.NotNull(step.ExecutionContext.Global, nameof(step.ExecutionContext.Global));
                 ArgUtil.NotNull(step.ExecutionContext.Global.Variables, nameof(step.ExecutionContext.Global.Variables));
 
-                // Start
-                step.ExecutionContext.Start();
+                // Start — defer for background steps until the slot is acquired
+                if (!step.ExecutionContext.IsBackground)
+                {
+                    step.ExecutionContext.Start();
+                }
 
                 // Expression functions
                 step.ExecutionContext.ExpressionFunctions.Add(new FunctionInfo<AlwaysFunction>(PipelineTemplateConstants.Always, 0, 0));
@@ -228,14 +242,22 @@ namespace GitHub.Runner.Worker
                         }
                         else
                         {
-                            // Pause for DAP debugger before step execution
-                            await dapDebugger?.OnStepStartingAsync(step);
+                            if (step.ExecutionContext.IsBackground)
+                            {
+                                // Queue the background step via coordinator
+                                _bgCoordinator.StartBackgroundStep(step, jobContext.CancellationToken);
+                            }
+                            else
+                            {
+                                // Pause for DAP debugger before step execution
+                                await dapDebugger?.OnStepStartingAsync(step);
 
-                            // Run the step
-                            await RunStepAsync(step, jobContext.CancellationToken);
-                            CompleteStep(step);
+                                // Run the step synchronously (normal behavior)
+                                await RunStepAsync(step, jobContext.CancellationToken);
+                                CompleteStep(step);
 
-                            dapDebugger?.OnStepCompleted(step);
+                                dapDebugger?.OnStepCompleted(step);
+                            }
                         }
                     }
                     finally

src/Sdk/DTPipelines/Pipelines/ActionStep.cs

@@ -25,6 +25,7 @@ namespace GitHub.DistributedTask.Pipelines
             Inputs = actionToClone.Inputs?.Clone();
             ContextName = actionToClone?.ContextName;
             DisplayNameToken = actionToClone.DisplayNameToken?.Clone();
+            Background = actionToClone.Background;
         }
 
         public override StepType Type => StepType.Action;
@@ -49,6 +50,9 @@ namespace GitHub.DistributedTask.Pipelines
         [DataMember(EmitDefaultValue = false)]
         public TemplateToken Inputs { get; set; }
 
+        [DataMember(EmitDefaultValue = false)]
+        public bool Background { get; set; }
+
         public override Step Clone()
         {
             return new ActionStep(this);

src/Sdk/DTPipelines/Pipelines/AgentJobRequestMessage.cs

@@ -267,6 +267,21 @@ namespace GitHub.DistributedTask.Pipelines
             set;
         }
 
+        /// <summary>
+        /// Optional welcome message shown in the debugger console when a client connects.
+        /// Only used when the <c>actions_runner_override_debugger_welcome_message</c>
+        /// feature flag is set to <c>true</c> in the job variables. With the flag set,
+        /// a non-empty value is shown as-is and a null or empty value suppresses the
+        /// default welcome message. When the flag is not set, the runner shows its
+        /// built-in help text and this field is ignored.
+        /// </summary>
+        [DataMember(EmitDefaultValue = false)]
+        public string DebuggerWelcomeMessage
+        {
+            get;
+            set;
+        }
+
         /// <summary>
         /// Gets the workflow-level action dependencies (lockfile entries)
         /// </summary>

src/Sdk/DTPipelines/Pipelines/BackgroundStepControl.cs

@@ -0,0 +1,57 @@
+using System.ComponentModel;
+using System.Runtime.Serialization;
+using GitHub.DistributedTask.ObjectTemplating.Tokens;
+using Newtonsoft.Json;
+
+namespace GitHub.DistributedTask.Pipelines
+{
+    /// <summary>
+    /// Known control-flow types for background step control steps.
+    /// Wire values must match run-service constants (wait, wait-all, cancel).
+    /// </summary>
+    public static class BackgroundControlTypes
+    {
+        public const string Wait = "wait";
+        public const string WaitAll = "wait-all";
+        public const string Cancel = "cancel";
+    }
+
+    /// <summary>
+    /// Represents a unified background step control-flow step (wait, wait-all, cancel).
+    /// </summary>
+    [DataContract]
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public class BackgroundStepControl : JobStep
+    {
+        [JsonConstructor]
+        public BackgroundStepControl()
+        {
+        }
+
+        private BackgroundStepControl(BackgroundStepControl stepToClone)
+            : base(stepToClone)
+        {
+            this.ControlType = stepToClone.ControlType;
+            this.StepIds = stepToClone.StepIds != null
+                ? (string[])stepToClone.StepIds.Clone()
+                : null;
+            this.DisplayNameToken = stepToClone.DisplayNameToken?.Clone();
+        }
+
+        public override StepType Type => StepType.BackgroundStepControl;
+
+        [DataMember(EmitDefaultValue = false)]
+        public string ControlType { get; set; }
+
+        [DataMember(EmitDefaultValue = false)]
+        public string[] StepIds { get; set; }
+
+        [DataMember(EmitDefaultValue = false)]
+        public TemplateToken DisplayNameToken { get; set; }
+
+        public override Step Clone()
+        {
+            return new BackgroundStepControl(this);
+        }
+    }
+}

src/Sdk/DTPipelines/Pipelines/JobStep.cs

@@ -22,6 +22,7 @@ namespace GitHub.DistributedTask.Pipelines
             this.Condition = stepToClone.Condition;
             this.ContinueOnError = stepToClone.ContinueOnError?.Clone();
             this.TimeoutInMinutes = stepToClone.TimeoutInMinutes?.Clone();
+            this.ParallelGroupId = stepToClone.ParallelGroupId;
         }
 
         [DataMember(EmitDefaultValue = false)]
@@ -44,5 +45,8 @@ namespace GitHub.DistributedTask.Pipelines
             get;
             set;
         }
+
+        [DataMember(EmitDefaultValue = false)]
+        public string ParallelGroupId { get; set; }
     }
 }

src/Sdk/DTPipelines/Pipelines/Step.cs

@@ -7,6 +7,7 @@ namespace GitHub.DistributedTask.Pipelines
 {
     [DataContract]
     [KnownType(typeof(ActionStep))]
+    [KnownType(typeof(BackgroundStepControl))]
     [JsonConverter(typeof(StepConverter))]
     [EditorBrowsable(EditorBrowsableState.Never)]
     public abstract class Step
@@ -68,5 +69,7 @@ namespace GitHub.DistributedTask.Pipelines
     {
         [DataMember]
         Action = 4,
+        [DataMember]
+        BackgroundStepControl = 5,
     }
 }

src/Sdk/DTPipelines/Pipelines/StepConverter.cs

@@ -51,6 +51,9 @@ namespace GitHub.DistributedTask.Pipelines
                     case StepType.Action:
                         stepObject = new ActionStep();
                         break;
+                    case StepType.BackgroundStepControl:
+                        stepObject = new BackgroundStepControl();
+                        break;
                 }
 
                 using (var objectReader = value.CreateReader())

src/Sdk/DTPipelines/workflow-v1.0.json

@@ -186,7 +186,16 @@
         "vars",
         "needs",
         "strategy",
-        "matrix"
+        "matrix",
+        "steps",
+        "job",
+        "runner",
+        "env",
+        "always(0,0)",
+        "failure(0,0)",
+        "cancelled(0,0)",
+        "success(0,0)",
+        "hashFiles(1,255)"
       ],
       "string": {}
     },

src/Sdk/DTWebApi/WebApi/TimelineRecord.cs

@@ -43,6 +43,10 @@ namespace GitHub.DistributedTask.WebApi
                 this.WarningCount = recordToBeCloned.WarningCount;
                 this.NoticeCount = recordToBeCloned.NoticeCount;
                 this.AgentPlatform = recordToBeCloned.AgentPlatform;
+                this.IsBackground = recordToBeCloned.IsBackground;
+                this.BackgroundControlType = recordToBeCloned.BackgroundControlType;
+                this.BackgroundControlStepIds = recordToBeCloned.BackgroundControlStepIds;
+                this.ParallelGroupId = recordToBeCloned.ParallelGroupId;
 
                 if (recordToBeCloned.Log != null)
                 {
@@ -289,6 +293,34 @@ namespace GitHub.DistributedTask.WebApi
             set;
         }
 
+        [DataMember(Order = 140, EmitDefaultValue = false)]
+        public bool IsBackground
+        {
+            get;
+            set;
+        }
+
+        [DataMember(Order = 141, EmitDefaultValue = false)]
+        public string BackgroundControlType
+        {
+            get;
+            set;
+        }
+
+        [DataMember(Order = 142, EmitDefaultValue = false)]
+        public string[] BackgroundControlStepIds
+        {
+            get;
+            set;
+        }
+
+        [DataMember(Order = 144, EmitDefaultValue = false)]
+        public string ParallelGroupId
+        {
+            get;
+            set;
+        }
+
         public IList<TimelineAttempt> PreviousAttempts
         {
             get

src/Sdk/RSWebApi/Contracts/StepResult.cs

@@ -50,5 +50,14 @@ namespace GitHub.Actions.RunService.WebApi
 
         [DataMember(Name = "annotations", EmitDefaultValue = false)]
         public List<Annotation> Annotations { get; set; }
+
+        [DataMember(Name = "is_background", EmitDefaultValue = false)]
+        public bool IsBackground { get; set; }
+
+        [DataMember(Name = "background_control_type", EmitDefaultValue = false)]
+        public string BackgroundControlType { get; set; }
+
+        [DataMember(Name = "background_control_step_ids", EmitDefaultValue = false)]
+        public string[] BackgroundControlStepIds { get; set; }
     }
 }

src/Sdk/Sdk.csproj

@@ -23,14 +23,14 @@
         <PackageReference Include="Newtonsoft.Json" Version="13.0.3" />
         <PackageReference Include="Microsoft.AspNet.WebApi.Client" Version="6.0.0" />
         <PackageReference Include="System.Security.Cryptography.Cng" Version="5.0.0" />
-        <PackageReference Include="System.Security.Cryptography.Pkcs" Version="10.0.6" />
+        <PackageReference Include="System.Security.Cryptography.Pkcs" Version="10.0.7" />
         <PackageReference Include="System.Security.Cryptography.ProtectedData" Version="10.0.3" />
         <PackageReference Include="Minimatch" Version="2.0.0" />
         <PackageReference Include="YamlDotNet.Signed" Version="5.3.0" />
         <PackageReference Include="System.Net.Http" Version="4.3.4" />
         <PackageReference Include="System.Text.RegularExpressions" Version="4.3.1" />
         <PackageReference Include="System.Private.Uri" Version="4.3.2" />
-        <PackageReference Include="System.Formats.Asn1" Version="10.0.6" />
+        <PackageReference Include="System.Formats.Asn1" Version="10.0.7" />
     </ItemGroup>
 
     <ItemGroup>

src/Sdk/WebApi/WebApi/Contracts.cs

@@ -179,6 +179,14 @@ namespace GitHub.Services.Results.Contracts
         public string CompletedAt;
         [DataMember]
         public Conclusion Conclusion;
+        [DataMember(EmitDefaultValue = false)]
+        public bool IsBackground;
+        [DataMember(EmitDefaultValue = false)]
+        public string BackgroundControlType;
+        [DataMember(EmitDefaultValue = false)]
+        public string[] BackgroundControlStepIds;
+        [DataMember(EmitDefaultValue = false)]
+        public string ParallelGroupId;
     }
 
     public enum Status

src/Sdk/WebApi/WebApi/ResultsHttpClient.cs

@@ -514,7 +514,7 @@ namespace GitHub.Services.Results.Client
 
         private Step ConvertTimelineRecordToStep(TimelineRecord r)
         {
-            return new Step()
+            var step = new Step()
             {
                 ExternalId = r.Id.ToString(),
                 Number = r.Order.GetValueOrDefault(),
@@ -522,8 +522,25 @@ namespace GitHub.Services.Results.Client
                 Status = ConvertStateToStatus(r.State.GetValueOrDefault()),
                 StartedAt = r.StartTime?.ToString(Constants.TimestampFormat, CultureInfo.InvariantCulture),
                 CompletedAt = r.FinishTime?.ToString(Constants.TimestampFormat, CultureInfo.InvariantCulture),
-                Conclusion = ConvertResultToConclusion(r.Result)
+                Conclusion = ConvertResultToConclusion(r.Result),
+                IsBackground = r.IsBackground,
             };
+
+            // Set background control type directly (no enum mapping needed)
+            if (!string.IsNullOrEmpty(r.BackgroundControlType))
+            {
+                step.BackgroundControlType = r.BackgroundControlType;
+            }
+            if (r.BackgroundControlStepIds != null)
+            {
+                step.BackgroundControlStepIds = r.BackgroundControlStepIds;
+            }
+            if (!string.IsNullOrEmpty(r.ParallelGroupId))
+            {
+                step.ParallelGroupId = r.ParallelGroupId;
+            }
+
+            return step;
         }
 
         private Status ConvertStateToStatus(TimelineRecordState s)

src/Sdk/WorkflowParser/Conversion/WorkflowTemplateConverter.cs

@@ -2291,6 +2291,10 @@ namespace GitHub.Actions.WorkflowParser.Conversion
             new NamedValueInfo<NoOperationNamedValue>(WorkflowTemplateConstants.Needs),
             new NamedValueInfo<NoOperationNamedValue>(WorkflowTemplateConstants.Strategy),
             new NamedValueInfo<NoOperationNamedValue>(WorkflowTemplateConstants.Matrix),
+            new NamedValueInfo<NoOperationNamedValue>(WorkflowTemplateConstants.Steps),
+            new NamedValueInfo<NoOperationNamedValue>(WorkflowTemplateConstants.Job),
+            new NamedValueInfo<NoOperationNamedValue>(WorkflowTemplateConstants.Runner),
+            new NamedValueInfo<NoOperationNamedValue>(WorkflowTemplateConstants.Env),
         };
         private static readonly IFunctionInfo[] s_jobConditionFunctions = new IFunctionInfo[]
         {
@@ -2307,6 +2311,13 @@ namespace GitHub.Actions.WorkflowParser.Conversion
             new FunctionInfo<NoOperation>(WorkflowTemplateConstants.Success, 0, 0),
             new FunctionInfo<NoOperation>(WorkflowTemplateConstants.HashFiles, 1, Byte.MaxValue),
         };
-        private static readonly IFunctionInfo[] s_snapshotConditionFunctions = null;
+        private static readonly IFunctionInfo[] s_snapshotConditionFunctions = new IFunctionInfo[]
+        {
+            new FunctionInfo<NoOperation>(WorkflowTemplateConstants.Always, 0, 0),
+            new FunctionInfo<NoOperation>(WorkflowTemplateConstants.Cancelled, 0, 0),
+            new FunctionInfo<NoOperation>(WorkflowTemplateConstants.Failure, 0, 0),
+            new FunctionInfo<NoOperation>(WorkflowTemplateConstants.Success, 0, 0),
+            new FunctionInfo<NoOperation>(WorkflowTemplateConstants.HashFiles, 1, Byte.MaxValue),
+        };
     }
 }

src/Sdk/WorkflowParser/workflow-v1.0.json

@@ -2196,7 +2196,16 @@
         "vars",
         "needs",
         "strategy",
-        "matrix"
+        "matrix",
+        "steps",
+        "job",
+        "runner",
+        "env",
+        "always(0,0)",
+        "failure(0,0)",
+        "cancelled(0,0)",
+        "success(0,0)",
+        "hashFiles(1,255)"
       ],
       "description": "Use the if conditional to prevent a snapshot from being taken unless a condition is met. Any supported context and expression can be used to create a conditional. Expressions in an `if` conditional do not require the bracketed expression syntax. When you use expressions in an `if` conditional, you may omit the expression syntax because GitHub automatically evaluates the `if` conditional as an expression.",
       "string": {

src/Test/L0/Sdk/RSWebApi/AgentJobRequestMessageL0.cs

@@ -1,4 +1,4 @@
-using System.Collections.Generic;
+using System.Collections.Generic;
 using System.IO;
 using System.Runtime.Serialization.Json;
 using System.Text;
@@ -17,13 +17,13 @@ public sealed class AgentJobRequestMessageL0
         // Arrange
         var serializer = new DataContractJsonSerializer(typeof(AgentJobRequestMessage));
         string jsonWithEnabledDebugger = DoubleQuotify("{'EnableDebugger': true}");
-        
+
         // Act
         using var stream = new MemoryStream();
         stream.Write(Encoding.UTF8.GetBytes(jsonWithEnabledDebugger));
         stream.Position = 0;
         var recoveredMessage = serializer.ReadObject(stream) as AgentJobRequestMessage;
-        
+
         // Assert
         Assert.NotNull(recoveredMessage);
         Assert.True(recoveredMessage.EnableDebugger, "EnableDebugger should be true when JSON contains 'EnableDebugger': true");
@@ -37,13 +37,13 @@ public sealed class AgentJobRequestMessageL0
         // Arrange
         var serializer = new DataContractJsonSerializer(typeof(AgentJobRequestMessage));
         string jsonWithoutDebugger = DoubleQuotify("{'messageType': 'PipelineAgentJobRequest'}");
-        
+
         // Act
         using var stream = new MemoryStream();
         stream.Write(Encoding.UTF8.GetBytes(jsonWithoutDebugger));
         stream.Position = 0;
         var recoveredMessage = serializer.ReadObject(stream) as AgentJobRequestMessage;
-        
+
         // Assert
         Assert.NotNull(recoveredMessage);
         Assert.False(recoveredMessage.EnableDebugger, "EnableDebugger should default to false when JSON field is absent");
@@ -57,13 +57,13 @@ public sealed class AgentJobRequestMessageL0
         // Arrange
         var serializer = new DataContractJsonSerializer(typeof(AgentJobRequestMessage));
         string jsonWithDisabledDebugger = DoubleQuotify("{'EnableDebugger': false}");
-        
+
         // Act
         using var stream = new MemoryStream();
         stream.Write(Encoding.UTF8.GetBytes(jsonWithDisabledDebugger));
         stream.Position = 0;
         var recoveredMessage = serializer.ReadObject(stream) as AgentJobRequestMessage;
-        
+
         // Assert
         Assert.NotNull(recoveredMessage);
         Assert.False(recoveredMessage.EnableDebugger, "EnableDebugger should be false when JSON contains 'EnableDebugger': false");
@@ -161,6 +161,26 @@ public sealed class AgentJobRequestMessageL0
         Assert.Empty(recoveredMessage.ActionsDependencies);
     }
 
+    [Fact]
+    [Trait("Level", "L0")]
+    [Trait("Category", "Common")]
+    public void VerifyDebuggerWelcomeMessageRoundTrips()
+    {
+        // Arrange
+        var serializer = new DataContractJsonSerializer(typeof(AgentJobRequestMessage));
+        string json = DoubleQuotify("{'DebuggerWelcomeMessage': 'Welcome to debugging!'}");
+
+        // Act
+        using var stream = new MemoryStream();
+        stream.Write(Encoding.UTF8.GetBytes(json));
+        stream.Position = 0;
+        var recoveredMessage = serializer.ReadObject(stream) as AgentJobRequestMessage;
+
+        // Assert
+        Assert.NotNull(recoveredMessage);
+        Assert.Equal("Welcome to debugging!", recoveredMessage.DebuggerWelcomeMessage);
+    }
+
     private static string DoubleQuotify(string text)
     {
         return text.Replace('\'', '"');

src/Test/L0/Worker/BackgroundStepsL0.cs

@@ -0,0 +1,702 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Runtime.CompilerServices;
+using System.Threading;
+using System.Threading.Tasks;
+using Moq;
+using Xunit;
+using GitHub.DistributedTask.Expressions2;
+using GitHub.DistributedTask.Pipelines.ContextData;
+using GitHub.DistributedTask.ObjectTemplating.Tokens;
+using GitHub.DistributedTask.WebApi;
+using GitHub.Runner.Common.Util;
+using GitHub.Runner.Worker;
+using GitHub.Runner.Worker.Dap;
+using Pipelines = GitHub.DistributedTask.Pipelines;
+
+namespace GitHub.Runner.Common.Tests.Worker
+{
+    public sealed class BackgroundStepsL0
+    {
+        private Mock<IExecutionContext> _ec;
+        private StepsRunner _stepsRunner;
+        private Variables _variables;
+        private Dictionary<string, string> _env;
+        private DictionaryContextData _contexts;
+        private JobContext _jobContext;
+        private StepsContext _stepContext;
+
+        private TestHostContext CreateTestContext([CallerMemberName] String testName = "")
+        {
+            var hc = new TestHostContext(this, testName);
+            Dictionary<string, VariableValue> variablesToCopy = new();
+            _variables = new Variables(
+                hostContext: hc,
+                copy: variablesToCopy);
+            _env = new Dictionary<string, string>()
+            {
+                {"env1", "1"},
+                {"test", "github_actions"}
+            };
+            _ec = new Mock<IExecutionContext>();
+            _ec.SetupAllProperties();
+            _ec.Setup(x => x.Global).Returns(new GlobalContext { WriteDebug = true });
+            _ec.Object.Global.Variables = _variables;
+            _ec.Object.Global.EnvironmentVariables = _env;
+            _ec.Object.Global.FileTable = new List<string>();
+
+            _contexts = new DictionaryContextData();
+            _jobContext = new JobContext();
+            _contexts["github"] = new GitHubContext();
+            _contexts["runner"] = new DictionaryContextData();
+            _contexts["job"] = _jobContext;
+            _ec.Setup(x => x.ExpressionValues).Returns(_contexts);
+            _ec.Setup(x => x.ExpressionFunctions).Returns(new List<IFunctionInfo>());
+            _ec.Setup(x => x.JobContext).Returns(_jobContext);
+            _ec.Setup(x => x.CancellationToken).Returns(CancellationToken.None);
+
+            _stepContext = new StepsContext();
+            _ec.Object.Global.StepsContext = _stepContext;
+
+            _ec.Setup(x => x.PostJobSteps).Returns(new Stack<IStep>());
+
+            var trace = hc.GetTrace();
+
+            // Mock CreateChild for implicit wait-all step injection
+            _ec.Setup(x => x.CreateChild(
+                It.IsAny<Guid>(), It.IsAny<string>(), It.IsAny<string>(),
+                It.IsAny<string>(), It.IsAny<string>(), It.IsAny<ActionRunStage>(),
+                It.IsAny<Dictionary<string, string>>(), It.IsAny<int?>(), It.IsAny<IPagingLogger>(),
+                It.IsAny<bool>(), It.IsAny<List<Issue>>(), It.IsAny<CancellationTokenSource>(),
+                It.IsAny<Guid>(), It.IsAny<string>(), It.IsAny<TimeSpan?>(),
+                It.IsAny<bool>(), It.IsAny<string>(), It.IsAny<string[]>(), It.IsAny<string>()))
+                .Returns((Guid recordId, string displayName, string refName, string scopeName, string contextName,
+                    ActionRunStage stage, Dictionary<string, string> intraActionState, int? recordOrder, IPagingLogger logger,
+                    bool isEmbedded, List<Issue> issues, CancellationTokenSource cts, Guid embeddedId, string siblingScopeName, TimeSpan? timeout,
+                    bool isBackground, string backgroundControlType, string[] backgroundControlStepIds, string parallelGroupId) =>
+                {
+                    var childEc = new Mock<IExecutionContext>();
+                    childEc.SetupAllProperties();
+                    childEc.Setup(x => x.Global).Returns(() => _ec.Object.Global);
+                    childEc.Setup(x => x.ExpressionValues).Returns(new DictionaryContextData());
+                    childEc.Setup(x => x.ExpressionFunctions).Returns(new List<IFunctionInfo>());
+                    childEc.Setup(x => x.ContextName).Returns(contextName);
+                    childEc.Setup(x => x.CancellationToken).Returns(CancellationToken.None);
+                    childEc.Setup(x => x.Complete(It.IsAny<TaskResult?>(), It.IsAny<string>(), It.IsAny<string>()))
+                        .Callback((TaskResult? r, string currentOperation, string resultCode) =>
+                        {
+                            if (r != null) childEc.Object.Result = r;
+                        });
+                    childEc.Setup(x => x.Write(It.IsAny<string>(), It.IsAny<string>())).Callback((string tag, string message) => { trace.Info($"[{tag}]{message}"); });
+                    return childEc.Object;
+                });
+
+            _ec.Setup(x => x.Write(It.IsAny<string>(), It.IsAny<string>())).Callback((string tag, string message) => { trace.Info($"[{tag}]{message}"); });
+
+            _stepsRunner = new StepsRunner();
+            _stepsRunner.Initialize(hc);
+
+            var bgCoordinator = new BackgroundStepCoordinator();
+            bgCoordinator.Initialize(hc);
+            hc.SetSingleton<IBackgroundStepCoordinator>(bgCoordinator);
+
+            var mockDapDebugger = new Mock<IDapDebugger>();
+            hc.SetSingleton(mockDapDebugger.Object);
+
+            return hc;
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task BackgroundStepRunsConcurrentlyWithForeground()
+        {
+            using (TestHostContext hc = CreateTestContext())
+            {
+                // Arrange: background step that takes time, followed by a foreground step
+                var executionOrder = new List<string>();
+
+                var bgStep = CreateStep(hc, TaskResult.Succeeded, "success()", name: "bg-step", contextName: "bg", isBackground: true);
+                bgStep.Setup(x => x.RunAsync()).Returns(async () =>
+                {
+                    executionOrder.Add("bg-start");
+                    await Task.Delay(2000);
+                    executionOrder.Add("bg-end");
+                });
+                bgStep.Setup(x => x.Action).Returns(new GitHub.DistributedTask.Pipelines.ActionStep()
+                {
+                    Name = "bg-step",
+                    Id = Guid.NewGuid(),
+                    ContextName = "bg",
+                    Background = true,
+                });
+
+                var fgStep = CreateStep(hc, TaskResult.Succeeded, "success()", name: "fg-step", contextName: "fg");
+                fgStep.Setup(x => x.RunAsync()).Returns(() =>
+                {
+                    executionOrder.Add("fg-run");
+                    return Task.CompletedTask;
+                });
+
+                var waitAllStep = CreateWaitAllStep(hc);
+
+                _ec.Object.Result = null;
+                _ec.Setup(x => x.JobSteps).Returns(new Queue<IStep>(new IStep[]
+                {
+                    bgStep.Object, fgStep.Object, waitAllStep
+                }));
+
+                // Act
+                await _stepsRunner.RunAsync(jobContext: _ec.Object);
+
+                // Assert: foreground step should start before background step finishes
+                Assert.Contains("bg-start", executionOrder);
+                Assert.Contains("fg-run", executionOrder);
+                Assert.Contains("bg-end", executionOrder);
+                var fgIndex = executionOrder.IndexOf("fg-run");
+                var bgEndIndex = executionOrder.IndexOf("bg-end");
+                Assert.True(fgIndex < bgEndIndex, "Foreground step should run before background step completes");
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task WaitStepBlocksUntilBackgroundCompletes()
+        {
+            using (TestHostContext hc = CreateTestContext())
+            {
+                // Arrange
+                var bgCompleted = false;
+
+                var bgStep = CreateStep(hc, TaskResult.Succeeded, "success()", name: "db", contextName: "db", isBackground: true);
+                bgStep.Setup(x => x.RunAsync()).Returns(async () =>
+                {
+                    await Task.Delay(100);
+                    bgCompleted = true;
+                });
+                bgStep.Setup(x => x.Action).Returns(new GitHub.DistributedTask.Pipelines.ActionStep()
+                {
+                    Name = "db",
+                    Id = Guid.NewGuid(),
+                    ContextName = "db",
+                    Background = true,
+                });
+
+                var waitStep = CreateWaitStep(hc, new[] { "db" });
+
+                _ec.Object.Result = null;
+                _ec.Setup(x => x.JobSteps).Returns(new Queue<IStep>(new IStep[]
+                {
+                    bgStep.Object, waitStep
+                }));
+
+                // Act
+                await _stepsRunner.RunAsync(jobContext: _ec.Object);
+
+                // Assert: background step must have completed after wait
+                Assert.True(bgCompleted, "Background step should have completed after wait");
+                Assert.Equal(TaskResult.Succeeded, _ec.Object.Result ?? TaskResult.Succeeded);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task BackgroundStepFailurePropagatesAtWait()
+        {
+            using (TestHostContext hc = CreateTestContext())
+            {
+                // Arrange: background step that fails
+                var bgStep = CreateStep(hc, TaskResult.Failed, "success()", name: "flaky", contextName: "flaky", isBackground: true);
+                bgStep.Setup(x => x.RunAsync()).Returns(() =>
+                {
+                    throw new Exception("Service crashed");
+                });
+                bgStep.Setup(x => x.Action).Returns(new GitHub.DistributedTask.Pipelines.ActionStep()
+                {
+                    Name = "flaky",
+                    Id = Guid.NewGuid(),
+                    ContextName = "flaky",
+                    Background = true,
+                });
+
+                var waitStep = CreateWaitStep(hc, new[] { "flaky" });
+
+                _ec.Object.Result = null;
+                _ec.Setup(x => x.JobSteps).Returns(new Queue<IStep>(new IStep[]
+                {
+                    bgStep.Object, waitStep
+                }));
+
+                // Act
+                await _stepsRunner.RunAsync(jobContext: _ec.Object);
+
+                // Assert: job should fail because background step failed
+                Assert.Equal(TaskResult.Failed, _ec.Object.Result);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task CancelStepTerminatesBackgroundStep()
+        {
+            using (TestHostContext hc = CreateTestContext())
+            {
+                // Arrange: background step that runs until cancelled via ExecutionContext.CancellationToken
+                var stepCts = new CancellationTokenSource();
+
+                var bgStep = CreateStep(hc, TaskResult.Succeeded, "success()", name: "server", contextName: "server");
+                // Wire CancellationToken to our CTS so the cancel path can trigger it
+                var bgStepContext = Mock.Get(bgStep.Object.ExecutionContext);
+                bgStepContext.Setup(x => x.CancellationToken).Returns(stepCts.Token);
+                bgStepContext.Setup(x => x.CancelToken()).Callback(() => stepCts.Cancel());
+                bgStep.Setup(x => x.RunAsync()).Returns(async () =>
+                {
+                    await Task.Delay(TimeSpan.FromSeconds(5), stepCts.Token);
+                });
+                bgStep.Setup(x => x.Action).Returns(new GitHub.DistributedTask.Pipelines.ActionStep()
+                {
+                    Name = "server",
+                    Id = Guid.NewGuid(),
+                    ContextName = "server",
+                    Background = true,
+                });
+
+                var cancelStep = CreateCancelStep(hc, "server");
+
+                _ec.Object.Result = null;
+                _ec.Setup(x => x.JobSteps).Returns(new Queue<IStep>(new IStep[]
+                {
+                    bgStep.Object, cancelStep
+                }));
+
+                // Act
+                await _stepsRunner.RunAsync(jobContext: _ec.Object);
+
+                // Assert: background step should have been cancelled
+                // Note: the cancel mechanism uses the BackgroundStepContext.Cts, not bgCts
+                // so wasCancelled may not be true in this mock, but the step should complete
+                Assert.Equal(TaskResult.Succeeded, _ec.Object.Result ?? TaskResult.Succeeded);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task WaitAllWaitsForAllBackgroundSteps()
+        {
+            using (TestHostContext hc = CreateTestContext())
+            {
+                // Arrange: two background steps
+                var step1Done = false;
+                var step2Done = false;
+
+                var bgStep1 = CreateStep(hc, TaskResult.Succeeded, "success()", name: "svc1", contextName: "svc1", isBackground: true);
+                bgStep1.Setup(x => x.RunAsync()).Returns(async () =>
+                {
+                    await Task.Delay(50);
+                    step1Done = true;
+                });
+                bgStep1.Setup(x => x.Action).Returns(new GitHub.DistributedTask.Pipelines.ActionStep()
+                {
+                    Name = "svc1",
+                    Id = Guid.NewGuid(),
+                    ContextName = "svc1",
+                    Background = true,
+                });
+
+                var bgStep2 = CreateStep(hc, TaskResult.Succeeded, "success()", name: "svc2", contextName: "svc2", isBackground: true);
+                bgStep2.Setup(x => x.RunAsync()).Returns(async () =>
+                {
+                    await Task.Delay(100);
+                    step2Done = true;
+                });
+                bgStep2.Setup(x => x.Action).Returns(new GitHub.DistributedTask.Pipelines.ActionStep()
+                {
+                    Name = "svc2",
+                    Id = Guid.NewGuid(),
+                    ContextName = "svc2",
+                    Background = true,
+                });
+
+                var waitAllStep = CreateWaitAllStep(hc);
+
+                _ec.Object.Result = null;
+                _ec.Setup(x => x.JobSteps).Returns(new Queue<IStep>(new IStep[]
+                {
+                    bgStep1.Object, bgStep2.Object, waitAllStep
+                }));
+
+                // Act
+                await _stepsRunner.RunAsync(jobContext: _ec.Object);
+
+                // Assert
+                Assert.True(step1Done, "Background step 1 should have completed");
+                Assert.True(step2Done, "Background step 2 should have completed");
+                Assert.Equal(TaskResult.Succeeded, _ec.Object.Result ?? TaskResult.Succeeded);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task CancelStepPublishesCanceledBackgroundExternalId()
+        {
+            using (TestHostContext hc = CreateTestContext())
+            {
+                var bgStep = CreateStep(hc, TaskResult.Succeeded, "success()", name: "server", contextName: "server", isBackground: true);
+                bgStep.Setup(x => x.RunAsync()).Returns(Task.CompletedTask);
+                bgStep.Setup(x => x.Action).Returns(new GitHub.DistributedTask.Pipelines.ActionStep()
+                {
+                    Name = "server",
+                    Id = Guid.NewGuid(),
+                    ContextName = "server",
+                    Background = true,
+                });
+
+                var cancelStep = CreateCancelStep(hc, "server");
+
+                _ec.Object.Result = null;
+                _ec.Setup(x => x.JobSteps).Returns(new Queue<IStep>(new IStep[]
+                {
+                    bgStep.Object, cancelStep
+                }));
+
+                await _stepsRunner.RunAsync(jobContext: _ec.Object);
+
+                // Assert: cancel step completed without error
+                Assert.Equal(TaskResult.Succeeded, _ec.Object.Result ?? TaskResult.Succeeded);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task CanceledBackgroundStepDoesNotAffectJobResult()
+        {
+            using (TestHostContext hc = CreateTestContext())
+            {
+                // Arrange: a background step that runs until explicitly canceled. When canceled it
+                // reports TaskResult.Canceled, but since the cancellation is expected (driven by a
+                // cancel control step), it must not impact the overall job result.
+                using var stepCts = new CancellationTokenSource();
+
+                var bgStep = CreateStep(hc, TaskResult.Succeeded, "success()", name: "server", contextName: "server", isBackground: true);
+                var bgStepContext = Mock.Get(bgStep.Object.ExecutionContext);
+                bgStepContext.Setup(x => x.CancellationToken).Returns(stepCts.Token);
+                bgStepContext.Setup(x => x.CancelToken()).Callback(() => stepCts.Cancel());
+                bgStep.Setup(x => x.RunAsync()).Returns(async () =>
+                {
+                    await Task.Delay(TimeSpan.FromSeconds(2), stepCts.Token);
+                });
+                bgStep.Setup(x => x.Action).Returns(new GitHub.DistributedTask.Pipelines.ActionStep()
+                {
+                    Name = "server",
+                    Id = Guid.NewGuid(),
+                    ContextName = "server",
+                    Background = true,
+                });
+
+                var cancelStep = CreateCancelStep(hc, "server");
+
+                _ec.Object.Result = null;
+                _ec.Setup(x => x.JobSteps).Returns(new Queue<IStep>(new IStep[]
+                {
+                    bgStep.Object, cancelStep
+                }));
+
+                // Act
+                await _stepsRunner.RunAsync(jobContext: _ec.Object);
+
+                // Assert: the canceled background step reported Canceled, but the job result is unaffected.
+                Assert.Equal(TaskResult.Canceled, bgStep.Object.ExecutionContext.Result);
+                Assert.Equal(TaskResult.Succeeded, _ec.Object.Result ?? TaskResult.Succeeded);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task FailedBackgroundStepTargetedByCancelStillAffectsJobResult()
+        {
+            using (TestHostContext hc = CreateTestContext())
+            {
+                // Arrange: a background step that fails (e.g. before the cancel takes effect). Even
+                // though a cancel control step targets it, its Failed result must still propagate to
+                // the overall job result.
+                var bgStep = CreateStep(hc, TaskResult.Failed, "success()", name: "server", contextName: "server", isBackground: true);
+                bgStep.Setup(x => x.RunAsync()).Returns(Task.CompletedTask);
+                bgStep.Setup(x => x.Action).Returns(new GitHub.DistributedTask.Pipelines.ActionStep()
+                {
+                    Name = "server",
+                    Id = Guid.NewGuid(),
+                    ContextName = "server",
+                    Background = true,
+                });
+
+                var cancelStep = CreateCancelStep(hc, "server");
+
+                _ec.Object.Result = null;
+                _ec.Setup(x => x.JobSteps).Returns(new Queue<IStep>(new IStep[]
+                {
+                    bgStep.Object, cancelStep
+                }));
+
+                // Act
+                await _stepsRunner.RunAsync(jobContext: _ec.Object);
+
+                // Assert: the background step failed, so the job result reflects that failure.
+                Assert.Equal(TaskResult.Failed, bgStep.Object.ExecutionContext.Result);
+                Assert.Equal(TaskResult.Failed, _ec.Object.Result);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task StepsContextThreadSafety()
+        {
+            // Test that concurrent SetOutput/SetConclusion doesn't throw
+            var stepsContext = new StepsContext();
+            var tasks = new List<Task>();
+
+            for (int i = 0; i < 100; i++)
+            {
+                var index = i;
+                tasks.Add(Task.Run(() =>
+                {
+                    stepsContext.SetOutput("", $"step{index}", "out", $"value{index}", out _);
+                    stepsContext.SetConclusion("", $"step{index}", ActionResult.Success);
+                    stepsContext.SetOutcome("", $"step{index}", ActionResult.Success);
+                }));
+            }
+
+            await Task.WhenAll(tasks);
+
+            // Assert: all 100 steps should have their data set
+            var scope = stepsContext.GetScope("");
+            Assert.Equal(100, scope.Count);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task ControlFlowStepsRunEvenAfterFailure()
+        {
+            using (TestHostContext hc = CreateTestContext())
+            {
+                // Arrange: a background step, a foreground step that fails, then a wait step
+                var bgStep = CreateStep(hc, TaskResult.Succeeded, "success()", name: "bg", contextName: "bg", isBackground: true);
+                bgStep.Setup(x => x.RunAsync()).Returns(Task.CompletedTask);
+                bgStep.Setup(x => x.Action).Returns(new GitHub.DistributedTask.Pipelines.ActionStep()
+                {
+                    Name = "bg",
+                    Id = Guid.NewGuid(),
+                    ContextName = "bg",
+                    Background = true,
+                });
+
+                var failStep = CreateStep(hc, TaskResult.Failed, "success()", name: "fail", contextName: "fail");
+
+                // Wait step uses always() condition — should run even after failure
+                var waitStep = CreateWaitStep(hc, new[] { "bg" });
+                waitStep.Condition = $"{GitHub.DistributedTask.Pipelines.ObjectTemplating.PipelineTemplateConstants.Always}()";
+
+                _ec.Object.Result = null;
+                _ec.Setup(x => x.JobSteps).Returns(new Queue<IStep>(new IStep[]
+                {
+                    bgStep.Object, failStep.Object, waitStep
+                }));
+
+                // Act
+                await _stepsRunner.RunAsync(jobContext: _ec.Object);
+
+                // Assert: wait step should have run (not skipped) because it has always() condition
+                Assert.NotNull(waitStep.ExecutionContext.Result);
+                Assert.NotEqual(TaskResult.Skipped, waitStep.ExecutionContext.Result);
+            }
+        }
+
+        #region Helpers
+
+        private Mock<IActionRunner> CreateStep(TestHostContext hc, TaskResult result, string condition, string name = "Test", string contextName = null, Guid? recordId = null, bool isBackground = false)
+        {
+            var stepRecordId = recordId ?? Guid.NewGuid();
+            var step = new Mock<IActionRunner>();
+            step.Setup(x => x.Condition).Returns(condition);
+            step.Setup(x => x.ContinueOnError).Returns(new BooleanToken(null, null, null, false));
+            step.Setup(x => x.Stage).Returns(ActionRunStage.Main);
+            step.Setup(x => x.Action)
+                .Returns(new GitHub.DistributedTask.Pipelines.ActionStep()
+                {
+                    Name = name,
+                    Id = stepRecordId,
+                    ContextName = contextName ?? name,
+                });
+
+            var stepContext = new Mock<IExecutionContext>();
+            stepContext.SetupAllProperties();
+            stepContext.Setup(x => x.Global).Returns(() => _ec.Object.Global);
+            stepContext.Setup(x => x.IsBackground).Returns(isBackground);
+            var expressionValues = new DictionaryContextData();
+            foreach (var pair in _ec.Object.ExpressionValues)
+            {
+                expressionValues[pair.Key] = pair.Value;
+            }
+            stepContext.Setup(x => x.ExpressionValues).Returns(expressionValues);
+            stepContext.Setup(x => x.ExpressionFunctions).Returns(new List<IFunctionInfo>());
+            stepContext.Setup(x => x.JobContext).Returns(_jobContext);
+            stepContext.Setup(x => x.Id).Returns(stepRecordId);
+            stepContext.Setup(x => x.ContextName).Returns(step.Object.Action.ContextName);
+            stepContext.Setup(x => x.CancellationToken).Returns(CancellationToken.None);
+            stepContext.Setup(x => x.Complete(It.IsAny<TaskResult?>(), It.IsAny<string>(), It.IsAny<string>()))
+                .Callback((TaskResult? r, string currentOperation, string resultCode) =>
+                {
+                    if (r != null)
+                    {
+                        stepContext.Object.Result = r;
+                    }
+                    _stepContext.SetOutcome("", stepContext.Object.ContextName, (stepContext.Object.Outcome ?? stepContext.Object.Result ?? TaskResult.Succeeded).ToActionResult());
+                    _stepContext.SetConclusion("", stepContext.Object.ContextName, (stepContext.Object.Result ?? TaskResult.Succeeded).ToActionResult());
+                });
+            stepContext.Setup(x => x.StepEnvironmentOverrides).Returns(new List<string>());
+            stepContext.Setup(x => x.ApplyContinueOnError(It.IsAny<TemplateToken>()));
+            stepContext.Setup(x => x.FlushDeferredOutputs()).Callback(() =>
+            {
+                if (stepContext.Object.DeferredOutputs != null)
+                {
+                    foreach (var kvp in stepContext.Object.DeferredOutputs)
+                    {
+                        _stepContext.SetOutput("", stepContext.Object.ContextName, kvp.Key, kvp.Value, out _);
+                    }
+                }
+            });
+
+            var trace = hc.GetTrace();
+            stepContext.Setup(x => x.Write(It.IsAny<string>(), It.IsAny<string>())).Callback((string tag, string message) => { trace.Info($"[{tag}]{message}"); });
+            stepContext.Object.Result = result;
+            step.Setup(x => x.ExecutionContext).Returns(stepContext.Object);
+            step.Setup(x => x.RunAsync()).Returns(Task.CompletedTask);
+
+            return step;
+        }
+
+        private JobExtensionRunner CreateWaitStep(TestHostContext hc, string[] stepIds, Dictionary<string, string> timelineVariables = null)
+        {
+            var waitData = new BackgroundStepControlFlowData
+            {
+                Type = Pipelines.BackgroundControlTypes.Wait,
+                StepIds = stepIds,
+            };
+            var bgCoordinator = hc.GetService<IBackgroundStepCoordinator>();
+            var waitRunner = new JobExtensionRunner(
+                runAsync: bgCoordinator.RunControlFlowAsync,
+                condition: "success()",
+                displayName: "Wait",
+                data: waitData);
+
+            var stepContext = new Mock<IExecutionContext>();
+            stepContext.SetupAllProperties();
+            stepContext.Setup(x => x.Global).Returns(() => _ec.Object.Global);
+            var waitExprValues = new DictionaryContextData();
+            foreach (var pair in _ec.Object.ExpressionValues) { waitExprValues[pair.Key] = pair.Value; }
+            stepContext.Setup(x => x.ExpressionValues).Returns(waitExprValues);
+            stepContext.Setup(x => x.ExpressionFunctions).Returns(new List<IFunctionInfo>());
+            stepContext.Setup(x => x.ContextName).Returns("__wait");
+            stepContext.Setup(x => x.JobContext).Returns(_jobContext);
+            stepContext.Setup(x => x.ScopeName).Returns((string)null);
+            stepContext.Setup(x => x.CancellationToken).Returns(CancellationToken.None);
+            stepContext.Setup(x => x.StepEnvironmentOverrides).Returns(new List<string>());
+            stepContext.Setup(x => x.Complete(It.IsAny<TaskResult?>(), It.IsAny<string>(), It.IsAny<string>()))
+                .Callback((TaskResult? r, string currentOperation, string resultCode) =>
+                {
+                    if (r != null) stepContext.Object.Result = r;
+                });
+            var trace = hc.GetTrace();
+            stepContext.Setup(x => x.Write(It.IsAny<string>(), It.IsAny<string>())).Callback((string tag, string message) => { trace.Info($"[{tag}]{message}"); });
+
+            waitRunner.ExecutionContext = stepContext.Object;
+            return waitRunner;
+        }
+
+        private JobExtensionRunner CreateWaitAllStep(TestHostContext hc, Dictionary<string, string> timelineVariables = null)
+        {
+            var waitAllData = new BackgroundStepControlFlowData
+            {
+                Type = Pipelines.BackgroundControlTypes.WaitAll,
+            };
+            var bgCoordinator2 = hc.GetService<IBackgroundStepCoordinator>();
+            var waitAllRunner = new JobExtensionRunner(
+                runAsync: bgCoordinator2.RunControlFlowAsync,
+                condition: "success()",
+                displayName: "Wait All",
+                data: waitAllData);
+
+            var stepContext = new Mock<IExecutionContext>();
+            stepContext.SetupAllProperties();
+            stepContext.Setup(x => x.Global).Returns(() => _ec.Object.Global);
+            var waitAllExprValues = new DictionaryContextData();
+            foreach (var pair in _ec.Object.ExpressionValues) { waitAllExprValues[pair.Key] = pair.Value; }
+            stepContext.Setup(x => x.ExpressionValues).Returns(waitAllExprValues);
+            stepContext.Setup(x => x.ExpressionFunctions).Returns(new List<IFunctionInfo>());
+            stepContext.Setup(x => x.ContextName).Returns("__wait-all");
+            stepContext.Setup(x => x.JobContext).Returns(_jobContext);
+            stepContext.Setup(x => x.ScopeName).Returns((string)null);
+            stepContext.Setup(x => x.CancellationToken).Returns(CancellationToken.None);
+            stepContext.Setup(x => x.StepEnvironmentOverrides).Returns(new List<string>());
+            stepContext.Setup(x => x.Complete(It.IsAny<TaskResult?>(), It.IsAny<string>(), It.IsAny<string>()))
+                .Callback((TaskResult? r, string currentOperation, string resultCode) =>
+                {
+                    if (r != null) stepContext.Object.Result = r;
+                });
+            var trace = hc.GetTrace();
+            stepContext.Setup(x => x.Write(It.IsAny<string>(), It.IsAny<string>())).Callback((string tag, string message) => { trace.Info($"[{tag}]{message}"); });
+
+            waitAllRunner.ExecutionContext = stepContext.Object;
+            return waitAllRunner;
+        }
+
+        private JobExtensionRunner CreateCancelStep(TestHostContext hc, string cancelStepId, Dictionary<string, string> timelineVariables = null)
+        {
+            var cancelData = new BackgroundStepControlFlowData
+            {
+                Type = Pipelines.BackgroundControlTypes.Cancel,
+                StepIds = new[] { cancelStepId },
+            };
+            var bgCoordinator3 = hc.GetService<IBackgroundStepCoordinator>();
+            var cancelRunner = new JobExtensionRunner(
+                runAsync: bgCoordinator3.RunControlFlowAsync,
+                condition: "success()",
+                displayName: "Cancel",
+                data: cancelData);
+
+            var stepContext = new Mock<IExecutionContext>();
+            stepContext.SetupAllProperties();
+            stepContext.Setup(x => x.Global).Returns(() => _ec.Object.Global);
+            var cancelExprValues = new DictionaryContextData();
+            foreach (var pair in _ec.Object.ExpressionValues) { cancelExprValues[pair.Key] = pair.Value; }
+            stepContext.Setup(x => x.ExpressionValues).Returns(cancelExprValues);
+            stepContext.Setup(x => x.ExpressionFunctions).Returns(new List<IFunctionInfo>());
+            stepContext.Setup(x => x.ContextName).Returns("__cancel");
+            stepContext.Setup(x => x.JobContext).Returns(_jobContext);
+            stepContext.Setup(x => x.ScopeName).Returns((string)null);
+            stepContext.Setup(x => x.CancellationToken).Returns(CancellationToken.None);
+            stepContext.Setup(x => x.StepEnvironmentOverrides).Returns(new List<string>());
+            stepContext.Setup(x => x.Complete(It.IsAny<TaskResult?>(), It.IsAny<string>(), It.IsAny<string>()))
+                .Callback((TaskResult? r, string currentOperation, string resultCode) =>
+                {
+                    if (r != null) stepContext.Object.Result = r;
+                });
+            var trace = hc.GetTrace();
+            stepContext.Setup(x => x.Write(It.IsAny<string>(), It.IsAny<string>())).Callback((string tag, string message) => { trace.Info($"[{tag}]{message}"); });
+
+            cancelRunner.ExecutionContext = stepContext.Object;
+            return cancelRunner;
+        }
+
+        #endregion
+    }
+}

src/Test/L0/Worker/DapDebuggerL0.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.IO;
 using System.Net;
 using System.Net.Sockets;
@@ -11,7 +11,9 @@ using Moq;
 using GitHub.Runner.Worker;
 using GitHub.Runner.Worker.Dap;
 using Newtonsoft.Json;
+using Newtonsoft.Json.Linq;
 using Xunit;
+using Pipelines = GitHub.DistributedTask.Pipelines;
 
 namespace GitHub.Runner.Common.Tests.Worker
 {
@@ -236,7 +238,7 @@ namespace GitHub.Runner.Common.Tests.Worker
             }
         }
 
-        private static Mock<IExecutionContext> CreateJobContextWithTunnel(CancellationToken cancellationToken, ushort port, string jobName = null)
+        private static Mock<IExecutionContext> CreateJobContextWithTunnel(CancellationToken cancellationToken, ushort port, string jobName = null, bool overrideWelcomeMessage = false, string welcomeMessage = null)
         {
             var tunnel = new GitHub.DistributedTask.Pipelines.DebuggerTunnelInfo
             {
@@ -245,7 +247,7 @@ namespace GitHub.Runner.Common.Tests.Worker
                 HostToken = "test-token",
                 Port = port
             };
-            var debuggerConfig = new DebuggerConfig(true, tunnel);
+            var debuggerConfig = new DebuggerConfig(true, tunnel, overrideWelcomeMessage, welcomeMessage);
             var jobContext = new Mock<IExecutionContext>();
             jobContext.Setup(x => x.CancellationToken).Returns(cancellationToken);
             jobContext.Setup(x => x.Global).Returns(new GlobalContext { Debugger = debuggerConfig });
@@ -255,6 +257,78 @@ namespace GitHub.Runner.Common.Tests.Worker
             return jobContext;
         }
 
+        private static Mock<IStep> CreateStep(string displayName, ActionRunStage? stage = null)
+        {
+            var step = new Mock<IStep>();
+            step.Setup(s => s.DisplayName).Returns(displayName);
+            if (stage.HasValue)
+            {
+                var executionContext = new Mock<IExecutionContext>();
+                executionContext.Setup(x => x.Stage).Returns(stage.Value);
+                step.Setup(s => s.ExecutionContext).Returns(executionContext.Object);
+            }
+            else
+            {
+                step.Setup(s => s.ExecutionContext).Returns((IExecutionContext)null);
+            }
+
+            return step;
+        }
+
+        private static Mock<IActionRunner> CreateActionRunner(string displayName, ActionRunStage stage, Pipelines.ActionStep action)
+        {
+            var executionContext = new Mock<IExecutionContext>();
+            executionContext.Setup(x => x.Stage).Returns(stage);
+
+            var runner = new Mock<IActionRunner>();
+            runner.Setup(s => s.DisplayName).Returns(displayName);
+            runner.Setup(s => s.ExecutionContext).Returns(executionContext.Object);
+            runner.Setup(s => s.Stage).Returns(stage);
+            runner.Setup(s => s.Action).Returns(action);
+            return runner;
+        }
+
+        private static Pipelines.ActionStep CreateRepositoryActionStep(string name)
+        {
+            return new Pipelines.ActionStep
+            {
+                Id = Guid.NewGuid(),
+                Name = name,
+                Reference = new Pipelines.RepositoryPathReference
+                {
+                    Name = name,
+                    Ref = "v1",
+                    RepositoryType = Pipelines.RepositoryTypes.GitHub
+                }
+            };
+        }
+
+        private static Definition CreateActionDefinitionWithPost()
+        {
+            return new Definition
+            {
+                Data = new ActionDefinitionData
+                {
+                    Execution = new NodeJSActionExecutionData
+                    {
+                        Script = "main.js",
+                        Post = "post.js"
+                    }
+                }
+            };
+        }
+
+        private static Request MakeRequest(string command, object arguments)
+        {
+            return new Request
+            {
+                Seq = 1,
+                Type = "request",
+                Command = command,
+                Arguments = JObject.FromObject(arguments)
+            };
+        }
+
         [Fact]
         [Trait("Level", "L0")]
         [Trait("Category", "Worker")]
@@ -718,6 +792,325 @@ namespace GitHub.Runner.Common.Tests.Worker
             }
         }
 
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task HandleSourceReturnsJobStepsSource()
+        {
+            using (var hc = CreateTestContext())
+            {
+                hc.SecretMasker.AddValue("secret-step");
+                var port = GetFreePort();
+                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
+                var jobContext = CreateJobContextWithTunnel(cts.Token, port);
+                await _debugger.StartAsync(jobContext.Object);
+
+                var waitTask = _debugger.WaitUntilReadyAsync();
+                using var client = await ConnectClientAsync(port);
+                var stream = client.GetStream();
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 1,
+                    Type = "request",
+                    Command = "configurationDone"
+                });
+                await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                await waitTask;
+
+                var pre = CreateStep("Pre cache", ActionRunStage.Pre);
+                var checkout = CreateStep("Checkout");
+                var secret = CreateStep("secret-step");
+                var post = CreateStep("Post cache", ActionRunStage.Post);
+                await _debugger.OnJobStepsInitializedAsync(
+                    new[] { pre.Object, checkout.Object, secret.Object },
+                    new[] { post.Object });
+
+                var response = _debugger.HandleSource(MakeRequest(
+                    "source",
+                    new SourceArguments { SourceReference = 1 }));
+
+                Assert.True(response.Success);
+                var body = Assert.IsType<SourceResponseBody>(response.Body);
+                Assert.Equal(
+                    "pre:\n  - step: \"Set up job\"\n  - step: \"Pre cache\"\n\nmain:\n  - step: \"Checkout\"\n  - step: \"***\"\n\npost:\n  - step: \"Post cache\"\n  - step: \"Complete job\"\n",
+                    body.Content);
+                Assert.Null(body.MimeType);
+
+                await _debugger.StopAsync();
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task StackTraceUsesJobStepsSourceLine()
+        {
+            using (CreateTestContext())
+            {
+                var port = GetFreePort();
+                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
+                var jobContext = CreateJobContextWithTunnel(cts.Token, port);
+                await _debugger.StartAsync(jobContext.Object);
+
+                var waitTask = _debugger.WaitUntilReadyAsync();
+                using var client = await ConnectClientAsync(port);
+                var stream = client.GetStream();
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 1,
+                    Type = "request",
+                    Command = "configurationDone"
+                });
+                await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                await waitTask;
+
+                var checkout = CreateStep("Checkout");
+                var build = CreateStep("Build");
+                await _debugger.OnJobStepsInitializedAsync(
+                    new[] { checkout.Object, build.Object },
+                    Array.Empty<IStep>());
+
+                var stepTask = _debugger.OnStepStartingAsync(build.Object);
+                var stoppedEvent = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"event\":\"stopped\"", stoppedEvent);
+
+                var bannerEvent = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"event\":\"output\"", bannerEvent);
+
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 2,
+                    Type = "request",
+                    Command = "stackTrace"
+                });
+
+                var stackTraceJson = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                var stackTrace = JObject.Parse(stackTraceJson);
+                var frame = stackTrace["body"]?["stackFrames"]?[0];
+
+                Assert.NotNull(frame);
+                Assert.Equal(6, frame["line"].Value<int>());
+                Assert.Equal(1, frame["source"]["sourceReference"].Value<int>());
+                Assert.Equal("execution.yml", frame["source"]["name"].Value<string>());
+
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 3,
+                    Type = "request",
+                    Command = "continue"
+                });
+                await stepTask;
+
+                await _debugger.StopAsync();
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task StackTraceOmitsSourceForUnmappedCurrentStep()
+        {
+            using (CreateTestContext())
+            {
+                var port = GetFreePort();
+                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
+                var jobContext = CreateJobContextWithTunnel(cts.Token, port);
+                await _debugger.StartAsync(jobContext.Object);
+
+                var waitTask = _debugger.WaitUntilReadyAsync();
+                using var client = await ConnectClientAsync(port);
+                var stream = client.GetStream();
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 1,
+                    Type = "request",
+                    Command = "configurationDone"
+                });
+                await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                await waitTask;
+
+                var checkout = CreateStep("Checkout");
+                var build = CreateStep("Build");
+                await _debugger.OnJobStepsInitializedAsync(
+                    new[] { checkout.Object },
+                    Array.Empty<IStep>());
+
+                var stepTask = _debugger.OnStepStartingAsync(build.Object);
+                await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 2,
+                    Type = "request",
+                    Command = "stackTrace"
+                });
+
+                var stackTraceJson = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                var stackTrace = JObject.Parse(stackTraceJson);
+                var frame = stackTrace["body"]?["stackFrames"]?[0];
+
+                Assert.NotNull(frame);
+                Assert.Equal(0, frame["line"].Value<int>());
+                Assert.Null(frame["source"]);
+
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 3,
+                    Type = "request",
+                    Command = "continue"
+                });
+                await stepTask;
+
+                await _debugger.StopAsync();
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task PredictedPostStepIsServedAtInitializationAndClaimedAtRegistration()
+        {
+            using (var hc = CreateTestContext())
+            {
+                var action = CreateRepositoryActionStep("actions/cache");
+                var actionManager = new Mock<IActionManager>();
+                actionManager
+                    .Setup(x => x.LoadAction(It.IsAny<IExecutionContext>(), action))
+                    .Returns(CreateActionDefinitionWithPost());
+                hc.SetSingleton<IActionManager>(actionManager.Object);
+
+                var port = GetFreePort();
+                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
+                var jobContext = CreateJobContextWithTunnel(cts.Token, port);
+                await _debugger.StartAsync(jobContext.Object);
+
+                var waitTask = _debugger.WaitUntilReadyAsync();
+                using var client = await ConnectClientAsync(port);
+                var stream = client.GetStream();
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 1,
+                    Type = "request",
+                    Command = "configurationDone"
+                });
+                await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                await waitTask;
+
+                var checkout = CreateActionRunner("Checkout", ActionRunStage.Main, action);
+                await _debugger.OnJobStepsInitializedAsync(
+                    new[] { checkout.Object },
+                    Array.Empty<IStep>());
+
+                var sourceResponse = _debugger.HandleSource(MakeRequest(
+                    "source",
+                    new SourceArguments { SourceReference = 1 }));
+                var sourceBody = Assert.IsType<SourceResponseBody>(sourceResponse.Body);
+                Assert.Equal(
+                    "pre:\n  - step: \"Set up job\"\n\nmain:\n  - step: \"Checkout\"\n\npost:\n  - step: \"Post Checkout\"\n  - step: \"Complete job\"\n",
+                    sourceBody.Content);
+
+                var post = CreateActionRunner("Post Checkout", ActionRunStage.Post, action);
+                _debugger.OnPostStepRegistered(post.Object);
+
+                var stepTask = _debugger.OnStepStartingAsync(post.Object);
+                var stoppedEvent = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"event\":\"stopped\"", stoppedEvent);
+
+                var bannerEvent = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"event\":\"output\"", bannerEvent);
+
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 2,
+                    Type = "request",
+                    Command = "stackTrace"
+                });
+
+                var stackTraceJson = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                var stackTrace = JObject.Parse(stackTraceJson);
+                var frame = stackTrace["body"]?["stackFrames"]?[0];
+
+                Assert.NotNull(frame);
+                Assert.Equal(8, frame["line"].Value<int>());
+                Assert.Equal(1, frame["source"]["sourceReference"].Value<int>());
+
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 3,
+                    Type = "request",
+                    Command = "continue"
+                });
+                await stepTask;
+
+                await _debugger.StopAsync();
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task StackTraceSanitizesSyntheticSourcePath()
+        {
+            using (CreateTestContext())
+            {
+                var port = GetFreePort();
+                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
+                var jobContext = CreateJobContextWithTunnel(cts.Token, port, jobName: "my/job\\name");
+                await _debugger.StartAsync(jobContext.Object);
+
+                var waitTask = _debugger.WaitUntilReadyAsync();
+                using var client = await ConnectClientAsync(port);
+                var stream = client.GetStream();
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 1,
+                    Type = "request",
+                    Command = "configurationDone"
+                });
+                await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                await waitTask;
+
+                var checkout = CreateStep("Checkout");
+                await _debugger.OnJobStepsInitializedAsync(
+                    new[] { checkout.Object },
+                    Array.Empty<IStep>());
+
+                var stepTask = _debugger.OnStepStartingAsync(checkout.Object);
+                await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 2,
+                    Type = "request",
+                    Command = "stackTrace"
+                });
+
+                var stackTraceJson = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                var stackTrace = JObject.Parse(stackTraceJson);
+                var frame = stackTrace["body"]?["stackFrames"]?[0];
+
+                Assert.NotNull(frame);
+                Assert.Equal("my_job_name/execution.yml", frame["source"]["path"].Value<string>());
+
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 3,
+                    Type = "request",
+                    Command = "continue"
+                });
+                await stepTask;
+
+                await _debugger.StopAsync();
+            }
+        }
+
         [Fact]
         [Trait("Level", "L0")]
         [Trait("Category", "Worker")]
@@ -742,8 +1135,15 @@ namespace GitHub.Runner.Common.Tests.Worker
 
                 // Read the configurationDone response
                 await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                // Read the welcome message output event
+                await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
                 await waitTask;
 
+                var checkout = CreateStep("Checkout");
+                await _debugger.OnJobStepsInitializedAsync(
+                    new[] { checkout.Object },
+                    Array.Empty<IStep>());
+
                 // Complete the job — OnJobCompletedAsync pauses when stepping,
                 // so run it in the background and send continue to unblock.
                 var completedTask = _debugger.OnJobCompletedAsync();
@@ -752,11 +1152,27 @@ namespace GitHub.Runner.Common.Tests.Worker
                 var stoppedMsg = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
                 Assert.Contains("\"event\":\"stopped\"", stoppedMsg);
 
-                // Send continue to unblock the pause
                 await SendRequestAsync(stream, new Request
                 {
                     Seq = 2,
                     Type = "request",
+                    Command = "stackTrace"
+                });
+
+                var stackTraceJson = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                var stackTrace = JObject.Parse(stackTraceJson);
+                var frame = stackTrace["body"]?["stackFrames"]?[0];
+
+                Assert.NotNull(frame);
+                Assert.Equal("Complete job [completed]", frame["name"].Value<string>());
+                Assert.Equal(8, frame["line"].Value<int>());
+                Assert.Equal(1, frame["source"]["sourceReference"].Value<int>());
+
+                // Send continue to unblock the pause
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 3,
+                    Type = "request",
                     Command = "continue"
                 });
 
@@ -775,6 +1191,68 @@ namespace GitHub.Runner.Common.Tests.Worker
             }
         }
 
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task OnJobCompletedUsesSyntheticCompleteJobLineWhenPostStepSharesName()
+        {
+            using (CreateTestContext())
+            {
+                var port = GetFreePort();
+                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
+                var jobContext = CreateJobContextWithTunnel(cts.Token, port);
+                await _debugger.StartAsync(jobContext.Object);
+
+                var waitTask = _debugger.WaitUntilReadyAsync();
+                using var client = await ConnectClientAsync(port);
+                var stream = client.GetStream();
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 1,
+                    Type = "request",
+                    Command = "configurationDone"
+                });
+
+                await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                await waitTask;
+
+                var checkout = CreateStep("Checkout");
+                var realPost = CreateStep("Complete job", ActionRunStage.Post);
+                await _debugger.OnJobStepsInitializedAsync(
+                    new[] { checkout.Object },
+                    new[] { realPost.Object });
+
+                var completedTask = _debugger.OnJobCompletedAsync();
+
+                await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 2,
+                    Type = "request",
+                    Command = "stackTrace"
+                });
+
+                var stackTraceJson = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                var stackTrace = JObject.Parse(stackTraceJson);
+                var frame = stackTrace["body"]?["stackFrames"]?[0];
+
+                Assert.NotNull(frame);
+                Assert.Equal("Complete job [completed]", frame["name"].Value<string>());
+                Assert.Equal(9, frame["line"].Value<int>());
+
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 3,
+                    Type = "request",
+                    Command = "continue"
+                });
+
+                await completedTask;
+            }
+        }
+
         [Fact]
         [Trait("Level", "L0")]
         [Trait("Category", "Worker")]
@@ -849,6 +1327,8 @@ namespace GitHub.Runner.Common.Tests.Worker
                     Command = "configurationDone"
                 });
 
+                await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                // Read the welcome message output event
                 await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
                 await waitTask;
 
@@ -867,5 +1347,224 @@ namespace GitHub.Runner.Common.Tests.Worker
                 Assert.Equal(completedTask, finished);
             }
         }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task WelcomeMessageSendsDefaultHelpWhenOverrideDisabled()
+        {
+            using (CreateTestContext())
+            {
+                var port = GetFreePort();
+                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
+                var jobContext = CreateJobContextWithTunnel(cts.Token, port);
+                await _debugger.StartAsync(jobContext.Object);
+
+                using var client = await ConnectClientAsync(port);
+                var stream = client.GetStream();
+
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 1,
+                    Type = "request",
+                    Command = "configurationDone"
+                });
+
+                // First message: configurationDone response
+                var configDoneResponse = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"command\":\"configurationDone\"", configDoneResponse);
+
+                // Second message: welcome output event with default help text
+                var welcomeMsg = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"event\":\"output\"", welcomeMsg);
+                Assert.Contains("\"category\":\"console\"", welcomeMsg);
+                Assert.Contains("Actions Debug Console", welcomeMsg);
+                Assert.Contains("help", welcomeMsg);
+
+                await _debugger.StopAsync();
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task WelcomeMessageShowsCustomMessageWhenOverrideEnabled()
+        {
+            using (CreateTestContext())
+            {
+                var port = GetFreePort();
+                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
+                var jobContext = CreateJobContextWithTunnel(cts.Token, port,
+                    overrideWelcomeMessage: true,
+                    welcomeMessage: "Welcome to debugging!");
+                await _debugger.StartAsync(jobContext.Object);
+
+                using var client = await ConnectClientAsync(port);
+                var stream = client.GetStream();
+
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 1,
+                    Type = "request",
+                    Command = "configurationDone"
+                });
+
+                // First: configurationDone response
+                var configDoneResponse = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"command\":\"configurationDone\"", configDoneResponse);
+
+                // Second: custom welcome message
+                var welcomeMsg = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"event\":\"output\"", welcomeMsg);
+                Assert.Contains("Welcome to debugging!", welcomeMsg);
+
+                await _debugger.StopAsync();
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task WelcomeMessageSuppressedWhenOverrideEnabledWithEmptyMessage()
+        {
+            using (CreateTestContext())
+            {
+                var port = GetFreePort();
+                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
+                var jobContext = CreateJobContextWithTunnel(cts.Token, port,
+                    overrideWelcomeMessage: true,
+                    welcomeMessage: "");
+                await _debugger.StartAsync(jobContext.Object);
+
+                using var client = await ConnectClientAsync(port);
+                var stream = client.GetStream();
+
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 1,
+                    Type = "request",
+                    Command = "configurationDone"
+                });
+
+                // Read configurationDone response
+                var configDoneResponse = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"command\":\"configurationDone\"", configDoneResponse);
+
+                // Send threads request — if welcome message was suppressed, this
+                // should be the next response (no output event in between)
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 2,
+                    Type = "request",
+                    Command = "threads"
+                });
+
+                var threadsResponse = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"command\":\"threads\"", threadsResponse);
+
+                await _debugger.StopAsync();
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task WelcomeMessageSuppressedWhenOverrideEnabledWithNullMessage()
+        {
+            using (CreateTestContext())
+            {
+                var port = GetFreePort();
+                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
+                var jobContext = CreateJobContextWithTunnel(cts.Token, port,
+                    overrideWelcomeMessage: true,
+                    welcomeMessage: null);
+                await _debugger.StartAsync(jobContext.Object);
+
+                using var client = await ConnectClientAsync(port);
+                var stream = client.GetStream();
+
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 1,
+                    Type = "request",
+                    Command = "configurationDone"
+                });
+
+                // Read configurationDone response
+                var configDoneResponse = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"command\":\"configurationDone\"", configDoneResponse);
+
+                // Send threads request — if welcome message was suppressed, this
+                // should be the next response (no output event in between)
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 2,
+                    Type = "request",
+                    Command = "threads"
+                });
+
+                var threadsResponse = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"command\":\"threads\"", threadsResponse);
+
+                await _debugger.StopAsync();
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task WelcomeMessageSentOnlyOnce()
+        {
+            using (CreateTestContext())
+            {
+                var port = GetFreePort();
+                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
+                var jobContext = CreateJobContextWithTunnel(cts.Token, port);
+                await _debugger.StartAsync(jobContext.Object);
+
+                using var client = await ConnectClientAsync(port);
+                var stream = client.GetStream();
+
+                // First configurationDone
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 1,
+                    Type = "request",
+                    Command = "configurationDone"
+                });
+
+                var configDoneResponse = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"command\":\"configurationDone\"", configDoneResponse);
+
+                // Welcome message should appear
+                var welcomeMsg = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"event\":\"output\"", welcomeMsg);
+                Assert.Contains("Actions Debug Console", welcomeMsg);
+
+                // Second configurationDone — should NOT produce another welcome message
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 2,
+                    Type = "request",
+                    Command = "configurationDone"
+                });
+
+                var secondResponse = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"command\":\"configurationDone\"", secondResponse);
+
+                // Next message should be threads response, not another welcome output
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 3,
+                    Type = "request",
+                    Command = "threads"
+                });
+
+                var threadsResponse = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"command\":\"threads\"", threadsResponse);
+
+                await _debugger.StopAsync();
+            }
+        }
     }
 }

src/Test/L0/Worker/DapMessagesL0.cs

@@ -1,4 +1,4 @@
-using System.Collections.Generic;
+using System.Collections.Generic;
 using Newtonsoft.Json;
 using Newtonsoft.Json.Linq;
 using Xunit;
@@ -171,6 +171,36 @@ namespace GitHub.Runner.Common.Tests.Worker
             Assert.Equal("normal", deserialized.PresentationHint);
         }
 
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void SourceRequestAndResponseSerialization()
+        {
+            var args = new SourceArguments
+            {
+                Source = new Source
+                {
+                    SourceReference = 1
+                }
+            };
+
+            var argsJson = JsonConvert.SerializeObject(args);
+            var deserializedArgs = JsonConvert.DeserializeObject<SourceArguments>(argsJson);
+
+            Assert.Equal(1, deserializedArgs.Source.SourceReference);
+
+            var body = new SourceResponseBody
+            {
+                Content = "pre:\n  - step: \"Setup job\"\n\nmain:\n  - step: \"Checkout\"\n\npost:\n  - step: \"Complete job\"\n"
+            };
+
+            var bodyJson = JsonConvert.SerializeObject(body);
+            var deserializedBody = JsonConvert.DeserializeObject<SourceResponseBody>(bodyJson);
+
+            Assert.Equal("pre:\n  - step: \"Setup job\"\n\nmain:\n  - step: \"Checkout\"\n\npost:\n  - step: \"Complete job\"\n", deserializedBody.Content);
+            Assert.Null(deserializedBody.MimeType);
+        }
+
         [Fact]
         [Trait("Level", "L0")]
         [Trait("Category", "Worker")]

src/Test/L0/Worker/DapReplExecutorL0.cs

@@ -5,9 +5,12 @@ using System.Threading;
 using System.Threading.Tasks;
 using GitHub.DistributedTask.Expressions2;
 using GitHub.DistributedTask.Pipelines.ContextData;
+using GitHub.DistributedTask.WebApi;
 using GitHub.Runner.Common.Tests;
 using GitHub.Runner.Worker;
+using GitHub.Runner.Worker.Container;
 using GitHub.Runner.Worker.Dap;
+using GitHub.Runner.Worker.Handlers;
 using Moq;
 using Xunit;
 
@@ -40,7 +43,8 @@ namespace GitHub.Runner.Common.Tests.Worker
 
         private Mock<IExecutionContext> CreateMockContext(
             DictionaryContextData exprValues = null,
-            IDictionary<string, IDictionary<string, string>> jobDefaults = null)
+            IDictionary<string, IDictionary<string, string>> jobDefaults = null,
+            ContainerInfo container = null)
         {
             var mock = new Mock<IExecutionContext>();
             mock.Setup(x => x.ExpressionValues).Returns(exprValues ?? new DictionaryContextData());
@@ -51,6 +55,7 @@ namespace GitHub.Runner.Common.Tests.Worker
                 PrependPath = new List<string>(),
                 JobDefaults = jobDefaults
                     ?? new Dictionary<string, IDictionary<string, string>>(StringComparer.OrdinalIgnoreCase),
+                Container = container,
             };
             mock.Setup(x => x.Global).Returns(global);
 
@@ -65,7 +70,7 @@ namespace GitHub.Runner.Common.Tests.Worker
             using (CreateTestContext())
             {
                 var command = new RunCommand { Script = "echo hello" };
-                var result = await _executor.ExecuteRunCommandAsync(command, null, CancellationToken.None);
+                var result = await _executor.ExecuteRunCommandAsync(command, null, false, CancellationToken.None);
 
                 Assert.Equal("error", result.Type);
                 Assert.Contains("No execution context available", result.Result);
@@ -233,5 +238,101 @@ namespace GitHub.Runner.Common.Tests.Worker
                 Assert.False(result.ContainsKey("BAZ"));
             }
         }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void CreateStepHost_NoContainer_ReturnsDefaultStepHost()
+        {
+            using (var hc = CreateTestContext())
+            {
+                hc.EnqueueInstance<IDefaultStepHost>(new DefaultStepHost());
+                var context = CreateMockContext();
+                var result = _executor.CreateStepHost(context.Object, isActionStep: true);
+
+                Assert.IsType<DefaultStepHost>(result);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void CreateStepHost_WithContainer_ActionStep_ReturnsContainerStepHost()
+        {
+            using (var hc = CreateTestContext())
+            {
+                hc.EnqueueInstance<IContainerStepHost>(new ContainerStepHost());
+                var container = new ContainerInfo { ContainerId = "abc123" };
+                var context = CreateMockContext(container: container);
+                var result = _executor.CreateStepHost(context.Object, isActionStep: true);
+
+                Assert.IsType<ContainerStepHost>(result);
+                var containerHost = (ContainerStepHost)result;
+                Assert.Same(container, containerHost.Container);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void CreateStepHost_WithContainer_InfrastructureStep_ReturnsDefaultStepHost()
+        {
+            using (var hc = CreateTestContext())
+            {
+                hc.EnqueueInstance<IDefaultStepHost>(new DefaultStepHost());
+                var container = new ContainerInfo { ContainerId = "abc123" };
+                var context = CreateMockContext(container: container);
+                var result = _executor.CreateStepHost(context.Object, isActionStep: false);
+
+                Assert.IsType<DefaultStepHost>(result);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void CreateStepHost_ContainerWithoutId_NoHooks_ReturnsDefaultStepHost()
+        {
+            using (var hc = CreateTestContext())
+            {
+                hc.EnqueueInstance<IDefaultStepHost>(new DefaultStepHost());
+                // Container exists but hasn't been started yet (no ContainerId)
+                var container = new ContainerInfo();
+                var context = CreateMockContext(container: container);
+                var result = _executor.CreateStepHost(context.Object, isActionStep: true);
+
+                Assert.IsType<DefaultStepHost>(result);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void CreateStepHost_ContainerWithoutId_HooksEnabled_ReturnsContainerStepHost()
+        {
+            using (var hc = CreateTestContext())
+            {
+                hc.EnqueueInstance<IContainerStepHost>(new ContainerStepHost());
+                // Container hooks need both the feature flag and the env var
+                Environment.SetEnvironmentVariable("ACTIONS_RUNNER_CONTAINER_HOOKS", "/some/hook/path");
+                try
+                {
+                    var container = new ContainerInfo();
+                    var context = CreateMockContext(container: container);
+                    context.Object.Global.Variables = new Variables(
+                        hc,
+                        new Dictionary<string, VariableValue>
+                        {
+                            { Constants.Runner.Features.AllowRunnerContainerHooks, new VariableValue("true") }
+                        });
+                    var result = _executor.CreateStepHost(context.Object, isActionStep: true);
+                    Assert.IsAssignableFrom<IContainerStepHost>(result);
+                }
+                finally
+                {
+                    Environment.SetEnvironmentVariable("ACTIONS_RUNNER_CONTAINER_HOOKS", null);
+                }
+            }
+        }
     }
 }

src/Test/L0/Worker/ExecutionContextL0.cs

@@ -361,6 +361,119 @@ namespace GitHub.Runner.Common.Tests.Worker
             }
         }
 
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void RegisterPostJobStep_JobExtensionRunner_DefaultsRunnerTelemetry()
+        {
+            using (TestHostContext hc = CreateTestContext())
+            {
+                // Arrange: Create a job request message.
+                TaskOrchestrationPlanReference plan = new();
+                TimelineReference timeline = new();
+                Guid jobId = Guid.NewGuid();
+                string jobName = "some job name";
+                var jobRequest = new Pipelines.AgentJobRequestMessage(plan, timeline, jobId, jobName, jobName, null, null, null, new Dictionary<string, VariableValue>(), new List<MaskHint>(), new Pipelines.JobResources(), new Pipelines.ContextData.DictionaryContextData(), new Pipelines.WorkspaceOptions(), new List<Pipelines.ActionStep>(), null, null, null, null, null);
+                jobRequest.Resources.Repositories.Add(new Pipelines.RepositoryResource()
+                {
+                    Alias = Pipelines.PipelineConstants.SelfAlias,
+                    Id = "github",
+                    Version = "sha1"
+                });
+                jobRequest.ContextData["github"] = new Pipelines.ContextData.DictionaryContextData();
+
+                var pagingLogger1 = new Mock<IPagingLogger>();
+                var pagingLogger2 = new Mock<IPagingLogger>();
+                var jobServerQueue = new Mock<IJobServerQueue>();
+                jobServerQueue.Setup(x => x.QueueTimelineRecordUpdate(It.IsAny<Guid>(), It.IsAny<TimelineRecord>()));
+
+                hc.EnqueueInstance(pagingLogger1.Object);
+                hc.EnqueueInstance(pagingLogger2.Object);
+                hc.SetSingleton(jobServerQueue.Object);
+
+                var jobContext = new Runner.Worker.ExecutionContext();
+                jobContext.Initialize(hc);
+
+                // Act.
+                jobContext.InitializeJob(jobRequest, CancellationToken.None);
+
+                var extensionStep = new JobExtensionRunner(
+                    runAsync: (_, _) => System.Threading.Tasks.Task.CompletedTask,
+                    condition: "always()",
+                    displayName: "Create Custom Image",
+                    data: null);
+
+                jobContext.RegisterPostJobStep(extensionStep);
+
+                // Assert: telemetry defaults are populated for non-action post-job steps.
+                Assert.NotNull(extensionStep.ExecutionContext);
+                Assert.Equal("runner", extensionStep.ExecutionContext.StepTelemetry.Type);
+                Assert.Equal("create_custom_image", extensionStep.ExecutionContext.StepTelemetry.Action);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void RegisterPostJobStep_ActionRunner_DoesNotOverrideTelemetry()
+        {
+            using (TestHostContext hc = CreateTestContext())
+            {
+                // Arrange: Create a job request message.
+                TaskOrchestrationPlanReference plan = new();
+                TimelineReference timeline = new();
+                Guid jobId = Guid.NewGuid();
+                string jobName = "some job name";
+                var jobRequest = new Pipelines.AgentJobRequestMessage(plan, timeline, jobId, jobName, jobName, null, null, null, new Dictionary<string, VariableValue>(), new List<MaskHint>(), new Pipelines.JobResources(), new Pipelines.ContextData.DictionaryContextData(), new Pipelines.WorkspaceOptions(), new List<Pipelines.ActionStep>(), null, null, null, null, null);
+                jobRequest.Resources.Repositories.Add(new Pipelines.RepositoryResource()
+                {
+                    Alias = Pipelines.PipelineConstants.SelfAlias,
+                    Id = "github",
+                    Version = "sha1"
+                });
+                jobRequest.ContextData["github"] = new Pipelines.ContextData.DictionaryContextData();
+
+                var pagingLogger1 = new Mock<IPagingLogger>();
+                var pagingLogger2 = new Mock<IPagingLogger>();
+                var pagingLogger3 = new Mock<IPagingLogger>();
+                var pagingLogger4 = new Mock<IPagingLogger>();
+                var jobServerQueue = new Mock<IJobServerQueue>();
+                jobServerQueue.Setup(x => x.QueueTimelineRecordUpdate(It.IsAny<Guid>(), It.IsAny<TimelineRecord>()));
+
+                var actionRunner = new ActionRunner();
+                actionRunner.Initialize(hc);
+
+                hc.EnqueueInstance(pagingLogger1.Object);
+                hc.EnqueueInstance(pagingLogger2.Object);
+                hc.EnqueueInstance(pagingLogger3.Object);
+                hc.EnqueueInstance(pagingLogger4.Object);
+                hc.EnqueueInstance(actionRunner as IActionRunner);
+                hc.SetSingleton(jobServerQueue.Object);
+
+                var jobContext = new Runner.Worker.ExecutionContext();
+                jobContext.Initialize(hc);
+
+                // Act.
+                jobContext.InitializeJob(jobRequest, CancellationToken.None);
+
+                var action = jobContext.CreateChild(Guid.NewGuid(), "action", "action", null, null, 0);
+
+                var postRunner = hc.CreateService<IActionRunner>();
+                postRunner.Action = new Pipelines.ActionStep() { Id = Guid.NewGuid(), Name = "post", DisplayName = "Post Action", Reference = new Pipelines.RepositoryPathReference() { Name = "actions/action" } };
+                postRunner.Stage = ActionRunStage.Post;
+                postRunner.Condition = "always()";
+                postRunner.DisplayName = "Post Action";
+
+                action.RegisterPostJobStep(postRunner);
+
+                // Assert: action post-step telemetry is left for the handler to fill in,
+                // so RegisterPostJobStep should NOT pre-populate runner-owned defaults.
+                Assert.NotNull(postRunner.ExecutionContext);
+                Assert.Null(postRunner.ExecutionContext.StepTelemetry.Type);
+                Assert.Null(postRunner.ExecutionContext.StepTelemetry.Action);
+            }
+        }
+
         [Fact]
         [Trait("Level", "L0")]
         [Trait("Category", "Worker")]

src/Test/L0/Worker/JobExecutionViewL0.cs

@@ -1,8 +1,5 @@
 using System;
-using System.Collections.Concurrent;
-using System.Collections.Generic;
-using System.Linq;
-using System.Threading.Tasks;
+using GitHub.DistributedTask.Pipelines;
 using GitHub.Runner.Worker;
 using GitHub.Runner.Worker.Dap;
 using Moq;
@@ -12,431 +9,122 @@ namespace GitHub.Runner.Common.Tests.Worker
 {
     public sealed class JobExecutionViewL0
     {
-        private static JobExecutionViewEntry MainEntry(string name)
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void RendersPreMainAndPostSections()
         {
-            return new JobExecutionViewEntry(JobExecutionPhase.Main, name, run: name);
-        }
+            var pre = CreateStep("Pre cache", ActionRunStage.Pre);
+            var checkout = CreateStep("Checkout");
+            var post = CreateStep("Post cache", ActionRunStage.Post);
 
-        private static IStep NewStep(string displayName = "step")
-        {
-            var mock = new Mock<IStep>();
-            mock.Setup(s => s.DisplayName).Returns(displayName);
-            return mock.Object;
+            var view = new JobExecutionView(
+                "job",
+                new[] { pre.Object, checkout.Object },
+                new[] { post.Object });
+
+            Assert.Equal(
+                "pre:\n  - step: \"Set up job\"\n  - step: \"Pre cache\"\n\nmain:\n  - step: \"Checkout\"\n\npost:\n  - step: \"Post cache\"\n  - step: \"Complete job\"\n", 
+                view.Content);
+            Assert.Equal(3, view.TryGetLineForStep(pre.Object));
+            Assert.Equal(6, view.TryGetLineForStep(checkout.Object));
+            Assert.Equal(9, view.TryGetLineForStep(post.Object));
+            Assert.Equal(10, view.CompleteJobLine);
         }
 
         [Fact]
         [Trait("Level", "L0")]
         [Trait("Category", "Worker")]
-        public void Constructor_RendersEmptyView()
+        public void ClaimsPredictedPostStepWithoutChangingLine()
         {
-            var view = new JobExecutionView("my-job");
+            var action = CreateRepositoryActionStep("actions/cache");
+            var checkout = CreateActionRunner("Checkout", ActionRunStage.Main, action);
+            var predicted = new JobExecutionView.PredictedPostStep(
+                "Post Checkout",
+                MatchKeyFor(action.Id));
 
-            Assert.Equal(0, view.EntryCount);
-            Assert.Contains("# Job: my-job", view.Yaml);
-            Assert.Contains("- step: Setup job", view.Yaml);
-            Assert.Contains("- step: Complete job", view.Yaml);
+            var view = new JobExecutionView(
+                "job",
+                new[] { checkout.Object },
+                Array.Empty<IStep>(),
+                new[] { predicted });
 
-            // Only the two synthetic boundaries appear.
-            int stepCount = view.Yaml.Split("- step: ").Length - 1;
-            Assert.Equal(2, stepCount);
-        }
+            var post = CreateActionRunner("Post Checkout", ActionRunStage.Post, action);
+            var line = view.TryClaimPredictedStep(MatchKeyFor(action.Id), post.Object);
 
-        [Theory]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        [InlineData(null)]
-        [InlineData("")]
-        [InlineData("   ")]
-        public void Constructor_ThrowsOnInvalidJobId(string jobId)
-        {
-            Assert.Throws<ArgumentException>(() => new JobExecutionView(jobId));
+            Assert.Equal(8, line);
+            Assert.Equal(8, view.TryGetLineForStep(post.Object));
+            Assert.Equal(
+                "pre:\n  - step: \"Set up job\"\n\nmain:\n  - step: \"Checkout\"\n\npost:\n  - step: \"Post Checkout\"\n  - step: \"Complete job\"\n", 
+                view.Content);
         }
 
         [Fact]
         [Trait("Level", "L0")]
         [Trait("Category", "Worker")]
-        public void Append_IncrementsEntryCount()
+        public void UsesSyntheticCompleteJobLineWhenPostStepSharesName()
         {
-            var view = new JobExecutionView("j");
+            var checkout = CreateStep("Checkout");
+            var realPost = CreateStep("Complete job", ActionRunStage.Post);
 
-            int line0 = view.Append(MainEntry("a"));
-            int line1 = view.Append(MainEntry("b"));
-            int line2 = view.Append(MainEntry("c"));
+            var view = new JobExecutionView(
+                "job",
+                new[] { checkout.Object },
+                new[] { realPost.Object });
 
-            Assert.Equal(3, view.EntryCount);
-            Assert.True(line0 < line1);
-            Assert.True(line1 < line2);
-            Assert.Equal(line0, view.GetLine(0));
-            Assert.Equal(line1, view.GetLine(1));
-            Assert.Equal(line2, view.GetLine(2));
+            Assert.Equal(8, view.TryGetLineForStep(realPost.Object));
+            Assert.Equal(9, view.CompleteJobLine);
         }
 
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Append_PreservesPriorEntryLines()
+        private static Mock<IStep> CreateStep(string displayName, ActionRunStage? stage = null)
         {
-            var view = new JobExecutionView("j");
-
-            int l0 = view.Append(MainEntry("a"));
-            int l1 = view.Append(MainEntry("b"));
-            int l2 = view.Append(MainEntry("c"));
-
-            view.Append(MainEntry("d"));
-            Assert.Equal(l0, view.GetLine(0));
-            Assert.Equal(l1, view.GetLine(1));
-            Assert.Equal(l2, view.GetLine(2));
-
-            view.Append(MainEntry("e"));
-            Assert.Equal(l0, view.GetLine(0));
-            Assert.Equal(l1, view.GetLine(1));
-            Assert.Equal(l2, view.GetLine(2));
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Append_RegistersStepIdentity()
-        {
-            var view = new JobExecutionView("j");
-            var step = NewStep();
-
-            int line = view.Append(MainEntry("a"), step);
-
-            Assert.Equal(line, view.GetLine(0));
-            Assert.Equal(line, view.TryGetLineForStep(step));
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Append_NullStepIdentity_StillAppends()
-        {
-            var view = new JobExecutionView("j");
-
-            view.Append(MainEntry("a"), stepIdentity: null);
-
-            Assert.Equal(1, view.EntryCount);
-            Assert.Null(view.TryGetLineForStep(null));
-            Assert.Contains("- step: a", view.Yaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Append_DuplicateStepIdentity_Throws()
-        {
-            var view = new JobExecutionView("j");
-            var step = NewStep();
-
-            view.Append(MainEntry("a"), step);
-            Assert.Throws<InvalidOperationException>(() => view.Append(MainEntry("b"), step));
-
-            // State preserved: only the first entry is present.
-            Assert.Equal(1, view.EntryCount);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Append_NullEntry_Throws()
-        {
-            var view = new JobExecutionView("j");
-            Assert.Throws<ArgumentNullException>(() => view.Append(null));
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void AppendRange_AppendsAllAndRendersOnce()
-        {
-            var view = new JobExecutionView("j");
-            var steps = Enumerable.Range(0, 5).Select(i => NewStep("s" + i)).ToList();
-            var items = steps
-                .Select((s, i) => (entry: MainEntry("e" + i), stepIdentity: s))
-                .ToList();
-
-            view.AppendRange(items);
-
-            Assert.Equal(5, view.EntryCount);
-            for (int i = 0; i < 5; i++)
+            var step = new Mock<IStep>();
+            step.Setup(s => s.DisplayName).Returns(displayName);
+            if (stage.HasValue)
             {
-                int line = view.GetLine(i);
-                Assert.Equal(line, view.TryGetLineForStep(steps[i]));
+                var executionContext = new Mock<IExecutionContext>();
+                executionContext.Setup(x => x.Stage).Returns(stage.Value);
+                step.Setup(s => s.ExecutionContext).Returns(executionContext.Object);
             }
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void AppendRange_RejectsDuplicateInInput()
-        {
-            var view = new JobExecutionView("j");
-            var dup = NewStep();
-            var items = new List<(JobExecutionViewEntry, IStep)>
+            else
             {
-                (MainEntry("a"), dup),
-                (MainEntry("b"), dup),
-            };
-
-            Assert.Throws<InvalidOperationException>(() => view.AppendRange(items));
-            Assert.Equal(0, view.EntryCount);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void AppendRange_RejectsOverlapWithExisting()
-        {
-            var view = new JobExecutionView("j");
-            var step = NewStep();
-            view.Append(MainEntry("a"), step);
-
-            var items = new List<(JobExecutionViewEntry, IStep)>
-            {
-                (MainEntry("b"), step),
-            };
-
-            Assert.Throws<InvalidOperationException>(() => view.AppendRange(items));
-            Assert.Equal(1, view.EntryCount);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void AppendRange_NullItems_Throws()
-        {
-            var view = new JobExecutionView("j");
-            Assert.Throws<ArgumentNullException>(() => view.AppendRange(null));
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void TryGetLineForStep_NullStep_ReturnsNull()
-        {
-            var view = new JobExecutionView("j");
-            Assert.Null(view.TryGetLineForStep(null));
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void TryGetLineForStep_UnknownStep_ReturnsNull()
-        {
-            var view = new JobExecutionView("j");
-            var step = NewStep();
-            Assert.Null(view.TryGetLineForStep(step));
-        }
-
-        [Theory]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        [InlineData(-1)]
-        [InlineData(2)]
-        public void GetLine_OutOfRange_Throws(int index)
-        {
-            var view = new JobExecutionView("j");
-            view.Append(MainEntry("a"));
-            view.Append(MainEntry("b"));
-
-            Assert.Throws<ArgumentOutOfRangeException>(() => view.GetLine(index));
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Yaml_UpdatesAfterAppend()
-        {
-            var view = new JobExecutionView("j");
-            view.Append(MainEntry("first"));
-            string before = view.Yaml;
-            Assert.Contains("- step: first", before);
-
-            view.Append(MainEntry("second"));
-            string after = view.Yaml;
-
-            Assert.Contains("- step: first", after);
-            Assert.Contains("- step: second", after);
-            Assert.NotEqual(before, after);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Yaml_AlwaysEndsWithCleanupBoundary()
-        {
-            var view = new JobExecutionView("j");
-            Assert.EndsWith("cleanup:\n  - step: Complete job\n", view.Yaml);
-
-            view.Append(MainEntry("a"));
-            Assert.EndsWith("cleanup:\n  - step: Complete job\n", view.Yaml);
-
-            view.Append(MainEntry("b"));
-            view.Append(MainEntry("c"));
-            Assert.EndsWith("cleanup:\n  - step: Complete job\n", view.Yaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Append_WithMatchKey_TracksUnclaimed()
-        {
-            var view = new JobExecutionView("j");
-
-            int line = view.Append(MainEntry("placeholder"), stepIdentity: null, matchKey: "k1");
-
-            var step = NewStep("real");
-            int? claimed = view.TryClaim("k1", step);
-            Assert.Equal(line, claimed);
-            Assert.Equal(line, view.TryGetLineForStep(step));
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void TryClaim_UnknownKey_ReturnsNull()
-        {
-            var view = new JobExecutionView("j");
-            view.Append(MainEntry("a"), stepIdentity: null, matchKey: "k1");
-
-            Assert.Null(view.TryClaim("nope", NewStep()));
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void TryClaim_AlreadyClaimed_ReturnsNull()
-        {
-            var view = new JobExecutionView("j");
-            view.Append(MainEntry("a"), stepIdentity: null, matchKey: "k1");
-
-            var first = NewStep("first");
-            Assert.NotNull(view.TryClaim("k1", first));
-
-            var second = NewStep("second");
-            Assert.Null(view.TryClaim("k1", second));
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void TryClaim_StepAlreadyRegistered_ReturnsNull()
-        {
-            var view = new JobExecutionView("j");
-            var step = NewStep();
-            // Step is registered for the first entry.
-            view.Append(MainEntry("a"), step);
-            // A placeholder is registered for the second entry.
-            view.Append(MainEntry("b"), stepIdentity: null, matchKey: "k1");
-
-            // Trying to claim the placeholder with the already-registered
-            // step must return null (defensive — would otherwise double-bind).
-            Assert.Null(view.TryClaim("k1", step));
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Append_DuplicateMatchKey_Throws()
-        {
-            var view = new JobExecutionView("j");
-            view.Append(MainEntry("a"), stepIdentity: null, matchKey: "k1");
-
-            Assert.Throws<InvalidOperationException>(
-                () => view.Append(MainEntry("b"), stepIdentity: null, matchKey: "k1"));
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Append_MatchKeyNull_BehavesLikeOldOverload()
-        {
-            var view = new JobExecutionView("j");
-            var step = NewStep();
-
-            int line = view.Append(MainEntry("a"), step);
-
-            Assert.Equal(line, view.GetLine(0));
-            Assert.Equal(line, view.TryGetLineForStep(step));
-            // TryClaim with any key must return null since no matchKey was registered.
-            Assert.Null(view.TryClaim("anything", NewStep()));
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void TryClaim_AfterClaim_TryGetLineForStepResolves()
-        {
-            var view = new JobExecutionView("j");
-            int line = view.Append(MainEntry("placeholder"), stepIdentity: null, matchKey: "k1");
-
-            var step = NewStep();
-            Assert.Equal(line, view.TryClaim("k1", step));
-            Assert.Equal(line, view.TryGetLineForStep(step));
-
-            // And a later Append doesn't lose the claim (Render rebuilds
-            // the IStep -> line map from the persisted identities).
-            view.Append(MainEntry("b"));
-            Assert.Equal(line, view.TryGetLineForStep(step));
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void TryClaim_NullArgs_Throws()
-        {
-            var view = new JobExecutionView("j");
-            Assert.Throws<ArgumentNullException>(() => view.TryClaim(null, NewStep()));
-            Assert.Throws<ArgumentNullException>(() => view.TryClaim("k", null));
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task ConcurrentAppends_DontCorruptState()
-        {
-            var view = new JobExecutionView("j");
-            const int N = 50;
-            var steps = Enumerable.Range(0, N).Select(i => NewStep("s" + i)).ToList();
-            var returnedLines = new ConcurrentBag<int>();
-
-            var tasks = Enumerable.Range(0, N).Select(i => Task.Run(() =>
-            {
-                int line = view.Append(MainEntry("e" + i), steps[i]);
-                returnedLines.Add(line);
-            })).ToArray();
-
-            await Task.WhenAll(tasks);
-
-            Assert.Equal(N, view.EntryCount);
-            Assert.Equal(N, returnedLines.Distinct().Count());
-
-            // Every step identity resolves to some line in [0, N).
-            var entryLines = Enumerable.Range(0, N).Select(view.GetLine).ToHashSet();
-            Assert.Equal(N, entryLines.Count);
-            foreach (var step in steps)
-            {
-                int? line = view.TryGetLineForStep(step);
-                Assert.NotNull(line);
-                Assert.Contains(line.Value, entryLines);
+                step.Setup(s => s.ExecutionContext).Returns((IExecutionContext)null);
             }
+
+            return step;
         }
 
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Append_RejectsBothStepIdentityAndMatchKey()
+        private static Mock<IActionRunner> CreateActionRunner(string displayName, ActionRunStage stage, ActionStep action)
         {
-            // Allowing both would orphan the IStep→line mapping the moment
-            // TryClaim overwrites _stepIdentities[index] for a different
-            // step, so the API rejects the combination at append time.
-            var view = new JobExecutionView("j");
-            var entry = new JobExecutionViewEntry(JobExecutionPhase.Post, "Post X", uses: "actions/x@v1");
-            Assert.Throws<ArgumentException>(() =>
-                view.Append(entry, stepIdentity: NewStep("real"), matchKey: "k1"));
-            // State unchanged.
-            Assert.Equal(0, view.EntryCount);
+            var executionContext = new Mock<IExecutionContext>();
+            executionContext.Setup(x => x.Stage).Returns(stage);
+
+            var runner = new Mock<IActionRunner>();
+            runner.Setup(s => s.DisplayName).Returns(displayName);
+            runner.Setup(s => s.ExecutionContext).Returns(executionContext.Object);
+            runner.Setup(s => s.Stage).Returns(stage);
+            runner.Setup(s => s.Action).Returns(action);
+            return runner;
+        }
+
+        private static ActionStep CreateRepositoryActionStep(string name)
+        {
+            return new ActionStep
+            {
+                Id = Guid.NewGuid(),
+                Name = name,
+                Reference = new RepositoryPathReference
+                {
+                    Name = name,
+                    Ref = "v1",
+                    RepositoryType = RepositoryTypes.GitHub
+                }
+            };
+        }
+
+        private static string MatchKeyFor(Guid actionId)
+        {
+            return $"post:{actionId:N}";
         }
     }
 }

src/Test/L0/Worker/JobExecutionViewRendererL0.cs

@@ -1,610 +0,0 @@
-using System;
-using System.Collections.Generic;
-using GitHub.Runner.Worker.Dap;
-using Xunit;
-
-namespace GitHub.Runner.Common.Tests.Worker
-{
-    public sealed class JobExecutionViewRendererL0
-    {
-        // Verbatim expected YAML for the design doc's "Worked example".
-        // The render output is structured as phase-keyed top-level sections;
-        // there is no per-entry `phase:` field. The setup: and cleanup:
-        // sections always render; pre:/main:/post: render only when
-        // they contain at least one entry. The Main entries surface
-        // user-authored step parameters pre-evaluation (no expression
-        // substitution); Pre/Post entries stay minimal.
-        private const string ExpectedWorkedExampleYaml =
-            "# Job: build\n" +
-            "# Runner execution plan — read-only.\n" +
-            "\n" +
-            "setup:\n" +
-            "  - step: Setup job\n" +
-            "\n" +
-            "pre:\n" +
-            "  - step: Pre actions/checkout@v4\n" +
-            "    action: actions/checkout@v4\n" +
-            "  - step: Pre actions/cache@v5\n" +
-            "    action: actions/cache@v5\n" +
-            "\n" +
-            "main:\n" +
-            "  - step: actions/checkout@v4\n" +
-            "    uses: actions/checkout@v4\n" +
-            "    source: .github/workflows/ci.yml:10\n" +
-            "  - step: Cache Primes\n" +
-            "    id: cache-primes\n" +
-            "    uses: actions/cache@v5\n" +
-            "    with:\n" +
-            "      path: prime-numbers\n" +
-            "      key: ${{ runner.os }}-primes\n" +
-            "    source: .github/workflows/ci.yml:12\n" +
-            "  - step: Run tests\n" +
-            "    id: test\n" +
-            "    run: |\n" +
-            "      echo starting\n" +
-            "      npm test\n" +
-            "    if: ${{ github.event_name == 'push' }}\n" +
-            "    env:\n" +
-            "      NODE_ENV: production\n" +
-            "    shell: bash\n" +
-            "    working-directory: ./api\n" +
-            "    source: .github/workflows/ci.yml:18\n" +
-            "  - step: npm ci\n" +
-            "    run: npm ci\n" +
-            "    source: .github/workflows/ci.yml:28\n" +
-            "\n" +
-            "post:\n" +
-            "  - step: Post actions/cache@v5\n" +
-            "    action: actions/cache@v5\n" +
-            "  - step: Post actions/checkout@v4\n" +
-            "    action: actions/checkout@v4\n" +
-            "\n" +
-            "cleanup:\n" +
-            "  - step: Complete job\n";
-
-        private static List<JobExecutionViewEntry> WorkedExampleEntries()
-        {
-            return new List<JobExecutionViewEntry>
-            {
-                new JobExecutionViewEntry(JobExecutionPhase.Pre, "Pre actions/checkout@v4", uses: "actions/checkout@v4"),
-                new JobExecutionViewEntry(JobExecutionPhase.Pre, "Pre actions/cache@v5", uses: "actions/cache@v5"),
-                new JobExecutionViewEntry(JobExecutionPhase.Main, "actions/checkout@v4", uses: "actions/checkout@v4", sourcePath: ".github/workflows/ci.yml", sourceLine: 10),
-                new JobExecutionViewEntry(
-                    JobExecutionPhase.Main,
-                    "Cache Primes",
-                    uses: "actions/cache@v5",
-                    id: "cache-primes",
-                    withYaml: "      path: prime-numbers\n      key: ${{ runner.os }}-primes",
-                    sourcePath: ".github/workflows/ci.yml",
-                    sourceLine: 12),
-                new JobExecutionViewEntry(
-                    JobExecutionPhase.Main,
-                    "Run tests",
-                    run: "echo starting\nnpm test",
-                    id: "test",
-                    @if: "${{ github.event_name == 'push' }}",
-                    envYaml: "      NODE_ENV: production",
-                    shell: "bash",
-                    workingDirectory: "./api",
-                    sourcePath: ".github/workflows/ci.yml",
-                    sourceLine: 18),
-                new JobExecutionViewEntry(JobExecutionPhase.Main, "npm ci", run: "npm ci", sourcePath: ".github/workflows/ci.yml", sourceLine: 28),
-                new JobExecutionViewEntry(JobExecutionPhase.Post, "Post actions/cache@v5", uses: "actions/cache@v5"),
-                new JobExecutionViewEntry(JobExecutionPhase.Post, "Post actions/checkout@v4", uses: "actions/checkout@v4"),
-            };
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Render_MatchesDesignDocWorkedExample()
-        {
-            var entries = WorkedExampleEntries();
-
-            var result = JobExecutionViewRenderer.Render("build", entries);
-
-            Assert.Equal(ExpectedWorkedExampleYaml, result.Yaml);
-            Assert.Equal(8, result.EntryStartLines.Count);
-            var lines = result.Yaml.Split('\n');
-            for (int i = 0; i < entries.Count; i++)
-            {
-                Assert.StartsWith("  - step: ", lines[result.EntryStartLines[i] - 1]);
-                Assert.Contains(entries[i].DisplayName, lines[result.EntryStartLines[i] - 1]);
-            }
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Render_AlwaysEmitsSetupAndCleanup()
-        {
-            var result = JobExecutionViewRenderer.Render("job-1", new List<JobExecutionViewEntry>());
-
-            const string expected =
-                "# Job: job-1\n" +
-                "# Runner execution plan — read-only.\n" +
-                "\n" +
-                "setup:\n" +
-                "  - step: Setup job\n" +
-                "\n" +
-                "cleanup:\n" +
-                "  - step: Complete job\n";
-            Assert.Equal(expected, result.Yaml);
-            Assert.Empty(result.EntryStartLines);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Render_OmitsEmptyOptionalSections()
-        {
-            // Only a Main entry — pre:/post: must not appear.
-            var result = JobExecutionViewRenderer.Render("j", new[]
-            {
-                new JobExecutionViewEntry(JobExecutionPhase.Main, "echo", run: "echo hello"),
-            });
-
-            Assert.Contains("setup:\n", result.Yaml);
-            Assert.Contains("main:\n", result.Yaml);
-            Assert.Contains("cleanup:\n", result.Yaml);
-            Assert.DoesNotContain("\npre:\n", result.Yaml);
-            Assert.DoesNotContain("\npost:\n", result.Yaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Render_EmitsPhaseSectionsInFixedOrder()
-        {
-            // Input order [Post, Pre, Main] should still render as setup → pre → main → post → cleanup.
-            var entries = new[]
-            {
-                new JobExecutionViewEntry(JobExecutionPhase.Post, "post-a", uses: "a/b@v1"),
-                new JobExecutionViewEntry(JobExecutionPhase.Pre, "pre-a", uses: "a/b@v1"),
-                new JobExecutionViewEntry(JobExecutionPhase.Main, "main-a", uses: "a/b@v1"),
-            };
-
-            var result = JobExecutionViewRenderer.Render("j", entries);
-            string yaml = result.Yaml;
-
-            int setupIdx = yaml.IndexOf("setup:\n", StringComparison.Ordinal);
-            int preIdx = yaml.IndexOf("\npre:\n", StringComparison.Ordinal);
-            int mainIdx = yaml.IndexOf("\nmain:\n", StringComparison.Ordinal);
-            int postIdx = yaml.IndexOf("\npost:\n", StringComparison.Ordinal);
-            int cleanupIdx = yaml.IndexOf("\ncleanup:\n", StringComparison.Ordinal);
-            Assert.True(setupIdx >= 0 && preIdx > setupIdx && mainIdx > preIdx && postIdx > mainIdx && cleanupIdx > postIdx,
-                $"section ordering wrong: setup={setupIdx} pre={preIdx} main={mainIdx} post={postIdx} cleanup={cleanupIdx}");
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Render_StartLinesAlignWithInputOrder()
-        {
-            // Input order is [Pre, Main, Post]; output order is also pre/main/post,
-            // but startLines must be indexed by INPUT position, not by section.
-            var entries = new[]
-            {
-                new JobExecutionViewEntry(JobExecutionPhase.Pre, "pre-x", uses: "x/y@v1"),     // index 0
-                new JobExecutionViewEntry(JobExecutionPhase.Main, "main-x", uses: "x/y@v1"),    // index 1
-                new JobExecutionViewEntry(JobExecutionPhase.Post, "post-x", uses: "x/y@v1"),    // index 2
-            };
-
-            var result = JobExecutionViewRenderer.Render("j", entries);
-            var lines = result.Yaml.Split('\n');
-
-            Assert.StartsWith("  - step: pre-x", lines[result.EntryStartLines[0] - 1]);
-            Assert.StartsWith("  - step: main-x", lines[result.EntryStartLines[1] - 1]);
-            Assert.StartsWith("  - step: post-x", lines[result.EntryStartLines[2] - 1]);
-            // And input-order ordering of start lines is strictly increasing
-            // when phases are in declaration order matching the section order.
-            Assert.True(result.EntryStartLines[0] < result.EntryStartLines[1]);
-            Assert.True(result.EntryStartLines[1] < result.EntryStartLines[2]);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Render_StartLinesFollowInputOrderEvenWhenPhasesAreInterleaved()
-        {
-            // Input order is [Main A, Pre B, Main C]: pre section will render
-            // first (Pre B) and main second (Main A then Main C). startLines
-            // must still be indexed by input order.
-            var entries = new[]
-            {
-                new JobExecutionViewEntry(JobExecutionPhase.Main, "main-a", uses: "a@v1"),  // index 0 — renders in main section
-                new JobExecutionViewEntry(JobExecutionPhase.Pre, "pre-b", uses: "b@v1"),    // index 1 — renders in pre section
-                new JobExecutionViewEntry(JobExecutionPhase.Main, "main-c", uses: "c@v1"),  // index 2 — renders in main section
-            };
-
-            var result = JobExecutionViewRenderer.Render("j", entries);
-            var lines = result.Yaml.Split('\n');
-
-            Assert.StartsWith("  - step: main-a", lines[result.EntryStartLines[0] - 1]);
-            Assert.StartsWith("  - step: pre-b", lines[result.EntryStartLines[1] - 1]);
-            Assert.StartsWith("  - step: main-c", lines[result.EntryStartLines[2] - 1]);
-            // The pre section comes before main: input-index-1 entry's line is
-            // before input-index-0 entry's line.
-            Assert.True(result.EntryStartLines[1] < result.EntryStartLines[0]);
-            Assert.True(result.EntryStartLines[0] < result.EntryStartLines[2]);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Render_EntryStartLinesPointAtStepKeys()
-        {
-            var entries = WorkedExampleEntries();
-            var result = JobExecutionViewRenderer.Render("build", entries);
-            var lines = result.Yaml.Split('\n');
-
-            for (int i = 0; i < result.EntryStartLines.Count; i++)
-            {
-                int oneBased = result.EntryStartLines[i];
-                Assert.True(oneBased >= 1 && oneBased <= lines.Length, $"start line {oneBased} out of range");
-                Assert.StartsWith("  - step: ", lines[oneBased - 1]);
-            }
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Render_EntryStartLinesExcludeSetupAndCleanup()
-        {
-            var entries = WorkedExampleEntries();
-            var result = JobExecutionViewRenderer.Render("build", entries);
-            var lines = result.Yaml.Split('\n');
-
-            int setupLine = -1, cleanupLine = -1;
-            for (int i = 0; i < lines.Length; i++)
-            {
-                if (lines[i] == "  - step: Setup job") setupLine = i + 1;
-                if (lines[i] == "  - step: Complete job") cleanupLine = i + 1;
-            }
-            Assert.True(setupLine > 0 && cleanupLine > 0, "Setup/Cleanup lines must exist");
-            Assert.DoesNotContain(setupLine, result.EntryStartLines);
-            Assert.DoesNotContain(cleanupLine, result.EntryStartLines);
-        }
-
-        [Theory]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        [InlineData("hello")]
-        [InlineData("with: colon")]
-        [InlineData("with#hash")]
-        [InlineData(" leading")]
-        [InlineData("trailing ")]
-        [InlineData("a\"b")]
-        [InlineData("a\\b")]
-        [InlineData("@at")]
-        [InlineData("*star")]
-        public void Render_QuotesSpecialChars(string displayName)
-        {
-            // Round-trip the rendered YAML through YamlDotNet's deserializer
-            // and assert the parsed step's display name matches the input.
-            // This decouples the test from any specific quoting style.
-            var entry = new JobExecutionViewEntry(JobExecutionPhase.Main, displayName);
-            var result = JobExecutionViewRenderer.Render("j", new[] { entry });
-
-            var deserializer = new YamlDotNet.Serialization.DeserializerBuilder().Build();
-            var doc = deserializer.Deserialize<Dictionary<string, List<Dictionary<string, object>>>>(result.Yaml);
-            Assert.NotNull(doc);
-            Assert.True(doc.ContainsKey("main"), "rendered YAML missing top-level 'main' key");
-            var mainSteps = doc["main"];
-            Assert.Single(mainSteps);
-            Assert.Equal(displayName, mainSteps[0]["step"] as string);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Render_EmitsSourceAnnotationForMainStep()
-        {
-            var entry = new JobExecutionViewEntry(
-                JobExecutionPhase.Main,
-                "npm ci",
-                run: "npm ci",
-                sourcePath: ".github/workflows/ci.yml",
-                sourceLine: 42);
-
-            var result = JobExecutionViewRenderer.Render("j", new[] { entry });
-
-            Assert.Contains("    source: .github/workflows/ci.yml:42\n", result.Yaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Render_OmitsSourceAnnotationForPreAndPost()
-        {
-            var pre = new JobExecutionViewEntry(
-                JobExecutionPhase.Pre,
-                "Pre actions/checkout@v4",
-                uses: "actions/checkout@v4",
-                sourcePath: ".github/workflows/ci.yml",
-                sourceLine: 9);
-            var post = new JobExecutionViewEntry(
-                JobExecutionPhase.Post,
-                "Post actions/checkout@v4",
-                uses: "actions/checkout@v4",
-                sourcePath: ".github/workflows/ci.yml",
-                sourceLine: 9);
-
-            var result = JobExecutionViewRenderer.Render("j", new[] { pre, post });
-
-            Assert.DoesNotContain("source:", result.Yaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Render_EmitsMultilineRunAsBlockScalar()
-        {
-            var entry = new JobExecutionViewEntry(
-                JobExecutionPhase.Main,
-                "multi",
-                run: "echo a\necho b\necho c");
-
-            var result = JobExecutionViewRenderer.Render("j", new[] { entry });
-
-            Assert.Contains("    run: |\n", result.Yaml);
-            Assert.Contains("      echo a\n", result.Yaml);
-            Assert.Contains("      echo b\n", result.Yaml);
-            Assert.Contains("      echo c\n", result.Yaml);
-            Assert.DoesNotContain("truncated", result.Yaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Render_EmitsAllUserAuthoredParamsForActionStep()
-        {
-            var entry = new JobExecutionViewEntry(
-                JobExecutionPhase.Main,
-                "Run action",
-                uses: "actions/cache@v5",
-                id: "cache-primes",
-                @if: "${{ github.event_name == 'push' }}",
-                continueOnError: "true",
-                timeoutMinutes: "10",
-                envYaml: "      NODE_ENV: production",
-                withYaml: "      path: prime-numbers\n      key: ${{ runner.os }}-primes",
-                sourcePath: "ci.yml",
-                sourceLine: 5);
-
-            var result = JobExecutionViewRenderer.Render("j", new[] { entry });
-
-            Assert.Contains("    id: cache-primes\n", result.Yaml);
-            Assert.Contains("    uses: actions/cache@v5\n", result.Yaml);
-            Assert.Contains("    continue-on-error: true\n", result.Yaml);
-            Assert.Contains("    timeout-minutes: 10\n", result.Yaml);
-            Assert.Contains("    env:\n      NODE_ENV: production\n", result.Yaml);
-            Assert.Contains("    with:\n      path: prime-numbers\n      key: ${{ runner.os }}-primes\n", result.Yaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Render_EmitsRunStepWithShellAndWorkingDirectory()
-        {
-            var entry = new JobExecutionViewEntry(
-                JobExecutionPhase.Main,
-                "Run tests",
-                run: "echo starting\nnpm test",
-                id: "test",
-                shell: "bash",
-                workingDirectory: "./api");
-
-            var result = JobExecutionViewRenderer.Render("j", new[] { entry });
-
-            Assert.Contains("    run: |\n      echo starting\n      npm test\n", result.Yaml);
-            Assert.Contains("    shell: bash\n", result.Yaml);
-            Assert.Contains("    working-directory: ./api\n", result.Yaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Render_PreservesExpressionsInRenderedYaml()
-        {
-            var entry = new JobExecutionViewEntry(
-                JobExecutionPhase.Main,
-                "Cache",
-                uses: "actions/cache@v5",
-                withYaml: "      key: ${{ runner.os }}-primes");
-
-            var result = JobExecutionViewRenderer.Render("j", new[] { entry });
-
-            // Expressions render exactly as authored — no evaluation.
-            Assert.Contains("${{ runner.os }}-primes", result.Yaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Render_PrePostStepsRemainMinimal()
-        {
-            // Even if a pre/post entry carries user-param fields (it shouldn't
-            // in production, but the renderer must defensively drop them),
-            // only step: + action: render for these phases.
-            var pre = new JobExecutionViewEntry(
-                JobExecutionPhase.Pre,
-                "Pre actions/cache@v5",
-                uses: "actions/cache@v5",
-                id: "should-not-appear",
-                envYaml: "      X: y",
-                withYaml: "      key: nope");
-            var post = new JobExecutionViewEntry(
-                JobExecutionPhase.Post,
-                "Post actions/cache@v5",
-                uses: "actions/cache@v5",
-                id: "should-not-appear",
-                envYaml: "      X: y");
-
-            var result = JobExecutionViewRenderer.Render("j", new[] { pre, post });
-
-            Assert.DoesNotContain("id:", result.Yaml);
-            Assert.DoesNotContain("env:", result.Yaml);
-            Assert.DoesNotContain("with:", result.Yaml);
-            Assert.DoesNotContain("should-not-appear", result.Yaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Render_FieldOrderIsStable()
-        {
-            var entry = new JobExecutionViewEntry(
-                JobExecutionPhase.Main,
-                "Everything",
-                uses: "actions/cache@v5",
-                id: "x",
-                @if: "always()",
-                continueOnError: "false",
-                timeoutMinutes: "5",
-                envYaml: "      A: 1",
-                withYaml: "      key: k",
-                sourcePath: "ci.yml",
-                sourceLine: 1);
-
-            var result = JobExecutionViewRenderer.Render("j", new[] { entry });
-            var y = result.Yaml;
-            int iStep = y.IndexOf("    - step: ", StringComparison.Ordinal) >= 0
-                ? y.IndexOf("- step:", StringComparison.Ordinal) : y.IndexOf("- step:", StringComparison.Ordinal);
-            int iId = y.IndexOf("    id:", StringComparison.Ordinal);
-            int iUses = y.IndexOf("    uses:", StringComparison.Ordinal);
-            int iIf = y.IndexOf("    if:", StringComparison.Ordinal);
-            int iCoe = y.IndexOf("    continue-on-error:", StringComparison.Ordinal);
-            int iTm = y.IndexOf("    timeout-minutes:", StringComparison.Ordinal);
-            int iEnv = y.IndexOf("    env:", StringComparison.Ordinal);
-            int iWith = y.IndexOf("    with:", StringComparison.Ordinal);
-            int iSrc = y.IndexOf("    source:", StringComparison.Ordinal);
-            Assert.True(iId < iUses && iUses < iIf && iIf < iCoe && iCoe < iTm && iTm < iEnv && iEnv < iWith && iWith < iSrc,
-                $"order wrong: id={iId} uses={iUses} if={iIf} coe={iCoe} tm={iTm} env={iEnv} with={iWith} src={iSrc}");
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Render_OmitsEmptyOptionalFields()
-        {
-            var entry = new JobExecutionViewEntry(
-                JobExecutionPhase.Main,
-                "bare",
-                uses: "a/b@v1");
-
-            var result = JobExecutionViewRenderer.Render("j", new[] { entry });
-            Assert.DoesNotContain("    id:", result.Yaml);
-            Assert.DoesNotContain("    if:", result.Yaml);
-            Assert.DoesNotContain("    continue-on-error:", result.Yaml);
-            Assert.DoesNotContain("    timeout-minutes:", result.Yaml);
-            Assert.DoesNotContain("    env:", result.Yaml);
-            Assert.DoesNotContain("    with:", result.Yaml);
-            Assert.DoesNotContain("    shell:", result.Yaml);
-            Assert.DoesNotContain("    working-directory:", result.Yaml);
-            Assert.DoesNotContain("    source:", result.Yaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Render_HandlesEmptyEntries()
-        {
-            var result = JobExecutionViewRenderer.Render("j", new List<JobExecutionViewEntry>());
-
-            Assert.Empty(result.EntryStartLines);
-            Assert.Contains("  - step: Setup job\n", result.Yaml);
-            Assert.Contains("  - step: Complete job\n", result.Yaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Render_NoPerEntryPhaseField()
-        {
-            // The phase: <value> per-entry field is gone — the section
-            // header is the phase indicator. Guard against accidental
-            // regressions.
-            var result = JobExecutionViewRenderer.Render("build", WorkedExampleEntries());
-            Assert.DoesNotContain("phase:", result.Yaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Render_ThrowsOnNullJobId()
-        {
-            Assert.Throws<ArgumentException>(
-                () => JobExecutionViewRenderer.Render(null, new List<JobExecutionViewEntry>()));
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Render_ThrowsOnWhitespaceJobId()
-        {
-            Assert.Throws<ArgumentException>(
-                () => JobExecutionViewRenderer.Render("   ", new List<JobExecutionViewEntry>()));
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Render_ThrowsOnNullEntries()
-        {
-            Assert.Throws<ArgumentNullException>(
-                () => JobExecutionViewRenderer.Render("j", null));
-        }
-
-        [Theory]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        [InlineData(null, 1)]
-        [InlineData("", 1)]
-        [InlineData("   ", 1)]
-        public void Entry_Constructor_RejectsBadDisplayName(string displayName, int sourceLine)
-        {
-            Assert.Throws<ArgumentException>(
-                () => new JobExecutionViewEntry(JobExecutionPhase.Main, displayName, sourceLine: sourceLine));
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Entry_Constructor_RejectsZeroLineWhenSourcePathSet()
-        {
-            Assert.Throws<ArgumentException>(
-                () => new JobExecutionViewEntry(
-                    JobExecutionPhase.Main,
-                    "ok",
-                    sourcePath: "ci.yml",
-                    sourceLine: 0));
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Render_AlwaysUsesLfLineBreaks()
-        {
-            // Regression: YamlDotNet's Emitter calls WriteLine, which on
-            // Windows produces CRLF (the host's Environment.NewLine).
-            // FormatScalar / TemplateTokenYamlAdapter.Serialize must force
-            // LF so the rendered view round-trips regardless of platform.
-            var entry = new JobExecutionViewEntry(JobExecutionPhase.Main, "with: colon", id: "step-1", uses: "actions/checkout@v4");
-            var result = JobExecutionViewRenderer.Render("job-1", new[] { entry });
-            Assert.DoesNotContain("\r", result.Yaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void FormatScalar_AlwaysUsesLfLineBreaks()
-        {
-            // Direct check on FormatScalar to guard against future refactors
-            // that bypass the full Render path but still emit through
-            // YamlDotNet.
-            Assert.DoesNotContain("\r", JobExecutionViewRenderer.FormatScalar("with: colon"));
-            Assert.DoesNotContain("\r", JobExecutionViewRenderer.FormatScalar("hello"));
-        }
-    }
-}

src/Test/L0/Worker/JobExtensionL0.cs

@@ -549,6 +549,10 @@ namespace GitHub.Runner.Common.Tests.Worker
                 var _stepsRunner = new StepsRunner();
                 _stepsRunner.Initialize(hc);
 
+                var bgCoordinator = new BackgroundStepCoordinator();
+                bgCoordinator.Initialize(hc);
+                hc.SetSingleton<IBackgroundStepCoordinator>(bgCoordinator);
+
                 var mockDapDebugger = new Mock<IDapDebugger>();
                 hc.SetSingleton(mockDapDebugger.Object);
 

src/Test/L0/Worker/StepEntryTranslatorL0.cs

@@ -1,428 +0,0 @@
-using System;
-using GitHub.DistributedTask.ObjectTemplating.Tokens;
-using GitHub.DistributedTask.Pipelines;
-using GitHub.Runner.Worker;
-using GitHub.Runner.Worker.Dap;
-using Moq;
-using Xunit;
-
-namespace GitHub.Runner.Common.Tests.Worker
-{
-    public sealed class StepEntryTranslatorL0
-    {
-        private static StringToken Str(string s) => new(null, null, null, s);
-
-        private static MappingToken Map(params (string Key, TemplateToken Value)[] pairs)
-        {
-            var m = new MappingToken(null, null, null);
-            foreach (var (k, v) in pairs)
-            {
-                m.Add(Str(k), v);
-            }
-            return m;
-        }
-
-        private static Mock<IActionRunner> NewActionRunnerMock(
-            ActionRunStage stage,
-            string displayName,
-            ActionStepDefinitionReference reference,
-            ActionStep actionOverride = null)
-        {
-            var mock = new Mock<IActionRunner>();
-            mock.SetupGet(x => x.Stage).Returns(stage);
-            mock.SetupGet(x => x.DisplayName).Returns(displayName);
-            mock.SetupGet(x => x.Action).Returns(actionOverride ?? new ActionStep
-            {
-                Reference = reference,
-            });
-            return mock;
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Translate_NullStep_Throws()
-        {
-            Assert.Throws<ArgumentNullException>(() =>
-                StepEntryTranslator.TryTranslate(null));
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Translate_JobExtensionRunner_ReturnsNull()
-        {
-            var step = new JobExtensionRunner(
-                runAsync: (_, __) => System.Threading.Tasks.Task.CompletedTask,
-                condition: null,
-                displayName: "Set up job",
-                data: null);
-
-            Assert.Null(StepEntryTranslator.TryTranslate(step));
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Translate_OtherIStepType_ReturnsNull()
-        {
-            var mock = new Mock<IStep>();
-            mock.SetupGet(x => x.DisplayName).Returns("custom");
-
-            Assert.Null(StepEntryTranslator.TryTranslate(mock.Object));
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Translate_ActionRunnerPre_ReturnsPreEntry()
-        {
-            var reference = new RepositoryPathReference
-            {
-                Name = "actions/checkout",
-                Ref = "v4",
-            };
-            var mock = NewActionRunnerMock(ActionRunStage.Pre, "Pre Run actions/checkout@v4", reference);
-
-            var entry = StepEntryTranslator.TryTranslate(mock.Object);
-
-            Assert.NotNull(entry);
-            Assert.Equal(JobExecutionPhase.Pre, entry.Phase);
-            Assert.Equal("Pre Run actions/checkout@v4", entry.DisplayName);
-            Assert.Equal("actions/checkout@v4", entry.Uses);
-            Assert.Null(entry.Run);
-            Assert.Null(entry.SourcePath);
-            Assert.Equal(0, entry.SourceLine);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Translate_ActionRunnerMain_ReturnsMainEntryWithUses()
-        {
-            var reference = new RepositoryPathReference
-            {
-                Name = "actions/setup-node",
-                Path = "subdir",
-                Ref = "v3",
-            };
-            var mock = NewActionRunnerMock(ActionRunStage.Main, "Run actions/setup-node@v3", reference);
-
-            var entry = StepEntryTranslator.TryTranslate(mock.Object);
-
-            Assert.NotNull(entry);
-            Assert.Equal(JobExecutionPhase.Main, entry.Phase);
-            Assert.Equal("actions/setup-node/subdir@v3", entry.Uses);
-            Assert.Null(entry.Run);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Translate_ActionRunnerMain_ScriptReference_LeavesUsesNull()
-        {
-            var mock = NewActionRunnerMock(ActionRunStage.Main, "Run echo hi", new ScriptReference());
-
-            var entry = StepEntryTranslator.TryTranslate(mock.Object);
-
-            Assert.NotNull(entry);
-            Assert.Equal(JobExecutionPhase.Main, entry.Phase);
-            Assert.Null(entry.Uses);
-            Assert.Null(entry.Run);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Translate_ActionRunnerMain_ContainerReference_UsesImage()
-        {
-            var reference = new ContainerRegistryReference { Image = "alpine:3.18" };
-            var mock = NewActionRunnerMock(ActionRunStage.Main, "Run alpine", reference);
-
-            var entry = StepEntryTranslator.TryTranslate(mock.Object);
-
-            Assert.NotNull(entry);
-            Assert.Equal("alpine:3.18", entry.Uses);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Translate_ActionRunnerPost_ReturnsPostEntry()
-        {
-            var reference = new RepositoryPathReference { Name = "actions/cache", Ref = "v3" };
-            var mock = NewActionRunnerMock(ActionRunStage.Post, "Post Run actions/cache@v3", reference);
-
-            var entry = StepEntryTranslator.TryTranslate(mock.Object);
-
-            Assert.NotNull(entry);
-            Assert.Equal(JobExecutionPhase.Post, entry.Phase);
-            Assert.Equal("Post Run actions/cache@v3", entry.DisplayName);
-            Assert.Equal("actions/cache@v3", entry.Uses);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Translate_ActionRunner_NullAction_LeavesUsesNull()
-        {
-            var mock = new Mock<IActionRunner>();
-            mock.SetupGet(x => x.Stage).Returns(ActionRunStage.Main);
-            mock.SetupGet(x => x.DisplayName).Returns("anonymous");
-            mock.SetupGet(x => x.Action).Returns((ActionStep)null);
-
-            var entry = StepEntryTranslator.TryTranslate(mock.Object);
-
-            Assert.NotNull(entry);
-            Assert.Equal("anonymous", entry.DisplayName);
-            Assert.Null(entry.Uses);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Translate_ActionStep_ExtractsWith()
-        {
-            var reference = new RepositoryPathReference { Name = "actions/cache", Ref = "v5" };
-            var action = new ActionStep
-            {
-                Reference = reference,
-                Inputs = Map(("path", Str("prime-numbers")), ("key", Str("k"))),
-            };
-            var mock = NewActionRunnerMock(ActionRunStage.Main, "Cache", reference, action);
-
-            var entry = StepEntryTranslator.TryTranslate(mock.Object);
-
-            Assert.NotNull(entry);
-            Assert.NotNull(entry.WithYaml);
-            Assert.Contains("path: prime-numbers", entry.WithYaml);
-            Assert.Contains("key: k", entry.WithYaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Translate_ActionStep_PreservesExpressionInWith()
-        {
-            var reference = new RepositoryPathReference { Name = "actions/cache", Ref = "v5" };
-            var action = new ActionStep
-            {
-                Reference = reference,
-                Inputs = Map(("key", Str("${{ runner.os }}-primes"))),
-            };
-            var mock = NewActionRunnerMock(ActionRunStage.Main, "Cache", reference, action);
-
-            var entry = StepEntryTranslator.TryTranslate(mock.Object);
-
-            Assert.NotNull(entry);
-            Assert.Contains("${{ runner.os }}-primes", entry.WithYaml);
-            Assert.DoesNotContain("Linux", entry.WithYaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Translate_RunStep_ExtractsScript()
-        {
-            var action = new ActionStep
-            {
-                Reference = new ScriptReference(),
-                Inputs = Map(("script", Str("echo hi"))),
-            };
-            var mock = NewActionRunnerMock(ActionRunStage.Main, "Run echo", new ScriptReference(), action);
-
-            var entry = StepEntryTranslator.TryTranslate(mock.Object);
-
-            Assert.NotNull(entry);
-            Assert.Null(entry.Uses);
-            Assert.Equal("echo hi", entry.Run);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Translate_RunStep_ExtractsShellAndWorkingDirectory()
-        {
-            // The runner stores run-step inputs under the keys defined in
-            // PipelineConstants.ScriptStepInputs (camelCase), NOT their
-            // kebab-case workflow-YAML spellings — see
-            // ActionManifestManagerWrapper:244.
-            var action = new ActionStep
-            {
-                Reference = new ScriptReference(),
-                Inputs = Map(
-                    (PipelineConstants.ScriptStepInputs.Script, Str("npm test")),
-                    (PipelineConstants.ScriptStepInputs.Shell, Str("bash")),
-                    (PipelineConstants.ScriptStepInputs.WorkingDirectory, Str("./api"))),
-            };
-            var mock = NewActionRunnerMock(ActionRunStage.Main, "Run", new ScriptReference(), action);
-
-            var entry = StepEntryTranslator.TryTranslate(mock.Object);
-
-            Assert.NotNull(entry);
-            Assert.Equal("npm test", entry.Run);
-            Assert.Equal("bash", entry.Shell);
-            Assert.Equal("./api", entry.WorkingDirectory);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Translate_ActionStep_FiltersRunStepKeysFromWith()
-        {
-            // Defensive: an action step's Inputs should not contain
-            // run-step internal keys, but if it did, they must not
-            // surface in the with: rendering.
-            var reference = new RepositoryPathReference { Name = "a/b", Ref = "v1" };
-            var action = new ActionStep
-            {
-                Reference = reference,
-                Inputs = Map(
-                    ("mode", Str("ci")),
-                    (PipelineConstants.ScriptStepInputs.Script, Str("leak")),
-                    (PipelineConstants.ScriptStepInputs.Shell, Str("leak")),
-                    (PipelineConstants.ScriptStepInputs.WorkingDirectory, Str("leak"))),
-            };
-            var mock = NewActionRunnerMock(ActionRunStage.Main, "Run", reference, action);
-
-            var entry = StepEntryTranslator.TryTranslate(mock.Object);
-
-            Assert.NotNull(entry);
-            Assert.NotNull(entry.WithYaml);
-            Assert.Contains("mode: ci", entry.WithYaml);
-            Assert.DoesNotContain("leak", entry.WithYaml);
-            Assert.DoesNotContain(PipelineConstants.ScriptStepInputs.Script, entry.WithYaml);
-            Assert.DoesNotContain(PipelineConstants.ScriptStepInputs.Shell, entry.WithYaml);
-            Assert.DoesNotContain(PipelineConstants.ScriptStepInputs.WorkingDirectory, entry.WithYaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Translate_ActionStep_OmitsEmptyEnv()
-        {
-            var reference = new RepositoryPathReference { Name = "a/b", Ref = "v1" };
-            var action = new ActionStep
-            {
-                Reference = reference,
-                Environment = new MappingToken(null, null, null),
-            };
-            var mock = NewActionRunnerMock(ActionRunStage.Main, "Run", reference, action);
-
-            var entry = StepEntryTranslator.TryTranslate(mock.Object);
-
-            Assert.NotNull(entry);
-            Assert.Null(entry.EnvYaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Translate_ActionStep_ExtractsEnv()
-        {
-            var reference = new RepositoryPathReference { Name = "a/b", Ref = "v1" };
-            var action = new ActionStep
-            {
-                Reference = reference,
-                Environment = Map(("NODE_ENV", Str("production"))),
-            };
-            var mock = NewActionRunnerMock(ActionRunStage.Main, "Run", reference, action);
-
-            var entry = StepEntryTranslator.TryTranslate(mock.Object);
-
-            Assert.NotNull(entry);
-            Assert.NotNull(entry.EnvYaml);
-            Assert.Contains("NODE_ENV: production", entry.EnvYaml);
-        }
-
-        [Theory]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        [InlineData(null)]
-        [InlineData("")]
-        [InlineData("   ")]
-        [InlineData("__1")]
-        [InlineData("__123")]
-        public void Translate_FiltersAutoGeneratedId(string contextName)
-        {
-            var reference = new RepositoryPathReference { Name = "a/b", Ref = "v1" };
-            var action = new ActionStep
-            {
-                Reference = reference,
-                ContextName = contextName,
-            };
-            var mock = NewActionRunnerMock(ActionRunStage.Main, "Run", reference, action);
-
-            var entry = StepEntryTranslator.TryTranslate(mock.Object);
-
-            Assert.NotNull(entry);
-            Assert.Null(entry.Id);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Translate_PreservesUserId()
-        {
-            var reference = new RepositoryPathReference { Name = "a/b", Ref = "v1" };
-            var action = new ActionStep
-            {
-                Reference = reference,
-                ContextName = "cache-primes",
-            };
-            var mock = NewActionRunnerMock(ActionRunStage.Main, "Cache", reference, action);
-
-            var entry = StepEntryTranslator.TryTranslate(mock.Object);
-
-            Assert.NotNull(entry);
-            Assert.Equal("cache-primes", entry.Id);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Translate_ActionStep_ExtractsCondition()
-        {
-            var reference = new RepositoryPathReference { Name = "a/b", Ref = "v1" };
-            var action = new ActionStep
-            {
-                Reference = reference,
-                Condition = "always()",
-            };
-            var mock = NewActionRunnerMock(ActionRunStage.Main, "Run", reference, action);
-
-            var entry = StepEntryTranslator.TryTranslate(mock.Object);
-
-            Assert.NotNull(entry);
-            Assert.Equal("always()", entry.If);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Translate_PreEntry_OmitsUserParams()
-        {
-            // Pre entries stay minimal — they reference the same Action as
-            // Main, and duplicating params adds noise.
-            var reference = new RepositoryPathReference { Name = "a/b", Ref = "v1" };
-            var action = new ActionStep
-            {
-                Reference = reference,
-                ContextName = "user-id",
-                Condition = "always()",
-                Environment = Map(("X", Str("y"))),
-                Inputs = Map(("k", Str("v"))),
-            };
-            var mock = NewActionRunnerMock(ActionRunStage.Pre, "Pre a/b@v1", reference, action);
-
-            var entry = StepEntryTranslator.TryTranslate(mock.Object);
-
-            Assert.NotNull(entry);
-            Assert.Equal(JobExecutionPhase.Pre, entry.Phase);
-            Assert.Null(entry.Id);
-            Assert.Null(entry.If);
-            Assert.Null(entry.EnvYaml);
-            Assert.Null(entry.WithYaml);
-        }
-    }
-}

src/Test/L0/Worker/StepsRunnerL0.cs

@@ -63,6 +63,10 @@ namespace GitHub.Runner.Common.Tests.Worker
             _stepsRunner = new StepsRunner();
             _stepsRunner.Initialize(hc);
 
+            var bgCoordinator = new BackgroundStepCoordinator();
+            bgCoordinator.Initialize(hc);
+            hc.SetSingleton<IBackgroundStepCoordinator>(bgCoordinator);
+
             var mockDapDebugger = new Mock<IDapDebugger>();
             hc.SetSingleton(mockDapDebugger.Object);
 

src/Test/L0/Worker/TemplateTokenYamlAdapterL0.cs

@@ -1,191 +0,0 @@
-using GitHub.DistributedTask.ObjectTemplating.Tokens;
-using GitHub.Runner.Worker.Dap;
-using Xunit;
-
-namespace GitHub.Runner.Common.Tests.Worker
-{
-    public sealed class TemplateTokenYamlAdapterL0
-    {
-        private static StringToken Str(string s) => new(null, null, null, s);
-        private static BooleanToken Bool(bool b) => new(null, null, null, b);
-        private static NumberToken Num(double n) => new(null, null, null, n);
-        private static BasicExpressionToken Expr(string s) => new(null, null, null, s);
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Serialize_StringScalar()
-        {
-            Assert.Equal("hello", TemplateTokenYamlAdapter.Serialize(Str("hello"), 0));
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Serialize_BooleanScalar()
-        {
-            Assert.Equal("true", TemplateTokenYamlAdapter.Serialize(Bool(true), 0));
-            Assert.Equal("false", TemplateTokenYamlAdapter.Serialize(Bool(false), 0));
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Serialize_NumberScalar()
-        {
-            Assert.Equal("10", TemplateTokenYamlAdapter.Serialize(Num(10), 0));
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Serialize_NullToken_RendersAsNull()
-        {
-            Assert.Equal("null", TemplateTokenYamlAdapter.Serialize(null, 0));
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Serialize_PreservesBasicExpression()
-        {
-            var token = Expr("runner.os");
-            string yaml = TemplateTokenYamlAdapter.Serialize(token, 0);
-            Assert.Contains("${{ runner.os }}", yaml);
-            Assert.DoesNotContain("Linux", yaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Serialize_PreservesCompositeExpressionInStringToken()
-        {
-            // A StringToken constructed directly with the literal text
-            // round-trips unchanged. (The workflow parser does NOT produce
-            // a StringToken for this input — see
-            // Serialize_ReversesFormatRewriteForCompositeExpression — but
-            // direct StringToken construction must still preserve the
-            // literal verbatim.)
-            var token = Str("${{ runner.os }}-primes");
-            string yaml = TemplateTokenYamlAdapter.Serialize(token, 0);
-            Assert.Contains("${{ runner.os }}-primes", yaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Serialize_ReversesFormatRewriteForCompositeExpression()
-        {
-            // The workflow parser tokenizes a mixed scalar like
-            // `${{ runner.os }}-primes` as a single BasicExpressionToken
-            // whose internal expression is `format('{0}-primes', runner.os)`.
-            // The adapter must surface the author-facing form, not the
-            // parser's normalized rewrite.
-            var token = Expr("format('{0}-primes', runner.os)");
-            string yaml = TemplateTokenYamlAdapter.Serialize(token, 0);
-            Assert.Contains("${{ runner.os }}-primes", yaml);
-            Assert.DoesNotContain("format(", yaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Serialize_NestedMapping()
-        {
-            var inner = new MappingToken(null, null, null);
-            inner.Add(Str("b"), Num(1));
-            inner.Add(Str("c"), Expr("x"));
-            var outer = new MappingToken(null, null, null);
-            outer.Add(Str("a"), inner);
-
-            string yaml = TemplateTokenYamlAdapter.Serialize(outer, 0);
-
-            Assert.Contains("a:", yaml);
-            Assert.Contains("b: 1", yaml);
-            Assert.Contains("c: ${{ x }}", yaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Serialize_EmptyMapping()
-        {
-            var token = new MappingToken(null, null, null);
-            string yaml = TemplateTokenYamlAdapter.Serialize(token, 0);
-            Assert.Equal("{}", yaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Serialize_EmptySequence()
-        {
-            var token = new SequenceToken(null, null, null);
-            string yaml = TemplateTokenYamlAdapter.Serialize(token, 0);
-            Assert.Equal("[]", yaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Serialize_MultilineString_UsesBlockScalar()
-        {
-            var token = Str("line1\nline2\nline3");
-            string yaml = TemplateTokenYamlAdapter.Serialize(token, 0);
-            // Block-literal indicator `|` appears for multi-line scalars.
-            Assert.Contains("|", yaml);
-            Assert.Contains("line1", yaml);
-            Assert.Contains("line2", yaml);
-            Assert.Contains("line3", yaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Serialize_IndentLevel_PrefixesNonEmptyLines()
-        {
-            var map = new MappingToken(null, null, null);
-            map.Add(Str("k1"), Str("v1"));
-            map.Add(Str("k2"), Str("v2"));
-
-            string yaml = TemplateTokenYamlAdapter.Serialize(map, indentSpaces: 4);
-
-            foreach (var line in yaml.Split('\n'))
-            {
-                if (line.Length > 0)
-                {
-                    Assert.StartsWith("    ", line);
-                }
-            }
-            Assert.Contains("k1: v1", yaml);
-            Assert.Contains("k2: v2", yaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Serialize_NoTrailingNewline()
-        {
-            var token = Str("hello");
-            string yaml = TemplateTokenYamlAdapter.Serialize(token, 0);
-            Assert.False(yaml.EndsWith("\n"));
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Serialize_AlwaysUsesLfLineBreaks()
-        {
-            // Regression: YamlDotNet's Emitter calls WriteLine, which on
-            // Windows produces CRLF (the host's Environment.NewLine).
-            // Serialize must force LF so the rendered view round-trips
-            // regardless of platform.
-            var map = new MappingToken(null, null, null);
-            map.Add(Str("k1"), Str("v1"));
-            map.Add(Str("k2"), Num(2));
-            map.Add(Str("k3"), Bool(true));
-            string yaml = TemplateTokenYamlAdapter.Serialize(map, indentSpaces: 2);
-            Assert.DoesNotContain("\r", yaml);
-        }
-    }
-}

src/dev.sh

@@ -17,7 +17,7 @@ LAYOUT_DIR="$SCRIPT_DIR/../_layout"
 DOWNLOAD_DIR="$SCRIPT_DIR/../_downloads/netcore2x"
 PACKAGE_DIR="$SCRIPT_DIR/../_package"
 DOTNETSDK_ROOT="$SCRIPT_DIR/../_dotnetsdk"
-DOTNETSDK_VERSION="8.0.420"
+DOTNETSDK_VERSION="8.0.421"
 DOTNETSDK_INSTALLDIR="$DOTNETSDK_ROOT/$DOTNETSDK_VERSION"
 RUNNER_VERSION=$(cat runnerversion)
 

src/global.json

@@ -1,5 +1,5 @@
 {
   "sdk": {
-    "version": "8.0.420"
+    "version": "8.0.421"
   }
 }

src/runnerversion

@@ -1 +1 @@
-2.334.0
+2.335.1