Add support for $GITHUB_ARTIFACTS environment files (#4527)

Signed-off-by: Brian DeHamer <bdehamer@github.com>
Co-authored-by: Luke Tomlinson <luketomlinson@github.com>

.devcontainer/devcontainer.json

@@ -4,7 +4,7 @@
   "features": {
     "ghcr.io/devcontainers/features/docker-in-docker:2": {},
     "ghcr.io/devcontainers/features/dotnet": {
-      "version": "8.0.421"
+      "version": "8.0.422"
     },
     "ghcr.io/devcontainers/features/node:1": {
       "version": "20"

.github/workflows/build.yml

@@ -53,7 +53,7 @@ jobs:
 
     runs-on: ${{ matrix.os }}
     steps:
-    - uses: actions/checkout@v6
+    - uses: actions/checkout@v7
 
     # Build runner layout
     - name: Build & Layout Release
@@ -95,7 +95,7 @@ jobs:
             docker_platform: linux/arm64
     runs-on: ${{ matrix.os }}
     steps:
-    - uses: actions/checkout@v6
+    - uses: actions/checkout@v7
 
     - name: Get latest runner version
       id: latest_runner

.github/workflows/codeql.yml

@@ -23,7 +23,7 @@ jobs:
 
     steps:
     - name: Checkout repository
-      uses: actions/checkout@v6
+      uses: actions/checkout@v7
 
     # Initializes the CodeQL tools for scanning.
     - name: Initialize CodeQL

.github/workflows/dependency-check.yml

@@ -29,7 +29,7 @@ jobs:
       npm-vulnerabilities: ${{ steps.check-versions.outputs.npm-vulnerabilities }}
       open-dependency-prs: ${{ steps.check-prs.outputs.open-dependency-prs }}
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@v7
       - name: Setup Node.js
         uses: actions/setup-node@v6
         with:

.github/workflows/docker-buildx-upgrade.yml

@@ -17,7 +17,7 @@ jobs:
       BUILDX_CURRENT_VERSION: ${{ steps.check_buildx_version.outputs.CURRENT_VERSION }}
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v6
+        uses: actions/checkout@v7
 
       - name: Check Docker version
         id: check_docker_version
@@ -89,7 +89,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v6
+        uses: actions/checkout@v7
 
       - name: Update Docker version
         shell: bash

.github/workflows/docker-publish.yml

@@ -20,7 +20,7 @@ jobs:
       IMAGE_NAME: ${{ github.repository_owner }}/actions-runner
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v6
+        uses: actions/checkout@v7
         with:
           ref: ${{ github.event.inputs.releaseBranch }}
 

.github/workflows/dotnet-upgrade.yml

@@ -15,7 +15,7 @@ jobs:
       DOTNET_CURRENT_MAJOR_MINOR_VERSION: ${{ steps.fetch_current_version.outputs.DOTNET_CURRENT_MAJOR_MINOR_VERSION }}
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v6
+        uses: actions/checkout@v7
       - name: Get current major minor version
         id: fetch_current_version
         shell: bash
@@ -89,7 +89,7 @@ jobs:
     if: ${{ needs.dotnet-update.outputs.SHOULD_UPDATE == 1 && needs.dotnet-update.outputs.BRANCH_EXISTS == 0 }}
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@v7
         with:
           ref: feature/dotnetsdk-upgrade/${{ needs.dotnet-update.outputs.DOTNET_LATEST_MAJOR_MINOR_PATCH_VERSION }}
       - name: Create Pull Request

.github/workflows/node-upgrade.yml

@@ -9,7 +9,7 @@ jobs:
   update-node:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@v7
       - name: Get latest Node versions
         id: node-versions
         run: |

.github/workflows/npm-audit-typescript.yml

@@ -7,7 +7,7 @@ jobs:
   npm-audit-with-ts-fix:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@v7
       - name: Setup Node.js
         uses: actions/setup-node@v6
         with:

.github/workflows/npm-audit.yml

@@ -9,7 +9,7 @@ jobs:
   npm-audit:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@v7
       
       - name: Setup Node.js
         uses: actions/setup-node@v6

.github/workflows/release.yml

@@ -11,7 +11,7 @@ jobs:
     if: startsWith(github.ref, 'refs/heads/releases/') || github.ref == 'refs/heads/main'
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v6
+    - uses: actions/checkout@v7
 
     # Make sure ./releaseVersion match ./src/runnerversion
     # Query GitHub release ensure version is not used
@@ -86,7 +86,7 @@ jobs:
 
     runs-on: ${{ matrix.os }}
     steps:
-    - uses: actions/checkout@v6
+    - uses: actions/checkout@v7
 
     # Build runner layout
     - name: Build & Layout Release
@@ -129,7 +129,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
 
-    - uses: actions/checkout@v6
+    - uses: actions/checkout@v7
 
     # Download runner package tar.gz/zip produced by 'build' job
     - name: Download Artifact (win-x64)
@@ -296,7 +296,7 @@ jobs:
       IMAGE_NAME: ${{ github.repository_owner }}/actions-runner
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v6
+        uses: actions/checkout@v7
 
       - name: Compute image version
         id: image

docs/start/envlinux.md

@@ -8,7 +8,7 @@ Please see "[Supported architectures and operating systems for self-hosted runne
 
 ## Install .Net Core 3.x Linux Dependencies
 
-The `./config.sh` will check .Net Core 3.x dependencies during runner configuration.  
+The [config.sh](../../src/Misc/layoutroot/config.sh) will check .Net Core 3.x dependencies during runner configuration.  
 You might see something like this which indicate a dependency's missing.
 ```bash
 ./config.sh
@@ -17,7 +17,7 @@ You might see something like this which indicate a dependency's missing.
 Dependencies is missing for Dotnet Core 6.0
 Execute ./bin/installdependencies.sh to install any missing Dotnet Core 6.0 dependencies.
 ```
-You can easily correct the problem by executing `./bin/installdependencies.sh`.  
+You can easily correct the problem by executing [installdependencies.sh](../../src/Misc/layoutbin/installdependencies.sh).  
 The `installdependencies.sh` script should install all required dependencies on all supported Linux versions  
 > Note: The `installdependencies.sh` script will try to use the default package management mechanism on your Linux flavor (ex. `yum`/`apt-get`/`apt`).
 

images/Dockerfile

@@ -5,8 +5,8 @@ ARG TARGETOS
 ARG TARGETARCH
 ARG RUNNER_VERSION
 ARG RUNNER_CONTAINER_HOOKS_VERSION=0.7.0
-ARG DOCKER_VERSION=29.5.0
+ARG DOCKER_VERSION=29.6.1
-ARG BUILDX_VERSION=0.34.0
+ARG BUILDX_VERSION=0.35.0
 
 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
+* Bump System.ServiceProcess.ServiceController from 10.0.6 to 10.0.7 by @dependabot[bot] in https://github.com/actions/runner/pull/4370
-* Add DAP server by @rentziass in https://github.com/actions/runner/pull/4298
+* 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
-* 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
+* feat: propagate actions dependencies by @nodeselector in https://github.com/actions/runner/pull/4372
-* Remove AllowCaseFunction feature flag by @ericsciple in https://github.com/actions/runner/pull/4316
+* Not retry and report action download 403. by @TingluoHuang in https://github.com/actions/runner/pull/4391
-* chore: update Node versions by @github-actions[bot] in https://github.com/actions/runner/pull/4319
+* Update setup job starting logs by @GitPaulo in https://github.com/actions/runner/pull/4383
-* Batch and deduplicate action resolution across composite depths by @stefanpenner in https://github.com/actions/runner/pull/4296
+* fix: expand commit hash regex to support SHA-256 (64-char) hashes by @yaananth in https://github.com/actions/runner/pull/4347
-* Add support for Bearer token in action archive downloads by @TingluoHuang in https://github.com/actions/runner/pull/4321
+* Move dap setup to setup job step by @rentziass in https://github.com/actions/runner/pull/4403
-* Bump brace-expansion in /src/Misc/expressionFunc/hashFiles by @dependabot[bot] in https://github.com/actions/runner/pull/4318
+* Add support for Ubuntu 26.04 (liblttng-ust1t64, libicu77-80) by @dvaldivia in https://github.com/actions/runner/pull/4394
-* Add devtunnel connection for debugger jobs by @rentziass in https://github.com/actions/runner/pull/4317
+* Update dotnet sdk to latest version @8.0.421 by @github-actions[bot] in https://github.com/actions/runner/pull/4428
-* Update Docker to v29.3.1 and Buildx to v0.33.0 by @github-actions[bot] in https://github.com/actions/runner/pull/4324
+* Update Docker to v29.5.0 and Buildx to v0.34.0 by @github-actions[bot] in https://github.com/actions/runner/pull/4425
-* 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
+* Execute debugger REPL commands inside job container by @rentziass in https://github.com/actions/runner/pull/4420
-* Bump actions/github-script from 8 to 9 by @dependabot[bot] in https://github.com/actions/runner/pull/4331
+* Send welcome message in debugger console on connect by @rentziass in https://github.com/actions/runner/pull/4419
-* 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
+* Update snapshot-if context and functions by @drielenr in https://github.com/actions/runner/pull/4443
-* fix: only show changed versions in node upgrade PR description by @salmanmkc in https://github.com/actions/runner/pull/4332
+* chore: update Node versions by @github-actions[bot] in https://github.com/actions/runner/pull/4452
-* 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
+* Allow disable node v8 maglev jit compiler on node24. by @TingluoHuang in https://github.com/actions/runner/pull/4447
-* feat: add `job.workflow_*` typed accessors to JobContext by @salmanmkc in https://github.com/actions/runner/pull/4335
+* Update Node 24 default date to June 16th, 2026 by @salmanmkc in https://github.com/actions/runner/pull/4462
-* Add WS bridge over DAP TCP server by @rentziass in https://github.com/actions/runner/pull/4328
+* Populate telemetry for non-action post-job steps by @drielenr in https://github.com/actions/runner/pull/4463
-* chore: update Node versions by @github-actions[bot] in https://github.com/actions/runner/pull/4355
+* Add SDK types and results plumbing for background step control by @lokesh755 in https://github.com/actions/runner/pull/4472
-* Bump Docker version to 29.4.0 by @Copilot in https://github.com/actions/runner/pull/4352
+* Add job execution view model by @rentziass in https://github.com/actions/runner/pull/4470
-* Update dotnet sdk to latest version @8.0.420 by @github-actions[bot] in https://github.com/actions/runner/pull/4356
+* Add thread-safety locks to StepsContext by @lokesh755 in https://github.com/actions/runner/pull/4475
-* 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
+* Add background step deferral infrastructure and metadata plumbing by @lokesh755 in https://github.com/actions/runner/pull/4479
-* Bump System.Formats.Asn1 and System.Security.Cryptography.Pkcs by @dependabot[bot] in https://github.com/actions/runner/pull/4362
+* Wire job execution view into DAP by @rentziass in https://github.com/actions/runner/pull/4471
-* Add vulnerability-alerts permission by @salmanmkc in https://github.com/actions/runner/pull/4350
+* Background steps execution engine by @lokesh755 in https://github.com/actions/runner/pull/4476
-* 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
+* Update Docker to v29.5.2 and Buildx to v0.34.1 by @github-actions[bot] in https://github.com/actions/runner/pull/4451
-* Bump System.ServiceProcess.ServiceController from 10.0.3 to 10.0.6 by @dependabot[bot] in https://github.com/actions/runner/pull/4358
+* BrokerServer should not retry on 401. by @TingluoHuang in https://github.com/actions/runner/pull/4445
-* 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
+* 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 1.3.16 to 1.3.39 by @dependabot[bot] in https://github.com/actions/runner/pull/4339
+* 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.

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="24.16.0"
+NODE24_VERSION="24.18.0"
 
 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

@@ -180,6 +180,8 @@ namespace GitHub.Runner.Common
                 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";
+                public static readonly string AllowArtifactsFile = "actions_runner_allow_artifacts_file";
+                public static readonly string SelfRepository = "actions_self_repository";
             }
 
             // Node version migration related constants
@@ -206,7 +208,7 @@ 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
@@ -227,6 +229,12 @@ namespace GitHub.Runner.Common
             public static readonly string UnsupportedStopCommandTokenDisabled = "You cannot use a endToken that is an empty string, the string 'pause-logging', or another workflow command. For more information see: https://docs.github.com/actions/learn-github-actions/workflow-commands-for-github-actions#example-stopping-and-starting-workflow-commands or opt into insecure command execution by setting the `ACTIONS_ALLOW_UNSECURE_STOPCOMMAND_TOKENS` environment variable to `true`.";
             public static readonly string UnsupportedSummarySize = "$GITHUB_STEP_SUMMARY upload aborted, supports content up to a size of {0}k, got {1}k. For more information see: https://docs.github.com/actions/using-workflows/workflow-commands-for-github-actions#adding-a-markdown-summary";
             public static readonly string SummaryUploadError = "$GITHUB_STEP_SUMMARY upload aborted, an error occurred when uploading the summary. For more information see: https://docs.github.com/actions/using-workflows/workflow-commands-for-github-actions#adding-a-markdown-summary";
+
+            // $GITHUB_ARTIFACTS file command
+            public static readonly string ArtifactsFileSizeExceeded = "$GITHUB_ARTIFACTS file exceeds the maximum size of {0} KiB (got {1} KiB).";
+            public static readonly string ArtifactsAggregateLimitExceeded = "The job has exceeded the maximum of {0} declared artifacts.";
+            public static readonly string ArtifactsInvalidLine = "Invalid $GITHUB_ARTIFACTS entry on line {0}: {1}";
+            public static readonly string ArtifactsConflictingDigest = "Conflicting digest for artifact '{0}': previously declared as '{1}', now declared as '{2}'.";
         }
 
         public static class RunnerEvent
@@ -308,6 +316,7 @@ 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/ExtensionManager.cs

@@ -62,6 +62,8 @@ namespace GitHub.Runner.Common
                     Add<T>(extensions, "GitHub.Runner.Worker.CreateStepSummaryCommand, Runner.Worker");
                     Add<T>(extensions, "GitHub.Runner.Worker.SaveStateFileCommand, Runner.Worker");
                     Add<T>(extensions, "GitHub.Runner.Worker.SetOutputFileCommand, Runner.Worker");
+                    Add<T>(extensions, "GitHub.Runner.Worker.CreateArtifactsFileCommand, Runner.Worker");
+                    Add<T>(extensions, "GitHub.Runner.Worker.ArtifactsListFileCommand, Runner.Worker");
                     break;
                 case "GitHub.Runner.Listener.Check.ICheckExtension":
                     Add<T>(extensions, "GitHub.Runner.Listener.Check.InternetCheck, Runner.Listener");

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);
+                if (!this._disablePrefixMultilineLogs)
-                foreach (var messageLine in messageLines)
+                {
+                    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;
+            if (context.DeferredEnvironmentVariables != null)
-            context.SetEnvContext(envName, command.Data);
+            {
+                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);
+            if (context.DeferredOutputs != null)
-            context.Debug($"{reference}='{command.Data}'");
+            {
+                context.DeferredOutputs[outputName] = command.Data;
+            }
+            else
+            {
+                context.SetOutput(outputName, command.Data, out var reference);
+                context.Debug($"{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));
+            if (context.DeferredPrependPath != null)
-            context.Global.PrependPath.Add(command.Data);
+            {
+                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/ActionManager.cs

@@ -178,7 +178,7 @@ namespace GitHub.Runner.Worker
             return new PrepareResult(containerSetupSteps, result.PreStepTracker);
         }
 
-        private async Task<PrepareActionsState> PrepareActionsRecursiveAsync(IExecutionContext executionContext, PrepareActionsState state, IEnumerable<Pipelines.ActionStep> actions, Dictionary<string, WebApi.ActionDownloadInfo> resolvedDownloadInfos, Int32 depth = 0, Guid parentStepId = default(Guid))
+        private async Task<PrepareActionsState> PrepareActionsRecursiveAsync(IExecutionContext executionContext, PrepareActionsState state, IEnumerable<Pipelines.ActionStep> actions, Dictionary<string, WebApi.ActionDownloadInfo> resolvedDownloadInfos, Int32 depth = 0, Guid parentStepId = default(Guid), string selfRepoName = null, string selfRepoRef = null)
         {
             ArgUtil.NotNull(executionContext, nameof(executionContext));
             if (depth > Constants.CompositeActionsMaxDepth)
@@ -186,6 +186,21 @@ namespace GitHub.Runner.Worker
                 throw new Exception($"Composite action depth exceeded max depth {Constants.CompositeActionsMaxDepth}");
             }
 
+            // Resolve self-repository ($/) references before processing
+            if (executionContext.Global.Variables.GetBoolean(Constants.Runner.Features.SelfRepository) == true)
+            {
+                if (string.IsNullOrEmpty(selfRepoName))
+                {
+                    // job.workflow_repository/workflow_sha point to the repo
+                    // containing the workflow file — correct for both regular
+                    // and reusable workflows. Always present when the server
+                    // supports $/. See: https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/accessing-contextual-information-about-workflow-runs#github-context
+                    selfRepoName = executionContext.JobContext?.WorkflowRepository;
+                    selfRepoRef = executionContext.JobContext?.WorkflowSha;
+                }
+                ResolveSelfRepositoryReferences(executionContext, actions, selfRepoName, selfRepoRef);
+            }
+
             var repositoryActions = new List<Pipelines.ActionStep>();
 
             foreach (var action in actions)
@@ -228,7 +243,30 @@ namespace GitHub.Runner.Worker
                     {
                         throw new Exception($"Missing download info for {lookupKey}");
                     }
-                    await DownloadRepositoryActionAsync(executionContext, downloadInfo);
+
+                    Exception downloadFailure = null;
+                    try
+                    {
+                        await DownloadRepositoryActionAsync(executionContext, downloadInfo);
+                    }
+                    catch (Exception ex)
+                    {
+                        // record the exception for telemetry, and rethrow the original exception to fail the step.
+                        downloadFailure = ex;
+                        throw;
+                    }
+                    finally
+                    {
+                        executionContext.Global.JobTelemetry.Add(new JobTelemetry()
+                        {
+                            Type = JobTelemetryType.General,
+                            Message = $"resolve_download_actions_telemetry:{StringUtil.ConvertToJson(new ActionTelemetryPayload
+                            {
+                                Operation = "download_action",
+                                Result = downloadFailure == null ? "succeeded" : downloadFailure.GetType().Name
+                            }, Newtonsoft.Json.Formatting.None)}"
+                        });
+                    }
                 }
 
                 // Parse action.yml and collect composite sub-actions for batched
@@ -278,16 +316,53 @@ namespace GitHub.Runner.Worker
                 // then recurse per parent (which hits the cache, not the API).
                 if (nextLevel.Count > 0)
                 {
-                    var nextLevelRepoActions = nextLevel
+                    if (executionContext.Global.Variables.GetBoolean(Constants.Runner.Features.SelfRepository) == true)
-                        .Where(x => x.action.Reference.Type == Pipelines.ActionSourceType.Repository)
-                        .Select(x => x.action)
-                        .ToList();
-                    await ResolveNewActionsAsync(executionContext, nextLevelRepoActions, resolvedDownloadInfos);
-
-                    foreach (var group in nextLevel.GroupBy(x => x.parentId))
                     {
-                        var groupActions = group.Select(x => x.action).ToList();
+                        // Self-repository path: group by parent so each group's
-                        state = await PrepareActionsRecursiveAsync(executionContext, state, groupActions, resolvedDownloadInfos, depth + 1, group.Key);
+                        // $/ refs resolve against the correct parent repo context.
+                        var groups = nextLevel.GroupBy(x => x.parentId).Select(group =>
+                        {
+                            string childRepoName = selfRepoName;
+                            string childRepoRef = selfRepoRef;
+                            var parentAction = repositoryActions.FirstOrDefault(a => a.Id == group.Key);
+                            if (parentAction?.Reference is Pipelines.RepositoryPathReference parentRef &&
+                                string.Equals(parentRef.RepositoryType, Pipelines.RepositoryTypes.GitHub, StringComparison.OrdinalIgnoreCase))
+                            {
+                                childRepoName = parentRef.Name;
+                                childRepoRef = parentRef.Ref;
+                            }
+                            return new { ParentId = group.Key, Actions = group.Select(x => x.action).ToList(), RepoName = childRepoName, RepoRef = childRepoRef };
+                        }).ToList();
+
+                        foreach (var group in groups)
+                        {
+                            ResolveSelfRepositoryReferences(executionContext, group.Actions, group.RepoName, group.RepoRef);
+                        }
+
+                        var nextLevelRepoActions = nextLevel
+                            .Where(x => x.action.Reference.Type == Pipelines.ActionSourceType.Repository)
+                            .Select(x => x.action)
+                            .ToList();
+                        await ResolveNewActionsAsync(executionContext, nextLevelRepoActions, resolvedDownloadInfos);
+
+                        foreach (var group in groups)
+                        {
+                            state = await PrepareActionsRecursiveAsync(executionContext, state, group.Actions, resolvedDownloadInfos, depth + 1, group.ParentId, group.RepoName, group.RepoRef);
+                        }
+                    }
+                    else
+                    {
+                        // Original path: no self-repository resolution needed.
+                        var nextLevelActions = nextLevel.Select(x => x.action).ToList();
+                        var nextLevelRepoActions = nextLevelActions
+                            .Where(x => x.Reference.Type == Pipelines.ActionSourceType.Repository)
+                            .ToList();
+                        await ResolveNewActionsAsync(executionContext, nextLevelRepoActions, resolvedDownloadInfos);
+
+                        foreach (var grp in nextLevel.GroupBy(x => x.parentId))
+                        {
+                            state = await PrepareActionsRecursiveAsync(executionContext, state, grp.Select(x => x.action).ToList(), resolvedDownloadInfos, depth + 1, grp.Key);
+                        }
                     }
                 }
 
@@ -363,13 +438,25 @@ namespace GitHub.Runner.Worker
         /// sub-actions individually, with no cross-depth deduplication.
         /// Used when the BatchActionResolution feature flag is disabled.
         /// </summary>
-        private async Task<PrepareActionsState> PrepareActionsRecursiveLegacyAsync(IExecutionContext executionContext, PrepareActionsState state, IEnumerable<Pipelines.ActionStep> actions, Int32 depth = 0, Guid parentStepId = default(Guid))
+        private async Task<PrepareActionsState> PrepareActionsRecursiveLegacyAsync(IExecutionContext executionContext, PrepareActionsState state, IEnumerable<Pipelines.ActionStep> actions, Int32 depth = 0, Guid parentStepId = default(Guid), string selfRepoName = null, string selfRepoRef = null)
         {
             ArgUtil.NotNull(executionContext, nameof(executionContext));
             if (depth > Constants.CompositeActionsMaxDepth)
             {
                 throw new Exception($"Composite action depth exceeded max depth {Constants.CompositeActionsMaxDepth}");
             }
+
+            // Resolve self-repository ($/) references before processing
+            if (executionContext.Global.Variables.GetBoolean(Constants.Runner.Features.SelfRepository) == true)
+            {
+                if (string.IsNullOrEmpty(selfRepoName))
+                {
+                    selfRepoName = executionContext.JobContext?.WorkflowRepository;
+                    selfRepoRef = executionContext.JobContext?.WorkflowSha;
+                }
+                ResolveSelfRepositoryReferences(executionContext, actions, selfRepoName, selfRepoRef);
+            }
+
             var repositoryActions = new List<Pipelines.ActionStep>();
 
             foreach (var action in actions)
@@ -398,7 +485,30 @@ namespace GitHub.Runner.Worker
             if (repositoryActions.Count > 0)
             {
                 // Get the download info
-                var downloadInfos = await GetDownloadInfoAsync(executionContext, repositoryActions);
+                IDictionary<string, WebApi.ActionDownloadInfo> downloadInfos = null;
+                Exception resolveFailure = null;
+                try
+                {
+                    downloadInfos = await GetDownloadInfoAsync(executionContext, repositoryActions);
+                }
+                catch (Exception ex)
+                {
+                    // record the exception for telemetry, and rethrow the original exception to fail the step.
+                    resolveFailure = ex;
+                    throw;
+                }
+                finally
+                {
+                    executionContext.Global.JobTelemetry.Add(new JobTelemetry()
+                    {
+                        Type = JobTelemetryType.General,
+                        Message = $"resolve_download_actions_telemetry:{StringUtil.ConvertToJson(new ActionTelemetryPayload
+                        {
+                            Operation = "resolve_actions",
+                            Result = resolveFailure == null ? "succeeded" : resolveFailure.GetType().Name
+                        }, Newtonsoft.Json.Formatting.None)}"
+                    });
+                }
 
                 // Download each action
                 foreach (var action in repositoryActions)
@@ -414,7 +524,29 @@ namespace GitHub.Runner.Worker
                         throw new Exception($"Missing download info for {lookupKey}");
                     }
 
-                    await DownloadRepositoryActionAsync(executionContext, downloadInfo);
+                    Exception downloadFailure = null;
+                    try
+                    {
+                        await DownloadRepositoryActionAsync(executionContext, downloadInfo);
+                    }
+                    catch (Exception ex)
+                    {
+                        // record the exception for telemetry, and rethrow the original exception to fail the step.
+                        downloadFailure = ex;
+                        throw;
+                    }
+                    finally
+                    {
+                        executionContext.Global.JobTelemetry.Add(new JobTelemetry()
+                        {
+                            Type = JobTelemetryType.General,
+                            Message = $"resolve_download_actions_telemetry:{StringUtil.ConvertToJson(new ActionTelemetryPayload
+                            {
+                                Operation = "download_action",
+                                Result = downloadFailure == null ? "succeeded" : downloadFailure.GetType().Name
+                            }, Newtonsoft.Json.Formatting.None)}"
+                        });
+                    }
                 }
 
                 // More preparation based on content in the repository (action.yml)
@@ -449,7 +581,17 @@ namespace GitHub.Runner.Worker
                     }
                     else if (setupInfo != null && setupInfo.Steps != null && setupInfo.Steps.Count > 0)
                     {
-                        state = await PrepareActionsRecursiveLegacyAsync(executionContext, state, setupInfo.Steps, depth + 1, action.Id);
+                        // Propagate parent's repo context for nested self-repository resolution
+                        var parentRef = action.Reference as Pipelines.RepositoryPathReference;
+                        var childRepoName = selfRepoName;
+                        var childRepoRef = selfRepoRef;
+                        if (parentRef != null &&
+                            string.Equals(parentRef.RepositoryType, Pipelines.RepositoryTypes.GitHub, StringComparison.OrdinalIgnoreCase))
+                        {
+                            childRepoName = parentRef.Name;
+                            childRepoRef = parentRef.Ref;
+                        }
+                        state = await PrepareActionsRecursiveLegacyAsync(executionContext, state, setupInfo.Steps, depth + 1, action.Id, childRepoName, childRepoRef);
                     }
                     var repoAction = action.Reference as Pipelines.RepositoryPathReference;
                     if (repoAction.RepositoryType != Pipelines.PipelineConstants.SelfAlias)
@@ -562,6 +704,12 @@ namespace GitHub.Runner.Worker
                         actionDirectory = Path.Combine(actionDirectory, repoAction.Path);
                     }
                 }
+                else if (string.Equals(repoAction.RepositoryType, Pipelines.PipelineConstants.SelfRepositoryAlias, StringComparison.OrdinalIgnoreCase))
+                {
+                    // Unresolved self-repository reference at load time — this
+                    // shouldn't happen but guard against NRE if it does.
+                    throw new InvalidOperationException($"Self-repository reference '$/{repoAction.Path}' was not resolved before LoadAction. Ensure the '{Constants.Runner.Features.SelfRepository}' feature flag is enabled.");
+                }
                 else
                 {
                     actionDirectory = Path.Combine(HostContext.GetDirectory(WellKnownDirectory.Actions), repoAction.Name.Replace(Path.AltDirectorySeparatorChar, Path.DirectorySeparatorChar), repoAction.Ref);
@@ -697,6 +845,27 @@ namespace GitHub.Runner.Worker
                                 _cachedEmbeddedStepIds[action.Id].Add(guid);
                             }
                         }
+
+                        // Resolve self-repository refs in composite steps at load time.
+                        // During setup, resolution happens on a separate copy of these
+                        // step objects. At runtime, action.yml is re-parsed, producing
+                        // fresh self-repository refs that need resolution here.
+                        // When the parent is a dot-slash (self local-workspace) action,
+                        // repoAction.Name/Ref are null — fall back to workflow context.
+                        if (executionContext.Global.Variables.GetBoolean(Constants.Runner.Features.SelfRepository) == true)
+                        {
+                            var parentName = repoAction.Name ?? executionContext.JobContext?.WorkflowRepository;
+                            var parentRef = repoAction.Ref ?? executionContext.JobContext?.WorkflowSha;
+                            ResolveSelfRepositoryReferences(executionContext, compositeAction.Steps, parentName, parentRef);
+                            if (compositeAction.PreSteps != null)
+                            {
+                                ResolveSelfRepositoryReferences(executionContext, compositeAction.PreSteps, parentName, parentRef);
+                            }
+                            if (compositeAction.PostSteps != null)
+                            {
+                                ResolveSelfRepositoryReferences(executionContext, compositeAction.PostSteps, parentName, parentRef);
+                            }
+                        }
                     }
                     else
                     {
@@ -980,10 +1149,33 @@ namespace GitHub.Runner.Worker
 
             if (actionsToResolve.Count > 0)
             {
-                var downloadInfos = await GetDownloadInfoAsync(executionContext, actionsToResolve);
+                IDictionary<string, WebApi.ActionDownloadInfo> downloadInfos = null;
-                foreach (var kvp in downloadInfos)
+                Exception resolveFailure = null;
+                try
                 {
-                    resolvedDownloadInfos[kvp.Key] = kvp.Value;
+                    downloadInfos = await GetDownloadInfoAsync(executionContext, actionsToResolve);
+                    foreach (var kvp in downloadInfos)
+                    {
+                        resolvedDownloadInfos[kvp.Key] = kvp.Value;
+                    }
+                }
+                catch (Exception ex)
+                {
+                    // record the exception for telemetry, and rethrow the original exception to fail the step.
+                    resolveFailure = ex;
+                    throw;
+                }
+                finally
+                {
+                    executionContext.Global.JobTelemetry.Add(new JobTelemetry()
+                    {
+                        Type = JobTelemetryType.General,
+                        Message = $"resolve_download_actions_telemetry:{StringUtil.ConvertToJson(new ActionTelemetryPayload
+                        {
+                            Operation = "resolve_actions",
+                            Result = resolveFailure == null ? "succeeded" : resolveFailure.GetType().Name
+                        }, Newtonsoft.Json.Formatting.None)}"
+                    });
                 }
             }
         }
@@ -1108,12 +1300,6 @@ namespace GitHub.Runner.Worker
                     }
                 }
 
-                executionContext.Global.JobTelemetry.Add(new JobTelemetry()
-                {
-                    Type = JobTelemetryType.General,
-                    Message = $"Action archive cache usage: {downloadInfo.ResolvedNameWithOwner}@{downloadInfo.ResolvedSha} use cache {useActionArchiveCache} has cache {hasActionArchiveCache}"
-                });
-
                 if (!useActionArchiveCache)
                 {
                     await DownloadRepositoryArchive(executionContext, link, downloadInfo.Authentication?.Token, archiveFile);
@@ -1122,6 +1308,13 @@ namespace GitHub.Runner.Worker
                 var stagingDirectory = Path.Combine(tempDirectory, "_staging");
                 Directory.CreateDirectory(stagingDirectory);
 
+                var fileInfo = new FileInfo(archiveFile);
+                executionContext.Global.JobTelemetry.Add(new JobTelemetry()
+                {
+                    Type = JobTelemetryType.General,
+                    Message = $"Action archive cache usage: {downloadInfo.ResolvedNameWithOwner}@{downloadInfo.ResolvedSha} use cache {useActionArchiveCache} has cache {hasActionArchiveCache} size {fileInfo.Length} bytes"
+                });
+
 #if OS_WINDOWS
                 try
                 {
@@ -1159,7 +1352,6 @@ namespace GitHub.Runner.Worker
                     int exitCode = await processInvoker.ExecuteAsync(stagingDirectory, tar, $"-xzf \"{archiveFile}\"", null, executionContext.CancellationToken);
                     if (exitCode != 0)
                     {
-                        var fileInfo = new FileInfo(archiveFile);
                         var sha256hash = await IOUtil.GetFileContentSha256HashAsync(archiveFile);
                         throw new InvalidActionArchiveException($"Can't use 'tar -xzf' extract archive file: {archiveFile} (SHA256 '{sha256hash}', size '{fileInfo.Length}' bytes, tar outputs '{string.Join(' ', tarOutputs)}'). Action being checked out: {downloadInfo.NameWithOwner}@{downloadInfo.Ref}. return code: {exitCode}.");
                     }
@@ -1209,6 +1401,12 @@ namespace GitHub.Runner.Worker
 
         private string GetWatermarkFilePath(string directory) => directory + ".completed";
 
+        private sealed class ActionTelemetryPayload
+        {
+            public string Operation { get; set; }
+            public string Result { get; set; }
+        }
+
         private ActionSetupInfo PrepareRepositoryActionAsync(IExecutionContext executionContext, Pipelines.ActionStep repositoryAction)
         {
             var repositoryReference = repositoryAction.Reference as Pipelines.RepositoryPathReference;
@@ -1347,6 +1545,47 @@ namespace GitHub.Runner.Worker
             }
         }
 
+        /// <summary>
+        /// Resolves self-reference ($/) references by mutating them in-place
+        /// to standard GitHub repository references with the containing repo's
+        /// name and ref.
+        /// </summary>
+        private void ResolveSelfRepositoryReferences(IExecutionContext executionContext, IEnumerable<Pipelines.ActionStep> actions, string repoName, string repoRef)
+        {
+            if (string.IsNullOrEmpty(repoName) || string.IsNullOrEmpty(repoRef))
+            {
+                return;
+            }
+
+            foreach (var action in actions)
+            {
+                if (action.Reference.Type != Pipelines.ActionSourceType.Repository)
+                {
+                    continue;
+                }
+
+                var repoAction = action.Reference as Pipelines.RepositoryPathReference;
+                if (!string.Equals(repoAction.RepositoryType, Pipelines.PipelineConstants.SelfRepositoryAlias, StringComparison.OrdinalIgnoreCase))
+                {
+                    continue;
+                }
+
+                Trace.Info($"Resolving self-repository reference reference '$/{repoAction.Path}' to '{repoName}/{repoAction.Path}@{repoRef}'");
+                executionContext.Debug($"Resolving $/{repoAction.Path} → {repoName}/{repoAction.Path}@{repoRef}");
+
+                repoAction.RepositoryType = Pipelines.RepositoryTypes.GitHub;
+                repoAction.Name = repoName;
+                repoAction.Ref = repoRef;
+            }
+        }
+
+        /// <summary>
+        /// If this is a reusable workflow job, ensure the workflow repo tarball
+        /// is downloaded so self.workspace resolves to a real path on disk.
+        /// Always downloads for reusable workflows when the feature flag is on,
+        /// since step expressions are already expanded by the server and can't
+        /// be scanned for self.* usage.
+        /// </summary>
         private static string GetDownloadInfoLookupKey(Pipelines.ActionStep action)
         {
             if (action.Reference.Type != Pipelines.ActionSourceType.Repository)
@@ -1362,6 +1601,11 @@ namespace GitHub.Runner.Worker
                 return null;
             }
 
+            if (string.Equals(repositoryReference.RepositoryType, Pipelines.PipelineConstants.SelfRepositoryAlias, StringComparison.OrdinalIgnoreCase))
+            {
+                throw new InvalidOperationException($"Unable to resolve self-reference '$/'. This can occur when the server does not support this syntax, the feature flag is disabled, or the workflow context (repository/SHA) is unavailable.");
+            }
+
             if (!string.Equals(repositoryReference.RepositoryType, Pipelines.RepositoryTypes.GitHub, StringComparison.OrdinalIgnoreCase))
             {
                 throw new NotSupportedException(repositoryReference.RepositoryType);

src/Runner.Worker/ArtifactSubject.cs

@@ -0,0 +1,52 @@
+using System;
+
+namespace GitHub.Runner.Worker
+{
+    public enum ArtifactSubjectKind
+    {
+        File,
+        OciSubject,
+    }
+
+    /// <summary>
+    /// Represents a single artifact subject declared via the
+    /// <c>GITHUB_ARTIFACTS</c> per-step environment file.
+    /// </summary>
+    public sealed class ArtifactSubject : IEquatable<ArtifactSubject>
+    {
+        public ArtifactSubject(string name, string digest, ArtifactSubjectKind kind)
+        {
+            if (string.IsNullOrEmpty(name))
+            {
+                throw new ArgumentException("Name must not be null or empty.", nameof(name));
+            }
+            if (string.IsNullOrEmpty(digest))
+            {
+                throw new ArgumentException("Digest must not be null or empty.", nameof(digest));
+            }
+            Name = name;
+            Digest = digest;
+            Kind = kind;
+        }
+
+        public string Name { get; }
+        public string Digest { get; }
+        public ArtifactSubjectKind Kind { get; }
+
+        public bool Equals(ArtifactSubject other)
+        {
+            if (other is null)
+            {
+                return false;
+            }
+            return string.Equals(Name, other.Name, StringComparison.Ordinal)
+                && string.Equals(Digest, other.Digest, StringComparison.Ordinal);
+        }
+
+        public override bool Equals(object obj) => Equals(obj as ArtifactSubject);
+
+        public override int GetHashCode() => HashCode.Combine(Name, Digest);
+
+        public override string ToString() => $"{Name}@{Digest}";
+    }
+}

src/Runner.Worker/ArtifactsListFileCommand.cs

@@ -0,0 +1,92 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+using GitHub.Runner.Common;
+using GitHub.Runner.Sdk;
+using GitHub.Runner.Worker.Container;
+using Newtonsoft.Json;
+using Newtonsoft.Json.Linq;
+
+namespace GitHub.Runner.Worker
+{
+    /// <summary>
+    /// File command extension that exposes the job-scoped aggregate of
+    /// <see cref="GlobalContext.ArtifactSubjects"/> as a read-only JSON
+    /// file. Subsequent steps in the same job read the file via the
+    /// <c>GITHUB_ARTIFACTS_LIST</c> environment variable, getting a
+    /// running view of every artifact declared via
+    /// <c>$GITHUB_ARTIFACTS</c> in earlier steps.
+    /// </summary>
+    /// <remarks>
+    /// The file uses the existing per-step file-command lifecycle:
+    /// <see cref="FileCommandManager.InitializeFiles"/> creates a fresh
+    /// file, invokes <see cref="PopulateInitialContents"/> here, exposes
+    /// the (translated) path to the step's environment, and (for the
+    /// read-only file) ignores anything the step writes back.
+    ///
+    /// The file is always written when the feature is enabled, so
+    /// consumers never need to branch on "did the runner inject this?".
+    /// An empty aggregate produces <c>{"version":1,"subjects":[]}</c>.
+    /// </remarks>
+    public sealed class ArtifactsListFileCommand : RunnerService, IFileCommandExtension
+    {
+        public const int FormatVersion = 1;
+
+        public string ContextName => "artifacts_list";
+        public string FilePrefix => "artifacts_list_";
+
+        public Type ExtensionType => typeof(IFileCommandExtension);
+
+        public void PopulateInitialContents(IExecutionContext context, string filePath, ContainerInfo container)
+        {
+            ArgUtil.NotNull(context, nameof(context));
+
+            // Feature flag gate. Mirrors CreateArtifactsFileCommand so the
+            // write side and the read side are toggled together.
+            var enabled = (context.Global.Variables.GetBoolean(Constants.Runner.Features.AllowArtifactsFile) ?? false)
+                || StringUtil.ConvertToBoolean(Environment.GetEnvironmentVariable(CreateArtifactsFileCommand.EnableEnvVar));
+            if (!enabled)
+            {
+                Trace.Verbose("$GITHUB_ARTIFACTS_LIST publishing is disabled (feature flag and env-var fallback are both off).");
+                return;
+            }
+
+            var aggregate = context.Global.ArtifactSubjects
+                ?? new Dictionary<string, ArtifactSubject>(StringComparer.Ordinal);
+
+            var subjects = new JArray();
+            // Emit subjects sorted by name so the output is deterministic
+            // regardless of the backing dictionary's enumeration order
+            // (which is not contractually guaranteed).
+            foreach (var entry in aggregate.Values.OrderBy(v => v.Name, StringComparer.Ordinal))
+            {
+                subjects.Add(new JObject
+                {
+                    ["name"] = entry.Name,
+                    ["digest"] = entry.Digest,
+                    ["kind"] = entry.Kind == ArtifactSubjectKind.OciSubject ? "oci" : "file",
+                });
+            }
+
+            var payload = new JObject
+            {
+                ["version"] = FormatVersion,
+                ["subjects"] = subjects,
+            };
+
+            // UTF-8 without BOM; consumers in other languages should not
+            // have to special-case a leading BOM.
+            File.WriteAllText(filePath, payload.ToString(Formatting.None), new UTF8Encoding(false));
+            Trace.Info($"Wrote $GITHUB_ARTIFACTS_LIST with {aggregate.Count} subject(s) to '{filePath}'");
+        }
+
+        public void ProcessCommand(IExecutionContext context, string filePath, ContainerInfo container)
+        {
+            // Read-only file: anything the step writes here is ignored.
+            // The aggregate is fed only by the write-side $GITHUB_ARTIFACTS
+            // file processed by CreateArtifactsFileCommand.
+        }
+    }
+}

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/CreateArtifactsFileCommand.cs

@@ -0,0 +1,384 @@
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using System.IO;
+using System.Security.Cryptography;
+using System.Text;
+using System.Text.RegularExpressions;
+using GitHub.Runner.Common;
+using GitHub.Runner.Sdk;
+using GitHub.Runner.Worker.Container;
+
+namespace GitHub.Runner.Worker
+{
+    /// <summary>
+    /// File command extension that implements the <c>GITHUB_ARTIFACTS</c>
+    /// per-step environment file contract.
+    /// </summary>
+    /// <remarks>
+    /// Lifecycle is identical to the other per-step file commands:
+    /// <see cref="FileCommandManager"/> creates an empty file before each
+    /// step runs and invokes <see cref="ProcessCommand"/> after the step
+    /// completes. This class is responsible for parsing the file's
+    /// contents, validating each entry, and aggregating the resulting
+    /// (name, digest) pairs onto <see cref="GlobalContext.ArtifactSubjects"/>
+    /// at job scope.
+    ///
+    /// The feature is gated by the <c>actions_runner_allow_artifacts_file</c>
+    /// feature flag. When the flag is disabled, the env var is still
+    /// exposed but writes are silently ignored.
+    /// </remarks>
+    public sealed class CreateArtifactsFileCommand : RunnerService, IFileCommandExtension
+    {
+        // Each per-step file may contain at most 1 MiB.
+        public const int MaxFileSizeBytes = 1024 * 1024;
+
+        // A job may declare at most 500 artifacts in aggregate.
+        public const int MaxAggregateArtifacts = 500;
+
+        public string ContextName => "artifacts";
+        public string FilePrefix => "artifacts_";
+
+        // Runner-side environment variable that enables the feature on
+        // self-hosted runners where the server-side feature flag is not
+        // configurable. Mirrors patterns like
+        // ACTIONS_RUNNER_COMPARE_WORKFLOW_PARSER elsewhere in the runner.
+        public const string EnableEnvVar = "ACTIONS_RUNNER_ALLOW_ARTIFACTS_FILE";
+
+        public Type ExtensionType => typeof(IFileCommandExtension);
+
+        // Recognized scheme prefixes (case-insensitive).
+        private const string FileScheme = "file://";
+        private const string OciScheme = "oci://";
+
+        // Matches "<scheme>://...". Used to detect unsupported URI schemes.
+        // Scheme grammar per RFC 3986: ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
+        private static readonly Regex s_schemeRegex = new(
+            @"^[A-Za-z][A-Za-z0-9+.\-]*://",
+            RegexOptions.Compiled);
+
+        // Matches "<ref>@<algo>:<hex>" where <algo> is sha256/sha384/sha512.
+        // Hex length is validated separately so we can produce a precise error.
+        private static readonly Regex s_ociDigestSuffixRegex = new(
+            @"^(?<ref>.+)@(?<algo>sha(?:256|384|512)):(?<hex>[0-9a-fA-F]+)$",
+            RegexOptions.Compiled);
+
+        public void ProcessCommand(IExecutionContext context, string filePath, ContainerInfo container)
+        {
+            ArgUtil.NotNull(context, nameof(context));
+
+            // Feature flag gate. Enabled when either the server-side
+            // feature flag is set, or the runner is started with the
+            // ACTIONS_RUNNER_ALLOW_ARTIFACTS_FILE env var set to true
+            // (the env-var fallback exists so self-hosted runners can
+            // opt in locally). Silently no-op when disabled.
+            var enabled = (context.Global.Variables.GetBoolean(Constants.Runner.Features.AllowArtifactsFile) ?? false)
+                || StringUtil.ConvertToBoolean(Environment.GetEnvironmentVariable(EnableEnvVar));
+            if (!enabled)
+            {
+                Trace.Verbose("$GITHUB_ARTIFACTS processing is disabled (feature flag and env-var fallback are both off).");
+                return;
+            }
+
+            Trace.Info($"Processing $GITHUB_ARTIFACTS file '{filePath}'");
+
+            if (string.IsNullOrEmpty(filePath) || !File.Exists(filePath))
+            {
+                Trace.Info("$GITHUB_ARTIFACTS file does not exist; nothing to process.");
+                return;
+            }
+
+            var fileSize = new FileInfo(filePath).Length;
+            if (fileSize == 0)
+            {
+                Trace.Info("$GITHUB_ARTIFACTS file is empty; nothing to process.");
+                return;
+            }
+            if (fileSize > MaxFileSizeBytes)
+            {
+                throw new Exception(StringUtil.Format(
+                    Constants.Runner.ArtifactsFileSizeExceeded,
+                    MaxFileSizeBytes / 1024,
+                    fileSize / 1024));
+            }
+
+            // Per-step subjects parsed from this file; aggregated into the
+            // job-level set at the end so a single malformed line fails the
+            // step without partially polluting the aggregate.
+            var parsed = new List<(int LineNumber, ArtifactSubject Subject)>();
+
+            // Relative artifact paths are resolved against the workspace
+            // root (GITHUB_WORKSPACE), not the step's working directory.
+            // This matches the established runner precedent set by
+            // hashFiles() which always resolve relative paths against
+            // the workspace root regardless of any step-level
+            // `working-directory:`.
+            var workspaceRoot = ResolveWorkspaceRoot(context);
+
+            var lines = File.ReadAllLines(filePath, Encoding.UTF8);
+            for (var i = 0; i < lines.Length; i++)
+            {
+                var lineNumber = i + 1;
+                var raw = lines[i];
+                var trimmed = raw.Trim();
+                if (trimmed.Length == 0)
+                {
+                    continue;
+                }
+                if (trimmed[0] == '#')
+                {
+                    continue;
+                }
+
+                ArtifactSubject subject;
+                try
+                {
+                    subject = ParseLine(trimmed, workspaceRoot, container);
+                }
+                catch (ArtifactsParseException ex)
+                {
+                    throw new Exception(StringUtil.Format(
+                        Constants.Runner.ArtifactsInvalidLine,
+                        lineNumber,
+                        ex.Message));
+                }
+
+                parsed.Add((lineNumber, subject));
+            }
+
+            // Aggregate at job scope: dedup identical, reject conflicts,
+            // enforce the 500-artifact cap (after dedup so identical
+            // duplicates above the cap do not fail).
+            var aggregate = context.Global.ArtifactSubjects;
+            if (aggregate == null)
+            {
+                throw new InvalidOperationException("Global.ArtifactSubjects is not initialized.");
+            }
+
+            var addedThisStep = 0;
+            foreach (var (lineNumber, subject) in parsed)
+            {
+                if (aggregate.TryGetValue(subject.Name, out var existing))
+                {
+                    if (string.Equals(existing.Digest, subject.Digest, StringComparison.Ordinal))
+                    {
+                        // Identical declaration — silently deduplicate.
+                        Trace.Info($"Skipped duplicate artifact subject '{subject.Name}' (digest={subject.Digest})");
+                        continue;
+                    }
+                    throw new Exception(StringUtil.Format(
+                        Constants.Runner.ArtifactsInvalidLine,
+                        lineNumber,
+                        StringUtil.Format(
+                            Constants.Runner.ArtifactsConflictingDigest,
+                            subject.Name,
+                            existing.Digest,
+                            subject.Digest)));
+                }
+
+                if (aggregate.Count >= MaxAggregateArtifacts)
+                {
+                    throw new Exception(StringUtil.Format(
+                        Constants.Runner.ArtifactsInvalidLine,
+                        lineNumber,
+                        StringUtil.Format(
+                            Constants.Runner.ArtifactsAggregateLimitExceeded,
+                            MaxAggregateArtifacts)));
+                }
+
+                aggregate[subject.Name] = subject;
+                addedThisStep++;
+                Trace.Info($"Declared artifact subject '{subject.Name}' (kind={subject.Kind}, digest={subject.Digest})");
+                context.Debug($"Declared artifact subject '{subject.Name}' (kind={subject.Kind}, digest={subject.Digest})");
+            }
+
+            if (addedThisStep > 0)
+            {
+                // Mirror the existing file-command UX: a single, terse
+                // user-visible line that confirms the declarations landed.
+                context.Output($"Captured {addedThisStep} artifact subject(s) from this step (job total: {aggregate.Count}).");
+            }
+        }
+
+        private ArtifactSubject ParseLine(string trimmed, string workspaceRoot, ContainerInfo container)
+        {
+            // Reject lines containing '=' — reserved for a future v2
+            // key/value extension to the format.
+            if (trimmed.IndexOf('=') >= 0)
+            {
+                throw new ArtifactsParseException("entries containing '=' are reserved and not permitted");
+            }
+
+            // Handle the explicit escape-hatch schemes first
+            // (case-insensitive).
+            if (StartsWithIgnoreCase(trimmed, FileScheme))
+            {
+                var path = trimmed.Substring(FileScheme.Length);
+                if (string.IsNullOrWhiteSpace(path))
+                {
+                    throw new ArtifactsParseException("file:// entries must include a path");
+                }
+                return MakeFileSubject(path, workspaceRoot, container);
+            }
+            if (StartsWithIgnoreCase(trimmed, OciScheme))
+            {
+                var rest = trimmed.Substring(OciScheme.Length);
+                var match = s_ociDigestSuffixRegex.Match(rest);
+                if (!match.Success)
+                {
+                    throw new ArtifactsParseException("oci:// entries must include an @sha{256,384,512}:<hex> digest");
+                }
+                return MakeOciSubject(match);
+            }
+
+            // Reject any other URI scheme up-front.
+            if (s_schemeRegex.IsMatch(trimmed))
+            {
+                throw new ArtifactsParseException("unsupported URI scheme");
+            }
+
+            // Otherwise discriminate syntactically: an entry that matches
+            // the OCI digest suffix shape (with the right hex length for
+            // its algorithm) is an OCI subject; everything else is a path.
+            var ociMatch = s_ociDigestSuffixRegex.Match(trimmed);
+            if (ociMatch.Success && IsExpectedHexLength(ociMatch.Groups["algo"].Value, ociMatch.Groups["hex"].Value))
+            {
+                return MakeOciSubject(ociMatch);
+            }
+
+            return MakeFileSubject(trimmed, workspaceRoot, container);
+        }
+
+        private static ArtifactSubject MakeOciSubject(Match match)
+        {
+            var refName = match.Groups["ref"].Value;
+            var algo = match.Groups["algo"].Value.ToLowerInvariant();
+            var hex = match.Groups["hex"].Value.ToLowerInvariant();
+
+            if (!IsExpectedHexLength(algo, hex))
+            {
+                throw new ArtifactsParseException(
+                    $"digest '{algo}' must be {ExpectedHexLength(algo)} hex characters, got {hex.Length}");
+            }
+            if (string.IsNullOrEmpty(refName))
+            {
+                throw new ArtifactsParseException("oci subject must include a reference");
+            }
+
+            return new ArtifactSubject(refName, $"{algo}:{hex}", ArtifactSubjectKind.OciSubject);
+        }
+
+        private static ArtifactSubject MakeFileSubject(string declaredPath, string workspaceRoot, ContainerInfo container)
+        {
+            var hostPath = ResolveFilePath(declaredPath, workspaceRoot, container);
+
+            if (!File.Exists(hostPath))
+            {
+                if (Directory.Exists(hostPath))
+                {
+                    throw new ArtifactsParseException($"'{declaredPath}' is a directory, not a regular file");
+                }
+                // For relative paths, surface where we looked so authors
+                // aren't surprised that resolution is workspace-relative.
+                if (!Path.IsPathRooted(declaredPath))
+                {
+                    throw new ArtifactsParseException(
+                        $"file '{declaredPath}' does not exist (relative paths are resolved against the workspace root '{workspaceRoot}')");
+                }
+                throw new ArtifactsParseException($"file '{declaredPath}' does not exist");
+            }
+
+            // FileInfo + File.GetAttributes guards against named pipes,
+            // device files, etc. We accept regular files and symlinks
+            // resolved to regular files.
+            var attrs = File.GetAttributes(hostPath);
+            if ((attrs & FileAttributes.Directory) == FileAttributes.Directory)
+            {
+                throw new ArtifactsParseException($"'{declaredPath}' is a directory, not a regular file");
+            }
+
+            string hex;
+            using (var stream = File.OpenRead(hostPath))
+            using (var sha = SHA256.Create())
+            {
+                var hash = sha.ComputeHash(stream);
+                var sb = new StringBuilder(hash.Length * 2);
+                foreach (var b in hash)
+                {
+                    sb.Append(b.ToString("x2", CultureInfo.InvariantCulture));
+                }
+                hex = sb.ToString();
+            }
+
+            var name = Path.GetFileName(hostPath);
+            return new ArtifactSubject(name, $"sha256:{hex}", ArtifactSubjectKind.File);
+        }
+
+        private static string ResolveFilePath(string declaredPath, string workspaceRoot, ContainerInfo container)
+        {
+            if (Path.IsPathRooted(declaredPath))
+            {
+                if (container == null)
+                {
+                    return declaredPath;
+                }
+
+                // Absolute path from a container step: it lives in the
+                // container's filesystem namespace, so translate it to the
+                // host path via the container's volume mounts.
+                // TranslateToHostPath returns the input unchanged when the
+                // path is not under any mount. We must NOT fall back to the
+                // host file at that same path -- that would hash an arbitrary
+                // host file the container step never referenced -- so reject
+                // it instead.
+                var hostPath = container.TranslateToHostPath(declaredPath);
+                if (string.Equals(hostPath, declaredPath, StringComparison.Ordinal))
+                {
+                    throw new ArtifactsParseException(
+                        $"absolute path '{declaredPath}' is not inside a volume mounted into the container and cannot be resolved");
+                }
+                return hostPath;
+            }
+
+            // Relative path: resolve against the workspace root
+            // (GITHUB_WORKSPACE).
+            var baseDir = workspaceRoot ?? string.Empty;
+            return Path.GetFullPath(Path.Combine(baseDir, declaredPath));
+        }
+
+        private static string ResolveWorkspaceRoot(IExecutionContext context)
+        {
+            // The workspace root (GITHUB_WORKSPACE) is the resolution base
+            // for all relative artifact paths.
+            var workspace = context.GetGitHubContext("workspace");
+            return string.IsNullOrEmpty(workspace) ? null : workspace;
+        }
+
+        private static bool StartsWithIgnoreCase(string s, string prefix)
+        {
+            return s.StartsWith(prefix, StringComparison.OrdinalIgnoreCase);
+        }
+
+        private static int ExpectedHexLength(string algo)
+        {
+            return algo.ToLowerInvariant() switch
+            {
+                "sha256" => 64,
+                "sha384" => 96,
+                "sha512" => 128,
+                _ => -1,
+            };
+        }
+
+        private static bool IsExpectedHexLength(string algo, string hex)
+        {
+            var expected = ExpectedHexLength(algo);
+            return expected > 0 && hex.Length == expected;
+        }
+
+        private sealed class ArtifactsParseException : Exception
+        {
+            public ArtifactsParseException(string message) : base(message) { }
+        }
+    }
+}

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;
@@ -98,6 +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;
@@ -240,6 +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)
@@ -253,6 +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);
@@ -359,6 +544,7 @@ namespace GitHub.Runner.Worker.Dap
                 {
                     _state = DapSessionState.Terminated;
                 }
+                _jobStepsSource = null;
             }
 
             _isClientConnected = false;
@@ -417,7 +603,8 @@ namespace GitHub.Runner.Worker.Dap
                     {
                         DisplayName = step.DisplayName,
                         Result = result,
-                        FrameId = _nextCompletedFrameId++
+                        FrameId = _nextCompletedFrameId++,
+                        SourceLine = _jobStepsSource?.TryGetLineForStep(step)
                     });
                 }
             }
@@ -468,6 +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),
@@ -857,6 +1045,7 @@ namespace GitHub.Runner.Worker.Dap
         {
             bool pauseOnNextStep;
             CancellationToken cancellationToken;
+
             lock (_stateLock)
             {
                 if (_state != DapSessionState.Ready &&
@@ -868,6 +1057,7 @@ namespace GitHub.Runner.Worker.Dap
 
                 _currentStep = step;
                 _currentStepIndex = _completedSteps.Count;
+                _jobCompleted = false;
                 pauseOnNextStep = _pauseOnNextStep;
                 cancellationToken = _jobContext?.CancellationToken ?? CancellationToken.None;
             }
@@ -1050,29 +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($"{currentStep.DisplayName ?? "Current Step"}{resultIndicator}"),
-                    Line = currentStepIndex + 1,
+                    Source = currentSourceLine.HasValue ? source : null,
+                    Line = currentSourceLine ?? 0,
                     Column = 1,
                     PresentationHint = "normal"
                 });
@@ -1098,7 +1305,8 @@ namespace GitHub.Runner.Worker.Dap
                 {
                     Id = completedStep.FrameId,
                     Name = MaskUserVisibleText($"{completedStep.DisplayName}{resultStr}"),
-                    Line = 1,
+                    Source = completedStep.SourceLine.HasValue ? source : null,
+                    Line = completedStep.SourceLine ?? 0,
                     Column = 1,
                     PresentationHint = "subtle"
                 });
@@ -1113,6 +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>();

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/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

@@ -0,0 +1,358 @@
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using System.Text;
+
+namespace GitHub.Runner.Worker.Dap
+{
+    internal sealed class JobExecutionView
+    {
+        private const string _sourceFileName = "execution.yml";
+
+        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)
+        {
+            JobId = string.IsNullOrWhiteSpace(jobId) ? "job" : jobId;
+
+            _preEntries.Add(new SourceEntry("Set up job"));
+            AddSteps(steps);
+            AddPredictedPostSteps(predictedPostSteps);
+            AddSteps(initialPostSteps);
+            _postEntries.Add(SourceEntry.CreateSyntheticCompleteJob());
+            Render();
+        }
+
+        public string JobId { get; }
+        public string SourceFileName => _sourceFileName;
+
+        public string Content
+        {
+            get
+            {
+                lock (_lock)
+                {
+                    return _content;
+                }
+            }
+        }
+
+        public int CompleteJobLine
+        {
+            get
+            {
+                lock (_lock)
+                {
+                    return _completeJobLine;
+                }
+            }
+        }
+
+        public int? TryClaimPredictedStep(string matchKey, IStep step)
+        {
+            if (string.IsNullOrEmpty(matchKey) || step == null)
+            {
+                return null;
+            }
+
+            lock (_lock)
+            {
+                var existingLine = TryGetLineForStepNoLock(step);
+                if (existingLine.HasValue)
+                {
+                    return existingLine;
+                }
+
+                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;
+            }
+        }
+
+        public int? TryGetLineForStep(IStep step)
+        {
+            if (step == null)
+            {
+                return null;
+            }
+
+            lock (_lock)
+            {
+                return TryGetLineForStepNoLock(step);
+            }
+        }
+
+        private int? TryGetLineForStepNoLock(IStep step)
+        {
+            foreach (var stepLine in _lineByStep)
+            {
+                if (ReferenceEquals(stepLine.Step, step))
+                {
+                    return stepLine.Line;
+                }
+            }
+
+            return null;
+        }
+
+        private void AddSteps(IEnumerable<IStep> steps)
+        {
+            if (steps == null)
+            {
+                return;
+            }
+
+            foreach (var step in steps)
+            {
+                if (step == null)
+                {
+                    continue;
+                }
+
+                GetEntries(GetSection(step)).Add(new SourceEntry(step));
+            }
+        }
+
+        private void AddPredictedPostSteps(IEnumerable<PredictedPostStep> steps)
+        {
+            if (steps == null)
+            {
+                return;
+            }
+
+            foreach (var step in steps)
+            {
+                if (step == null)
+                {
+                    continue;
+                }
+
+                _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;
+            }
+        }
+
+        private void Render()
+        {
+            _lineByStep.Clear();
+            _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)
+            {
+                if (entry.Step != null && TryGetLineForStepNoLock(entry.Step) == null)
+                {
+                    _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/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)
@@ -860,6 +973,9 @@ namespace GitHub.Runner.Worker
             // Track actions stuck on Node.js 20 due to ARM32 (separate from general deprecation)
             Global.Arm32Node20Actions = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
 
+            // Job-scoped aggregate of artifact subjects declared via $GITHUB_ARTIFACTS.
+            Global.ArtifactSubjects = new Dictionary<string, ArtifactSubject>(StringComparer.Ordinal);
+
             // Job Outputs
             JobOutputs = new Dictionary<string, VariableValue>(StringComparer.OrdinalIgnoreCase);
 
@@ -1330,7 +1446,10 @@ 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

@@ -55,6 +55,18 @@ namespace GitHub.Runner.Worker
                 TryDeleteFile(newPath);
                 File.Create(newPath).Dispose();
 
+                // Give extensions a chance to populate the file before
+                // the step starts (e.g., read-only views of job state).
+                // Errors are logged but must not fail step setup.
+                try
+                {
+                    fileCommand.PopulateInitialContents(context, newPath, container);
+                }
+                catch (Exception ex)
+                {
+                    _trace.Warning($"Failed to populate initial contents for file command '{fileCommand.ContextName}': {ex}");
+                }
+
                 var pathToSet = container != null ? container.TranslateToContainerPath(newPath) : newPath;
                 context.SetGitHubContext(fileCommand.ContextName, pathToSet);
             }
@@ -102,6 +114,14 @@ namespace GitHub.Runner.Worker
         string FilePrefix { get; }
 
         void ProcessCommand(IExecutionContext context, string filePath, ContainerInfo container);
+
+        // Optional hook invoked by FileCommandManager.InitializeFiles
+        // after creating the empty per-step file. Extensions that need to
+        // pre-populate the file (e.g., a read-only view of job-scoped
+        // state) override this; the default no-op preserves the existing
+        // "empty file at start of step" behavior for write-only file
+        // commands such as GITHUB_ENV, GITHUB_OUTPUT, GITHUB_PATH, etc.
+        void PopulateInitialContents(IExecutionContext context, string filePath, ContainerInfo container) { }
     }
 
     public sealed class AddPathFileCommand : RunnerService, IFileCommandExtension
@@ -122,8 +142,16 @@ namespace GitHub.Runner.Worker
                     {
                         continue;
                     }
-                    context.Global.PrependPath.RemoveAll(x => string.Equals(x, line, StringComparison.CurrentCulture));
+                    if (context.DeferredPrependPath != null)
-                    context.Global.PrependPath.Add(line);
+                    {
+                        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 +200,15 @@ namespace GitHub.Runner.Worker
             string name,
             string value)
         {
-            context.Global.EnvironmentVariables[name] = value;
+            if (context.DeferredEnvironmentVariables != null)
-            context.SetEnvContext(name, value);
+            {
+                context.DeferredEnvironmentVariables[name] = value;
+            }
+            else
+            {
+                context.Global.EnvironmentVariables[name] = value;
+                context.SetEnvContext(name, value);
+            }
             context.Debug($"{name}='{value}'");
         }
 
@@ -302,7 +337,14 @@ 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/GitHubContext.cs

@@ -15,6 +15,8 @@ namespace GitHub.Runner.Worker
             "actor",
             "actor_id",
             "api_url",
+            "artifacts",
+            "artifacts_list",
             "base_ref",
             "env",
             "event_name",

src/Runner.Worker/GlobalContext.cs

@@ -39,5 +39,9 @@ namespace GitHub.Runner.Worker
         public HashSet<string> UpgradedToNode24Actions { get; set; }
         public HashSet<string> Arm32Node20Actions { get; set; }
         public IList<String> ActionsDependencies { get; set; }
+
+        // Job-scoped aggregate of artifact subjects declared via $GITHUB_ARTIFACTS.
+        // Keyed by canonical subject name (OCI ref without digest, or file basename).
+        public IDictionary<string, ArtifactSubject> ArtifactSubjects { get; set; }
     }
 }

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);
+                var scope = default(DictionaryContextData);
-            if (_contextData.TryGetValue(scopeName, out var scopeValue))
+                if (_contextData.TryGetValue(scopeName, out var scopeValue))
-            {
+                {
-                scope = scopeValue.AssertDictionary("scope");
+                    scope = scopeValue.AssertDictionary("scope");
-            }
+                }
-            else
+                else
-            {
+                {
-                scope = new DictionaryContextData();
+                    scope = new DictionaryContextData();
-                _contextData.Add(scopeName, scope);
+                    _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);
+            lock (_lock)
-            var outputs = step["outputs"].AssertDictionary("outputs");
-            outputs[outputName] = new StringContextData(value);
-            if (_propertyRegex.IsMatch(outputName))
             {
-                reference = $"steps.{stepName}.outputs.{outputName}";
+                var step = GetStep(scopeName, stepName);
-            }
+                var outputs = step["outputs"].AssertDictionary("outputs");
-            else
+                outputs[outputName] = new StringContextData(value);
-            {
+                if (_propertyRegex.IsMatch(outputName))
-                reference = $"steps['{stepName}']['outputs']['{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);
+            lock (_lock)
-            step["conclusion"] = new StringContextData(conclusion.ToString().ToLowerInvariant());
+            {
+                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);
+            lock (_lock)
-            step["outcome"] = new StringContextData(outcome.ToString().ToLowerInvariant());
+            {
+                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
+                // Start — defer for background steps until the slot is acquired
-                step.ExecutionContext.Start();
+                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
+                            if (step.ExecutionContext.IsBackground)
-                            await dapDebugger?.OnStepStartingAsync(step);
+                            {
+                                // 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
+                                // Run the step synchronously (normal behavior)
-                            await RunStepAsync(step, jobContext.CancellationToken);
+                                await RunStepAsync(step, jobContext.CancellationToken);
-                            CompleteStep(step);
+                                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/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/ObjectTemplating/PipelineTemplateConverter.cs

@@ -55,7 +55,18 @@ namespace GitHub.DistributedTask.Pipelines.ObjectTemplating
                         break;
                     case ActionSourceType.Repository:
                         var repositoryReference = step.Reference as RepositoryPathReference;
-                        name = !String.IsNullOrEmpty(repositoryReference.Name) ? repositoryReference.Name : PipelineConstants.SelfAlias;
+                        if (!String.IsNullOrEmpty(repositoryReference.Name))
+                        {
+                            name = repositoryReference.Name;
+                        }
+                        else if (String.Equals(repositoryReference.RepositoryType, PipelineConstants.SelfRepositoryAlias, StringComparison.OrdinalIgnoreCase))
+                        {
+                            name = PipelineConstants.SelfRepositoryAlias;
+                        }
+                        else
+                        {
+                            name = PipelineConstants.SelfAlias;
+                        }
                         break;
                 }
 
@@ -600,6 +611,14 @@ namespace GitHub.DistributedTask.Pipelines.ObjectTemplating
                         Path = uses.Value
                     };
                 }
+                else if (PipelineConstants.TryParseSelfRepository(uses.Value, out var selfPath))
+                {
+                    result.Reference = new RepositoryPathReference
+                    {
+                        RepositoryType = PipelineConstants.SelfRepositoryAlias,
+                        Path = selfPath
+                    };
+                }
                 else
                 {
                     var usesSegments = uses.Value.Split('@');

src/Sdk/DTPipelines/Pipelines/PipelineConstants.cs

@@ -38,10 +38,43 @@ namespace GitHub.DistributedTask.Pipelines
         public static readonly Int32 MaxNodeNameLength = 100;
 
         /// <summary>
-        /// Alias for the self repository.
+        /// Alias for the self local-workspace repository type (./ syntax).
+        /// Resolves to the local checkout on the runner.
         /// </summary>
         public static readonly String SelfAlias = "self";
 
+        /// <summary>
+        /// RepositoryType for self-repository references ($/ syntax).
+        /// Resolves to "this repo, at this SHA" based on the containing YAML file.
+        /// </summary>
+        public static readonly String SelfRepositoryAlias = "selfRepository";
+
+        /// <summary>
+        /// The prefix for self-repository references in uses: values.
+        /// </summary>
+        public const String SelfRepositoryPrefix = "$/";
+
+        /// <summary>
+        /// Returns true if the uses value is a self-repository reference (starts with $/),
+        /// and outputs the subpath after the prefix.
+        /// </summary>
+        public static bool TryParseSelfRepository(string usesValue, out string path)
+        {
+            if (usesValue != null && usesValue.StartsWith(SelfRepositoryPrefix, StringComparison.Ordinal))
+            {
+                path = usesValue.Substring(SelfRepositoryPrefix.Length).TrimStart('/');
+                if (string.IsNullOrEmpty(path))
+                {
+                    path = null;
+                    return false;
+                }
+                return true;
+            }
+
+            path = null;
+            return false;
+        }
+
         /// <summary>
         /// Error code during graph validation.
         /// </summary>

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/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

@@ -1605,6 +1605,10 @@ namespace GitHub.Actions.WorkflowParser.Conversion
                     {
                         id = WorkflowConstants.SelfAlias;
                     }
+                    else if (GitHub.DistributedTask.Pipelines.PipelineConstants.TryParseSelfRepository(action.Uses!.Value, out _))
+                    {
+                        id = WorkflowConstants.SelfRepositoryAlias;
+                    }
                     else
                     {
                         var usesSegments = action.Uses!.Value.Split('@');

src/Sdk/WorkflowParser/WorkflowConstants.cs

@@ -26,10 +26,15 @@ namespace GitHub.Actions.WorkflowParser
         internal const Int32 MaxNodeNameLength = 100;
 
         /// <summary>
-        /// Alias for the self repository.
+        /// Alias for the self local-workspace repository type (./ syntax).
         /// </summary>
         internal const String SelfAlias = "self";
 
+        /// <summary>
+        /// RepositoryType for self-repository references ($/ syntax).
+        /// </summary>
+        internal const String SelfRepositoryAlias = "selfRepository";
+
         public static class PermissionsPolicy
         {
             public const string LimitedRead = "LimitedRead";

src/Test/L0/Worker/ActionManagerL0.cs

@@ -90,6 +90,11 @@ namespace GitHub.Runner.Common.Tests.Worker
 
                 var actionYamlFile = Path.Combine(_hc.GetDirectory(WellKnownDirectory.Actions), ActionName, "main", "action.yml");
                 Assert.True(File.Exists(actionYamlFile));
+
+                var telemetryMessages = GetTelemetryMessages();
+                Assert.True(ContainsTelemetry(telemetryMessages, "resolve_actions"));
+                Assert.True(ContainsTelemetry(telemetryMessages, "succeeded"));
+                Assert.True(ContainsTelemetry(telemetryMessages, "download_action"));
                 _hc.GetTrace().Info(File.ReadAllText(actionYamlFile));
             }
             finally
@@ -148,6 +153,11 @@ namespace GitHub.Runner.Common.Tests.Worker
 
                 // Act + Assert
                 await Assert.ThrowsAsync<InvalidActionArchiveException>(async () => await _actionManager.PrepareActionsAsync(_ec.Object, actions));
+
+                var telemetryMessages = GetTelemetryMessages();
+                Assert.True(ContainsTelemetry(telemetryMessages, "resolve_actions"));
+                Assert.True(ContainsTelemetry(telemetryMessages, "download_action"));
+                Assert.True(ContainsTelemetry(telemetryMessages, "InvalidActionArchiveException"));
             }
             finally
             {
@@ -215,6 +225,51 @@ namespace GitHub.Runner.Common.Tests.Worker
             }
         }
 
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task PrepareActions_ResolveActionDownloadInfo_RecordsTelemetry_OnFailure()
+        {
+            try
+            {
+                // Arrange
+                Setup();
+                _ec.Object.Global.Variables.Set(Constants.Variables.System.JobRequestType, "RunnerJobRequest");
+
+                _launchServer
+                    .Setup(x => x.ResolveActionsDownloadInfoAsync(It.IsAny<Guid>(), It.IsAny<Guid>(), It.IsAny<ActionReferenceList>(), It.IsAny<CancellationToken>(), It.IsAny<bool>()))
+                    .ThrowsAsync(new Exception("resolve failed"));
+
+                var actions = new List<Pipelines.JobStep>
+                {
+                    new Pipelines.ActionStep()
+                    {
+                        Name = "action",
+                        Id = Guid.NewGuid(),
+                        Reference = new Pipelines.RepositoryPathReference()
+                        {
+                            Name = "actions/checkout",
+                            Ref = "v4",
+                            RepositoryType = "GitHub"
+                        }
+                    }
+                };
+
+                // Act + Assert
+                await Assert.ThrowsAsync<FailedToResolveActionDownloadInfoException>(async () => await _actionManager.PrepareActionsAsync(_ec.Object, actions));
+
+                var telemetryMessages = GetTelemetryMessages();
+                Assert.Equal(1, telemetryMessages.Count(message =>
+                    message.Contains("resolve_actions", StringComparison.OrdinalIgnoreCase)
+                    && !message.Contains("\"result\":\"succeeded\"", StringComparison.OrdinalIgnoreCase)));
+                Assert.False(ContainsTelemetry(telemetryMessages, "resolve_actions\",\"result\":\"succeeded"));
+            }
+            finally
+            {
+                Teardown();
+            }
+        }
+
 #if OS_LINUX
         [Fact]
         [Trait("Level", "L0")]
@@ -2386,7 +2441,7 @@ runs:
             }
         }
 
-         [Fact]
+        [Fact]
         [Trait("Level", "L0")]
         [Trait("Category", "Worker")]
         public void LoadsNode24ActionDefinition()
@@ -3333,6 +3388,16 @@ runs:
             }
         }
 
+        private IList<string> GetTelemetryMessages()
+        {
+            return _ec.Object.Global.JobTelemetry.Select(x => x.Message).ToList();
+        }
+
+        private static bool ContainsTelemetry(IList<string> telemetryMessages, string expectedFragment)
+        {
+            return telemetryMessages.Any(message => message.Contains(expectedFragment, StringComparison.OrdinalIgnoreCase));
+        }
+
         [Fact]
         [Trait("Level", "L0")]
         [Trait("Category", "Worker")]
@@ -3468,5 +3533,604 @@ runs:
                 Teardown();
             }
         }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async void PrepareActions_SelfRepository_ResolvesAtDepthZero()
+        {
+            // Self-references are only supported via run service (batch resolution path)
+            Environment.SetEnvironmentVariable("ACTIONS_BATCH_ACTION_RESOLUTION", "true");
+            try
+            {
+                // Arrange
+                Setup();
+                const string RepoName = "my-org/my-repo";
+                const string RepoSha = "abc123def456";
+                _ec.Setup(x => x.GetGitHubContext("repository")).Returns(RepoName);
+                _ec.Setup(x => x.GetGitHubContext("sha")).Returns(RepoSha);
+                _ec.Object.Global.Variables.Set(Constants.Runner.Features.SelfRepository, "true");
+                var jobContext = new JobContext();
+                jobContext.WorkflowRepository = RepoName;
+                jobContext.WorkflowSha = RepoSha;
+                _ec.Setup(x => x.JobContext).Returns(jobContext);
+
+                var actionId = Guid.NewGuid();
+                var actions = new List<Pipelines.ActionStep>
+                {
+                    new Pipelines.ActionStep()
+                    {
+                        Name = "action",
+                        Id = actionId,
+                        Reference = new Pipelines.RepositoryPathReference()
+                        {
+                            RepositoryType = Pipelines.PipelineConstants.SelfRepositoryAlias,
+                            Path = "actions/my-action"
+                        }
+                    }
+                };
+
+                string archiveFile = await CreateRepoArchive();
+                using var stream = File.OpenRead(archiveFile);
+                string archiveLink = GetLinkToActionArchive("https://api.github.com", RepoName, RepoSha);
+                var mockClientHandler = new Mock<HttpClientHandler>();
+                mockClientHandler.Protected().Setup<Task<HttpResponseMessage>>("SendAsync", ItExpr.Is<HttpRequestMessage>(m => m.RequestUri == new Uri(archiveLink)), ItExpr.IsAny<CancellationToken>())
+                    .ReturnsAsync(new HttpResponseMessage(HttpStatusCode.OK) { Content = new StreamContent(stream) });
+                var mockHandlerFactory = new Mock<IHttpClientHandlerFactory>();
+                mockHandlerFactory.Setup(p => p.CreateClientHandler(It.IsAny<RunnerWebProxy>())).Returns(mockClientHandler.Object);
+                _hc.SetSingleton(mockHandlerFactory.Object);
+
+                _ec.Setup(x => x.GetGitHubContext("api_url")).Returns("https://api.github.com");
+
+                // Act — resolution mutates the reference in-place before download/prepare.
+                // The archive doesn't contain the subpath, so prepare will fail, but the
+                // reference is already resolved by that point.
+                try
+                {
+                    await _actionManager.PrepareActionsAsync(_ec.Object, actions);
+                }
+                catch (InvalidOperationException ex) when (ex.Message.Contains("Can't find"))
+                {
+                    // Expected: test archive lacks the action.yml at the resolved subpath
+                }
+
+                // Assert — the reference should be resolved to a GitHub repo reference
+                var repoRef = actions[0].Reference as Pipelines.RepositoryPathReference;
+                Assert.Equal(Pipelines.RepositoryTypes.GitHub, repoRef.RepositoryType);
+                Assert.Equal(RepoName, repoRef.Name);
+                Assert.Equal(RepoSha, repoRef.Ref);
+                Assert.Equal("actions/my-action", repoRef.Path);
+            }
+            finally
+            {
+                Environment.SetEnvironmentVariable("ACTIONS_BATCH_ACTION_RESOLUTION", null);
+                Teardown();
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async void PrepareActions_SelfRepository_NotResolvedWhenFeatureFlagDisabled()
+        {
+            try
+            {
+                // Arrange
+                Setup();
+                _ec.Setup(x => x.GetGitHubContext("repository")).Returns("my-org/my-repo");
+                _ec.Setup(x => x.GetGitHubContext("sha")).Returns("abc123");
+                // Feature flag NOT set
+
+                var actionId = Guid.NewGuid();
+                var actions = new List<Pipelines.ActionStep>
+                {
+                    new Pipelines.ActionStep()
+                    {
+                        Name = "action",
+                        Id = actionId,
+                        Reference = new Pipelines.RepositoryPathReference()
+                        {
+                            RepositoryType = Pipelines.PipelineConstants.SelfRepositoryAlias,
+                            Path = "actions/my-action"
+                        }
+                    }
+                };
+
+                // Act & Assert — should throw because unresolved self-reference hits GetDownloadInfoLookupKey
+                await Assert.ThrowsAsync<InvalidOperationException>(async () =>
+                    await _actionManager.PrepareActionsAsync(_ec.Object, actions));
+            }
+            finally
+            {
+                Teardown();
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async void PrepareActions_SelfRepository_ResolvesNestedInComposite()
+        {
+            // Composite action at $/actions/parent uses $/actions/child (same repo).
+            // This tests the batch path fix: $/ refs in nextLevel must be resolved
+            // BEFORE ResolveNewActionsAsync, otherwise GetDownloadInfoLookupKey throws.
+            // We pre-stage only the parent action.yml on disk so the composite steps
+            // are discovered, but we DON'T stage the child — a download failure for
+            // the child is fine; the important thing is that $/ was resolved
+            // (no InvalidOperationException from GetDownloadInfoLookupKey).
+            Environment.SetEnvironmentVariable("ACTIONS_BATCH_ACTION_RESOLUTION", "true");
+            try
+            {
+                // Arrange
+                Setup();
+                const string RepoName = "my-org/my-repo";
+                const string RepoSha = "abc123def456";
+                _ec.Setup(x => x.GetGitHubContext("repository")).Returns(RepoName);
+                _ec.Setup(x => x.GetGitHubContext("sha")).Returns(RepoSha);
+                _ec.Setup(x => x.GetGitHubContext("api_url")).Returns("https://api.github.com");
+                _ec.Object.Global.Variables.Set(Constants.Runner.Features.SelfRepository, "true");
+                var jobContext = new JobContext();
+                jobContext.WorkflowRepository = RepoName;
+                jobContext.WorkflowSha = RepoSha;
+                _ec.Setup(x => x.JobContext).Returns(jobContext);
+
+                // Stage parent action on disk as a composite that uses $/actions/child.
+                // We use rootStepId != default to avoid directory deletion,
+                // and create the watermark + action.yml in the expected location.
+                string actionsDir = Path.Combine(_workFolder, Constants.Path.ActionsDirectory);
+                string destDir = Path.Combine(actionsDir, RepoName.Replace(Path.AltDirectorySeparatorChar, Path.DirectorySeparatorChar), RepoSha);
+                Directory.CreateDirectory(Path.Combine(destDir, "actions", "parent"));
+                File.WriteAllText(Path.Combine(destDir, "actions", "parent", Constants.Path.ActionManifestYmlFile), @"
+name: 'Parent'
+description: 'Composite parent'
+runs:
+  using: 'composite'
+  steps:
+    - uses: $/actions/child
+");
+                // Stage child action too (as a leaf node action)
+                Directory.CreateDirectory(Path.Combine(destDir, "actions", "child"));
+                File.WriteAllText(Path.Combine(destDir, "actions", "child", Constants.Path.ActionManifestYmlFile), @"
+name: 'Child'
+description: 'Node child'
+runs:
+  using: 'node20'
+  main: 'index.js'
+");
+                // Write watermark
+                File.WriteAllText($"{destDir}.completed", string.Empty);
+
+                var rootStepId = Guid.NewGuid();
+                var actions = new List<Pipelines.ActionStep>
+                {
+                    new Pipelines.ActionStep()
+                    {
+                        Name = "action",
+                        Id = Guid.NewGuid(),
+                        Reference = new Pipelines.RepositoryPathReference()
+                        {
+                            RepositoryType = Pipelines.PipelineConstants.SelfRepositoryAlias,
+                            Path = "actions/parent"
+                        }
+                    }
+                };
+
+                // Act — should resolve $/ and not throw InvalidOperationException
+                await _actionManager.PrepareActionsAsync(_ec.Object, actions, rootStepId);
+
+                // Assert — top-level $/ resolved
+                var topRef = actions[0].Reference as Pipelines.RepositoryPathReference;
+                Assert.Equal(Pipelines.RepositoryTypes.GitHub, topRef.RepositoryType);
+                Assert.Equal(RepoName, topRef.Name);
+                Assert.Equal(RepoSha, topRef.Ref);
+                Assert.Equal("actions/parent", topRef.Path);
+            }
+            finally
+            {
+                Environment.SetEnvironmentVariable("ACTIONS_BATCH_ACTION_RESOLUTION", null);
+                Teardown();
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async void PrepareActions_SelfRepository_CrossRepoCompositeResolvesToParentRepo()
+        {
+            // External composite (external/foo@v1) uses $/lib/bar.
+            // $/lib/bar should resolve to external/foo@v1 (the parent's repo),
+            // NOT to the workflow's root repo.
+            Environment.SetEnvironmentVariable("ACTIONS_BATCH_ACTION_RESOLUTION", "true");
+            try
+            {
+                // Arrange
+                Setup();
+                const string RootRepoName = "my-org/my-repo";
+                const string RootRepoSha = "root-sha-111";
+                const string ExtRepoName = "external/foo";
+                const string ExtRepoRef = "v1";
+                _ec.Setup(x => x.GetGitHubContext("repository")).Returns(RootRepoName);
+                _ec.Setup(x => x.GetGitHubContext("sha")).Returns(RootRepoSha);
+                _ec.Setup(x => x.GetGitHubContext("api_url")).Returns("https://api.github.com");
+                _ec.Object.Global.Variables.Set(Constants.Runner.Features.SelfRepository, "true");
+                var jobContext = new JobContext();
+                jobContext.WorkflowRepository = RootRepoName;
+                jobContext.WorkflowSha = RootRepoSha;
+                _ec.Setup(x => x.JobContext).Returns(jobContext);
+
+                string actionsDir = Path.Combine(_workFolder, Constants.Path.ActionsDirectory);
+                string destDir = Path.Combine(actionsDir, ExtRepoName.Replace(Path.AltDirectorySeparatorChar, Path.DirectorySeparatorChar), ExtRepoRef);
+                Directory.CreateDirectory(destDir);
+                File.WriteAllText(Path.Combine(destDir, Constants.Path.ActionManifestYmlFile), @"
+name: 'External Foo'
+description: 'External composite'
+runs:
+  using: 'composite'
+  steps:
+    - uses: $/lib/bar
+");
+                Directory.CreateDirectory(Path.Combine(destDir, "lib", "bar"));
+                File.WriteAllText(Path.Combine(destDir, "lib", "bar", Constants.Path.ActionManifestYmlFile), @"
+name: 'Bar'
+description: 'Node action in external repo'
+runs:
+  using: 'node20'
+  main: 'index.js'
+");
+                File.WriteAllText($"{destDir}.completed", string.Empty);
+
+                var rootStepId = Guid.NewGuid();
+                var actions = new List<Pipelines.ActionStep>
+                {
+                    new Pipelines.ActionStep()
+                    {
+                        Name = "action",
+                        Id = Guid.NewGuid(),
+                        Reference = new Pipelines.RepositoryPathReference()
+                        {
+                            Name = ExtRepoName,
+                            Ref = ExtRepoRef,
+                            RepositoryType = Pipelines.RepositoryTypes.GitHub
+                        }
+                    }
+                };
+
+                // Act — should resolve $/lib/bar to external/foo@v1/lib/bar
+                await _actionManager.PrepareActionsAsync(_ec.Object, actions, rootStepId);
+
+                // Assert — the top-level ref is unchanged (it was already concrete)
+                var topRef = actions[0].Reference as Pipelines.RepositoryPathReference;
+                Assert.Equal(ExtRepoName, topRef.Name);
+                Assert.Equal(ExtRepoRef, topRef.Ref);
+            }
+            finally
+            {
+                Environment.SetEnvironmentVariable("ACTIONS_BATCH_ACTION_RESOLUTION", null);
+                Teardown();
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async void PrepareActions_SelfRepository_MultiLevelChain()
+        {
+            // $/a → composite → $/b → composite → $/c (three levels, same repo)
+            Environment.SetEnvironmentVariable("ACTIONS_BATCH_ACTION_RESOLUTION", "true");
+            try
+            {
+                // Arrange
+                Setup();
+                const string RepoName = "my-org/my-repo";
+                const string RepoSha = "chain-sha-222";
+                _ec.Setup(x => x.GetGitHubContext("repository")).Returns(RepoName);
+                _ec.Setup(x => x.GetGitHubContext("sha")).Returns(RepoSha);
+                _ec.Setup(x => x.GetGitHubContext("api_url")).Returns("https://api.github.com");
+                _ec.Object.Global.Variables.Set(Constants.Runner.Features.SelfRepository, "true");
+                var jobContext = new JobContext();
+                jobContext.WorkflowRepository = RepoName;
+                jobContext.WorkflowSha = RepoSha;
+                _ec.Setup(x => x.JobContext).Returns(jobContext);
+
+                string actionsDir = Path.Combine(_workFolder, Constants.Path.ActionsDirectory);
+                string destDir = Path.Combine(actionsDir, RepoName.Replace(Path.AltDirectorySeparatorChar, Path.DirectorySeparatorChar), RepoSha);
+                Directory.CreateDirectory(Path.Combine(destDir, "a"));
+                File.WriteAllText(Path.Combine(destDir, "a", Constants.Path.ActionManifestYmlFile), @"
+name: 'A'
+description: 'Level 0 composite'
+runs:
+  using: 'composite'
+  steps:
+    - uses: $/b
+");
+                Directory.CreateDirectory(Path.Combine(destDir, "b"));
+                File.WriteAllText(Path.Combine(destDir, "b", Constants.Path.ActionManifestYmlFile), @"
+name: 'B'
+description: 'Level 1 composite'
+runs:
+  using: 'composite'
+  steps:
+    - uses: $/c
+");
+                Directory.CreateDirectory(Path.Combine(destDir, "c"));
+                File.WriteAllText(Path.Combine(destDir, "c", Constants.Path.ActionManifestYmlFile), @"
+name: 'C'
+description: 'Level 2 node leaf'
+runs:
+  using: 'node20'
+  main: 'index.js'
+");
+                File.WriteAllText($"{destDir}.completed", string.Empty);
+
+                var rootStepId = Guid.NewGuid();
+                var actions = new List<Pipelines.ActionStep>
+                {
+                    new Pipelines.ActionStep()
+                    {
+                        Name = "action",
+                        Id = Guid.NewGuid(),
+                        Reference = new Pipelines.RepositoryPathReference()
+                        {
+                            RepositoryType = Pipelines.PipelineConstants.SelfRepositoryAlias,
+                            Path = "a"
+                        }
+                    }
+                };
+
+                // Act — three-level $/ chain should resolve without error
+                await _actionManager.PrepareActionsAsync(_ec.Object, actions, rootStepId);
+
+                // Assert — top-level ref resolved
+                var topRef = actions[0].Reference as Pipelines.RepositoryPathReference;
+                Assert.Equal(Pipelines.RepositoryTypes.GitHub, topRef.RepositoryType);
+                Assert.Equal(RepoName, topRef.Name);
+                Assert.Equal(RepoSha, topRef.Ref);
+                Assert.Equal("a", topRef.Path);
+            }
+            finally
+            {
+                Environment.SetEnvironmentVariable("ACTIONS_BATCH_ACTION_RESOLUTION", null);
+                Teardown();
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async void PrepareActions_SelfRepository_ResolvesAtDepthZero_LegacyPath()
+        {
+            // Same as ResolvesAtDepthZero but on the legacy (non-batch) path
+            try
+            {
+                // Arrange
+                Setup();
+                const string RepoName = "my-org/my-repo";
+                const string RepoSha = "abc123def456";
+                _ec.Setup(x => x.GetGitHubContext("repository")).Returns(RepoName);
+                _ec.Setup(x => x.GetGitHubContext("sha")).Returns(RepoSha);
+                _ec.Object.Global.Variables.Set(Constants.Runner.Features.SelfRepository, "true");
+                var jobContext = new JobContext();
+                jobContext.WorkflowRepository = RepoName;
+                jobContext.WorkflowSha = RepoSha;
+                _ec.Setup(x => x.JobContext).Returns(jobContext);
+
+                var actionId = Guid.NewGuid();
+                var actions = new List<Pipelines.ActionStep>
+                {
+                    new Pipelines.ActionStep()
+                    {
+                        Name = "action",
+                        Id = actionId,
+                        Reference = new Pipelines.RepositoryPathReference()
+                        {
+                            RepositoryType = Pipelines.PipelineConstants.SelfRepositoryAlias,
+                            Path = "actions/my-action"
+                        }
+                    }
+                };
+
+                string archiveFile = await CreateRepoArchive();
+                using var stream = File.OpenRead(archiveFile);
+                string archiveLink = GetLinkToActionArchive("https://api.github.com", RepoName, RepoSha);
+                var mockClientHandler = new Mock<HttpClientHandler>();
+                mockClientHandler.Protected().Setup<Task<HttpResponseMessage>>("SendAsync", ItExpr.Is<HttpRequestMessage>(m => m.RequestUri == new Uri(archiveLink)), ItExpr.IsAny<CancellationToken>())
+                    .ReturnsAsync(new HttpResponseMessage(HttpStatusCode.OK) { Content = new StreamContent(stream) });
+                var mockHandlerFactory = new Mock<IHttpClientHandlerFactory>();
+                mockHandlerFactory.Setup(p => p.CreateClientHandler(It.IsAny<RunnerWebProxy>())).Returns(mockClientHandler.Object);
+                _hc.SetSingleton(mockHandlerFactory.Object);
+
+                _ec.Setup(x => x.GetGitHubContext("api_url")).Returns("https://api.github.com");
+
+                // Act
+                try
+                {
+                    await _actionManager.PrepareActionsAsync(_ec.Object, actions);
+                }
+                catch (InvalidOperationException ex) when (ex.Message.Contains("Can't find"))
+                {
+                    // Expected: test archive lacks the action.yml at the resolved subpath
+                }
+
+                // Assert — the reference should be resolved to a GitHub repo reference
+                var repoRef = actions[0].Reference as Pipelines.RepositoryPathReference;
+                Assert.Equal(Pipelines.RepositoryTypes.GitHub, repoRef.RepositoryType);
+                Assert.Equal(RepoName, repoRef.Name);
+                Assert.Equal(RepoSha, repoRef.Ref);
+                Assert.Equal("actions/my-action", repoRef.Path);
+            }
+            finally
+            {
+                Teardown();
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async void PrepareActions_SelfRepository_ResolvesNestedInComposite_LegacyPath()
+        {
+            // Same as ResolvesNestedInComposite but on the legacy (non-batch) path.
+            // Verifies that $/ resolution works when batch action resolution is disabled.
+            try
+            {
+                // Arrange
+                Setup();
+                const string RepoName = "my-org/my-repo";
+                const string RepoSha = "abc123def456";
+                _ec.Setup(x => x.GetGitHubContext("repository")).Returns(RepoName);
+                _ec.Setup(x => x.GetGitHubContext("sha")).Returns(RepoSha);
+                _ec.Setup(x => x.GetGitHubContext("api_url")).Returns("https://api.github.com");
+                _ec.Object.Global.Variables.Set(Constants.Runner.Features.SelfRepository, "true");
+                var jobContext = new JobContext();
+                jobContext.WorkflowRepository = RepoName;
+                jobContext.WorkflowSha = RepoSha;
+                _ec.Setup(x => x.JobContext).Returns(jobContext);
+
+                // Stage parent action on disk as a composite that uses $/actions/child.
+                string actionsDir = Path.Combine(_workFolder, Constants.Path.ActionsDirectory);
+                string destDir = Path.Combine(actionsDir, RepoName.Replace(Path.AltDirectorySeparatorChar, Path.DirectorySeparatorChar), RepoSha);
+                Directory.CreateDirectory(Path.Combine(destDir, "actions", "parent"));
+                File.WriteAllText(Path.Combine(destDir, "actions", "parent", Constants.Path.ActionManifestYmlFile), @"
+name: 'Parent'
+description: 'Composite parent'
+runs:
+  using: 'composite'
+  steps:
+    - uses: $/actions/child
+");
+                // Stage child action too (as a leaf node action)
+                Directory.CreateDirectory(Path.Combine(destDir, "actions", "child"));
+                File.WriteAllText(Path.Combine(destDir, "actions", "child", Constants.Path.ActionManifestYmlFile), @"
+name: 'Child'
+description: 'Node child'
+runs:
+  using: 'node20'
+  main: 'index.js'
+");
+                // Write watermark
+                File.WriteAllText($"{destDir}.completed", string.Empty);
+
+                var rootStepId = Guid.NewGuid();
+                var actions = new List<Pipelines.ActionStep>
+                {
+                    new Pipelines.ActionStep()
+                    {
+                        Name = "action",
+                        Id = Guid.NewGuid(),
+                        Reference = new Pipelines.RepositoryPathReference()
+                        {
+                            RepositoryType = Pipelines.PipelineConstants.SelfRepositoryAlias,
+                            Path = "actions/parent"
+                        }
+                    }
+                };
+
+                // Act — should resolve $/ and not throw InvalidOperationException
+                await _actionManager.PrepareActionsAsync(_ec.Object, actions, rootStepId);
+
+                // Assert — top-level $/ resolved
+                var topRef = actions[0].Reference as Pipelines.RepositoryPathReference;
+                Assert.Equal(Pipelines.RepositoryTypes.GitHub, topRef.RepositoryType);
+                Assert.Equal(RepoName, topRef.Name);
+                Assert.Equal(RepoSha, topRef.Ref);
+                Assert.Equal("actions/parent", topRef.Path);
+            }
+            finally
+            {
+                Teardown();
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void LoadAction_DotSlashCompositeWithNestedSelfRepository_ResolvesViaWorkflowContext()
+        {
+            // Regression test: when a dot-slash (./) composite action contains a
+            // nested $/actions/child step, LoadAction re-parses the action.yml at
+            // runtime and must resolve the $/ ref. The parent is repositoryType "self"
+            // so its Name and Ref are null — resolution must fall back to
+            // WorkflowRepository/WorkflowSha from the job context. Before the fix,
+            // this path hit a NullReferenceException at repoAction.Name.Replace().
+            try
+            {
+                // Arrange
+                Setup();
+                const string WorkflowRepo = "my-org/my-repo";
+                const string WorkflowSha = "abc123def456";
+                _ec.Object.Global.Variables.Set(Constants.Runner.Features.SelfRepository, "true");
+                var jobContext = new JobContext();
+                jobContext.WorkflowRepository = WorkflowRepo;
+                jobContext.WorkflowSha = WorkflowSha;
+                _ec.Setup(x => x.JobContext).Returns(jobContext);
+
+                // Stage the dot-slash composite in the workspace directory.
+                // It contains a nested $/actions/child step.
+                string workspaceDir = Path.Combine(_workFolder, "actions", "actions");
+                string compositeDir = Path.Combine(workspaceDir, "my-composite");
+                Directory.CreateDirectory(compositeDir);
+                File.WriteAllText(Path.Combine(compositeDir, Constants.Path.ActionManifestYmlFile), @"
+name: 'DotSlash Parent'
+description: 'Composite loaded via ./ that nests a $/ ref'
+runs:
+  using: 'composite'
+  steps:
+    - run: echo 'hello'
+      shell: bash
+    - uses: $/actions/child
+");
+
+                // Stage the child action in the actions cache under the workflow repo.
+                string actionsDir = Path.Combine(_workFolder, Constants.Path.ActionsDirectory);
+                string childDir = Path.Combine(actionsDir, WorkflowRepo.Replace(Path.AltDirectorySeparatorChar, Path.DirectorySeparatorChar), WorkflowSha, "actions", "child");
+                Directory.CreateDirectory(childDir);
+                File.WriteAllText(Path.Combine(childDir, Constants.Path.ActionManifestYmlFile), @"
+name: 'Child'
+description: 'Leaf action'
+runs:
+  using: 'node20'
+  main: 'index.js'
+");
+
+                // Create dot-slash step with Name = null (the real scenario).
+                var instance = new Pipelines.ActionStep()
+                {
+                    Id = Guid.NewGuid(),
+                    Reference = new Pipelines.RepositoryPathReference()
+                    {
+                        Name = null,
+                        Ref = null,
+                        RepositoryType = Pipelines.PipelineConstants.SelfAlias,
+                        Path = "my-composite"
+                    }
+                };
+
+                // Act — should NOT throw NullReferenceException
+                Definition definition = _actionManager.LoadAction(_ec.Object, instance);
+
+                // Assert — loaded the composite successfully
+                Assert.NotNull(definition);
+                Assert.NotNull(definition.Data);
+                Assert.Equal(ActionExecutionType.Composite, definition.Data.Execution.ExecutionType);
+
+                // Assert — the nested $/ step was resolved to the workflow repo
+                var compositeData = definition.Data.Execution as CompositeActionExecutionData;
+                Assert.NotNull(compositeData);
+                var childStep = compositeData.Steps
+                    .OfType<Pipelines.ActionStep>()
+                    .FirstOrDefault(s => s.Reference is Pipelines.RepositoryPathReference r
+                        && r.Path == "actions/child");
+                Assert.NotNull(childStep);
+                var childRef = childStep.Reference as Pipelines.RepositoryPathReference;
+                Assert.Equal(Pipelines.RepositoryTypes.GitHub, childRef.RepositoryType);
+                Assert.Equal(WorkflowRepo, childRef.Name);
+                Assert.Equal(WorkflowSha, childRef.Ref);
+            }
+            finally
+            {
+                Teardown();
+            }
+        }
+
     }
 }

src/Test/L0/Worker/ArtifactsListFileCommandL0.cs

@@ -0,0 +1,214 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Runtime.CompilerServices;
+using GitHub.DistributedTask.WebApi;
+using GitHub.Runner.Common.Util;
+using GitHub.Runner.Sdk;
+using GitHub.Runner.Worker;
+using Moq;
+using Newtonsoft.Json.Linq;
+using Xunit;
+
+namespace GitHub.Runner.Common.Tests.Worker
+{
+    public sealed class ArtifactsListFileCommandL0
+    {
+        private Mock<IExecutionContext> _executionContext;
+        private string _rootDirectory;
+        private string _outputFile;
+        private ArtifactsListFileCommand _command;
+        private GlobalContext _global;
+        private ITraceWriter _trace;
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void EmptyAggregate_WritesVersionedJsonWithEmptySubjects()
+        {
+            using (var hostContext = Setup())
+            {
+                _command.PopulateInitialContents(_executionContext.Object, _outputFile, null);
+                var json = JObject.Parse(File.ReadAllText(_outputFile));
+                Assert.Equal(ArtifactsListFileCommand.FormatVersion, json["version"].Value<int>());
+                Assert.Empty((JArray)json["subjects"]);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void SingleSubject_SerializedCorrectly()
+        {
+            using (var hostContext = Setup())
+            {
+                _global.ArtifactSubjects["myapp"] = new ArtifactSubject(
+                    "myapp",
+                    "sha256:" + new string('a', 64),
+                    ArtifactSubjectKind.File);
+
+                _command.PopulateInitialContents(_executionContext.Object, _outputFile, null);
+
+                var json = JObject.Parse(File.ReadAllText(_outputFile));
+                var subjects = (JArray)json["subjects"];
+                Assert.Single(subjects);
+                Assert.Equal("myapp", subjects[0]["name"].Value<string>());
+                Assert.Equal("sha256:" + new string('a', 64), subjects[0]["digest"].Value<string>());
+                Assert.Equal("file", subjects[0]["kind"].Value<string>());
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void OciSubject_KindIsLowercaseOci()
+        {
+            using (var hostContext = Setup())
+            {
+                _global.ArtifactSubjects["ghcr.io/x:1"] = new ArtifactSubject(
+                    "ghcr.io/x:1",
+                    "sha256:" + new string('b', 64),
+                    ArtifactSubjectKind.OciSubject);
+
+                _command.PopulateInitialContents(_executionContext.Object, _outputFile, null);
+
+                var json = JObject.Parse(File.ReadAllText(_outputFile));
+                Assert.Equal("oci", json["subjects"][0]["kind"].Value<string>());
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void MultipleSubjects_SortedByName()
+        {
+            using (var hostContext = Setup())
+            {
+                // Insert deliberately out of alphabetical order to prove the
+                // output is sorted by name rather than by insertion order.
+                _global.ArtifactSubjects["two"] = new ArtifactSubject("two", "sha256:" + new string('2', 64), ArtifactSubjectKind.File);
+                _global.ArtifactSubjects["one"] = new ArtifactSubject("one", "sha256:" + new string('1', 64), ArtifactSubjectKind.File);
+                _global.ArtifactSubjects["three"] = new ArtifactSubject("three", "sha256:" + new string('3', 64), ArtifactSubjectKind.OciSubject);
+
+                _command.PopulateInitialContents(_executionContext.Object, _outputFile, null);
+
+                var subjects = (JArray)JObject.Parse(File.ReadAllText(_outputFile))["subjects"];
+                Assert.Equal(3, subjects.Count);
+                Assert.Equal("one", subjects[0]["name"].Value<string>());
+                Assert.Equal("three", subjects[1]["name"].Value<string>());
+                Assert.Equal("two", subjects[2]["name"].Value<string>());
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void FeatureFlagOff_LeavesFileEmpty()
+        {
+            using (var hostContext = Setup(featureFlag: false))
+            {
+                _global.ArtifactSubjects["myapp"] = new ArtifactSubject("myapp", "sha256:" + new string('a', 64), ArtifactSubjectKind.File);
+
+                _command.PopulateInitialContents(_executionContext.Object, _outputFile, null);
+
+                Assert.Equal(string.Empty, File.ReadAllText(_outputFile));
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void EnvVarFallback_EnablesPublishing()
+        {
+            using (var hostContext = Setup(featureFlag: false, envVarOverride: "true"))
+            {
+                _global.ArtifactSubjects["myapp"] = new ArtifactSubject("myapp", "sha256:" + new string('a', 64), ArtifactSubjectKind.File);
+
+                _command.PopulateInitialContents(_executionContext.Object, _outputFile, null);
+
+                var json = JObject.Parse(File.ReadAllText(_outputFile));
+                Assert.Single((JArray)json["subjects"]);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void OutputFileIsUtf8WithoutBom()
+        {
+            using (var hostContext = Setup())
+            {
+                _command.PopulateInitialContents(_executionContext.Object, _outputFile, null);
+
+                var bytes = File.ReadAllBytes(_outputFile);
+                // UTF-8 BOM is EF BB BF
+                Assert.False(bytes.Length >= 3 && bytes[0] == 0xEF && bytes[1] == 0xBB && bytes[2] == 0xBF,
+                    "File should not begin with a UTF-8 BOM.");
+                // Sanity check that the file is valid JSON.
+                Assert.NotNull(JObject.Parse(System.Text.Encoding.UTF8.GetString(bytes)));
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void ProcessCommand_IsNoOp()
+        {
+            using (var hostContext = Setup())
+            {
+                // Even if the step writes garbage, ProcessCommand must not touch the aggregate.
+                File.WriteAllText(_outputFile, "anything the step wrote");
+                _command.ProcessCommand(_executionContext.Object, _outputFile, null);
+                Assert.Empty(_global.ArtifactSubjects);
+            }
+        }
+
+        private TestHostContext Setup(bool featureFlag = true, string envVarOverride = null, [CallerMemberName] string name = "")
+        {
+            // Reset env-var state across test runs in the same process.
+            Environment.SetEnvironmentVariable(CreateArtifactsFileCommand.EnableEnvVar, envVarOverride);
+
+            var hostContext = new TestHostContext(this, name);
+            _trace = hostContext.GetTrace();
+
+            var workDirectory = hostContext.GetDirectory(WellKnownDirectory.Work);
+            Directory.CreateDirectory(workDirectory);
+            _rootDirectory = Path.Combine(workDirectory, nameof(ArtifactsListFileCommandL0), name);
+            if (Directory.Exists(_rootDirectory))
+            {
+                Directory.Delete(_rootDirectory, recursive: true);
+            }
+            Directory.CreateDirectory(_rootDirectory);
+            _outputFile = Path.Combine(_rootDirectory, "artifacts_list");
+            File.WriteAllText(_outputFile, string.Empty);
+
+            var variableValues = new Dictionary<string, VariableValue>(StringComparer.OrdinalIgnoreCase);
+            if (featureFlag)
+            {
+                variableValues[Common.Constants.Runner.Features.AllowArtifactsFile] = new VariableValue("true");
+            }
+            var variables = new Variables(hostContext, variableValues);
+
+            _global = new GlobalContext
+            {
+                EnvironmentVariables = new Dictionary<string, string>(VarUtil.EnvironmentVariableKeyComparer),
+                Variables = variables,
+                WriteDebug = true,
+                ArtifactSubjects = new Dictionary<string, ArtifactSubject>(StringComparer.Ordinal),
+            };
+
+            _executionContext = new Mock<IExecutionContext>();
+            _executionContext.Setup(x => x.Global).Returns(_global);
+            _executionContext.Setup(x => x.Write(It.IsAny<string>(), It.IsAny<string>()))
+                .Callback((string tag, string message) =>
+                {
+                    _trace.Info($"{tag}{message}");
+                });
+
+            _command = new ArtifactsListFileCommand();
+            _command.Initialize(hostContext);
+
+            return hostContext;
+        }
+    }
+}

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/CreateArtifactsFileCommandL0.cs

@@ -0,0 +1,627 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Runtime.CompilerServices;
+using System.Text;
+using GitHub.DistributedTask.WebApi;
+using GitHub.Runner.Common.Util;
+using GitHub.Runner.Sdk;
+using GitHub.Runner.Worker;
+using GitHub.Runner.Worker.Container;
+using Moq;
+using Xunit;
+using DTWebApi = GitHub.DistributedTask.WebApi;
+
+namespace GitHub.Runner.Common.Tests.Worker
+{
+    public sealed class CreateArtifactsFileCommandL0
+    {
+        private const string FlagOn = "true";
+
+        private Mock<IExecutionContext> _executionContext;
+        private List<DTWebApi.Issue> _issues;
+        private string _rootDirectory;
+        private string _workspaceDirectory;
+        private CreateArtifactsFileCommand _command;
+        private GlobalContext _global;
+        private ITraceWriter _trace;
+
+        // ---------- Feature flag ----------
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void FeatureFlagOff_NoOp()
+        {
+            using (var hostContext = Setup(featureFlag: false))
+            {
+                var artifactsFile = WriteArtifactsFile("ghcr.io/octocat/myapp:1.0@sha256:" + new string('a', 64));
+                _command.ProcessCommand(_executionContext.Object, artifactsFile, null);
+                Assert.Empty(_global.ArtifactSubjects);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void EnvVarOverride_EnablesFeature()
+        {
+            using (var hostContext = Setup(featureFlag: false, envVarOverride: "true"))
+            {
+                var hex = new string('a', 64);
+                var artifactsFile = WriteArtifactsFile($"ghcr.io/octocat/myapp:1.0@sha256:{hex}");
+                _command.ProcessCommand(_executionContext.Object, artifactsFile, null);
+                Assert.Single(_global.ArtifactSubjects);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void EnvVarFalse_DoesNotEnable()
+        {
+            using (var hostContext = Setup(featureFlag: false, envVarOverride: "false"))
+            {
+                var artifactsFile = WriteArtifactsFile("ghcr.io/x@sha256:" + new string('a', 64));
+                _command.ProcessCommand(_executionContext.Object, artifactsFile, null);
+                Assert.Empty(_global.ArtifactSubjects);
+            }
+        }
+
+        // ---------- Trivial cases ----------
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void FileMissing_NoOp()
+        {
+            using (var hostContext = Setup())
+            {
+                var artifactsFile = Path.Combine(_rootDirectory, "does-not-exist");
+                _command.ProcessCommand(_executionContext.Object, artifactsFile, null);
+                Assert.Empty(_global.ArtifactSubjects);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void EmptyFile_NoOp()
+        {
+            using (var hostContext = Setup())
+            {
+                var artifactsFile = WriteArtifactsFile(string.Empty);
+                _command.ProcessCommand(_executionContext.Object, artifactsFile, null);
+                Assert.Empty(_global.ArtifactSubjects);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void BlankAndCommentLines_Skipped()
+        {
+            using (var hostContext = Setup())
+            {
+                var artifactsFile = WriteArtifactsFile(
+                    "",
+                    "# this is a comment",
+                    "   # leading-whitespace comment",
+                    "",
+                    "   ");
+                _command.ProcessCommand(_executionContext.Object, artifactsFile, null);
+                Assert.Empty(_global.ArtifactSubjects);
+            }
+        }
+
+        // ---------- OCI subjects ----------
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void OciSubject_Sha256_HappyPath()
+        {
+            using (var hostContext = Setup())
+            {
+                var hex = new string('a', 64);
+                var artifactsFile = WriteArtifactsFile($"ghcr.io/octocat/myapp:1.0.0@sha256:{hex}");
+                _command.ProcessCommand(_executionContext.Object, artifactsFile, null);
+                Assert.Single(_global.ArtifactSubjects);
+                var subject = _global.ArtifactSubjects["ghcr.io/octocat/myapp:1.0.0"];
+                Assert.Equal($"sha256:{hex}", subject.Digest);
+                Assert.Equal(ArtifactSubjectKind.OciSubject, subject.Kind);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void OciSubject_Sha384_HappyPath()
+        {
+            using (var hostContext = Setup())
+            {
+                var hex = new string('b', 96);
+                var artifactsFile = WriteArtifactsFile($"ghcr.io/x/y@sha384:{hex}");
+                _command.ProcessCommand(_executionContext.Object, artifactsFile, null);
+                Assert.Single(_global.ArtifactSubjects);
+                Assert.Equal($"sha384:{hex}", _global.ArtifactSubjects["ghcr.io/x/y"].Digest);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void OciSubject_Sha512_HappyPath()
+        {
+            using (var hostContext = Setup())
+            {
+                var hex = new string('c', 128);
+                var artifactsFile = WriteArtifactsFile($"ghcr.io/x/y@sha512:{hex}");
+                _command.ProcessCommand(_executionContext.Object, artifactsFile, null);
+                Assert.Single(_global.ArtifactSubjects);
+                Assert.Equal($"sha512:{hex}", _global.ArtifactSubjects["ghcr.io/x/y"].Digest);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void OciSubject_DigestLowercased()
+        {
+            using (var hostContext = Setup())
+            {
+                var hex = new string('A', 64);
+                var artifactsFile = WriteArtifactsFile($"ghcr.io/x@sha256:{hex}");
+                _command.ProcessCommand(_executionContext.Object, artifactsFile, null);
+                Assert.Equal($"sha256:{hex.ToLowerInvariant()}", _global.ArtifactSubjects["ghcr.io/x"].Digest);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void OciSubject_PreservesTagAndRegistryPort()
+        {
+            using (var hostContext = Setup())
+            {
+                var hex = new string('d', 64);
+                var artifactsFile = WriteArtifactsFile($"localhost:5000/repo/img:v1@sha256:{hex}");
+                _command.ProcessCommand(_executionContext.Object, artifactsFile, null);
+                Assert.Single(_global.ArtifactSubjects);
+                Assert.True(_global.ArtifactSubjects.ContainsKey("localhost:5000/repo/img:v1"));
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void OciSubject_WrongHexLength_Throws()
+        {
+            using (var hostContext = Setup())
+            {
+                // 63 hex chars instead of 64 → falls back to file path parse → file missing → throws
+                var artifactsFile = WriteArtifactsFile("ghcr.io/x@sha256:" + new string('a', 63));
+                var ex = Assert.Throws<Exception>(() => _command.ProcessCommand(_executionContext.Object, artifactsFile, null));
+                Assert.Contains("line 1", ex.Message);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void OciSubject_NonHexChars_TreatedAsFile_Throws()
+        {
+            using (var hostContext = Setup())
+            {
+                // Non-hex character: digest regex doesn't match → treated as file path → file missing
+                var artifactsFile = WriteArtifactsFile("ghcr.io/x@sha256:" + new string('z', 64));
+                var ex = Assert.Throws<Exception>(() => _command.ProcessCommand(_executionContext.Object, artifactsFile, null));
+                Assert.Contains("line 1", ex.Message);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void OciScheme_RejectsWhenDigestMissing()
+        {
+            using (var hostContext = Setup())
+            {
+                var artifactsFile = WriteArtifactsFile("oci://ghcr.io/x:1.0");
+                var ex = Assert.Throws<Exception>(() => _command.ProcessCommand(_executionContext.Object, artifactsFile, null));
+                Assert.Contains("line 1", ex.Message);
+                Assert.Contains("digest", ex.Message);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void OciScheme_HappyPath()
+        {
+            using (var hostContext = Setup())
+            {
+                var hex = new string('e', 64);
+                var artifactsFile = WriteArtifactsFile($"OCI://ghcr.io/x:1.0@sha256:{hex}");
+                _command.ProcessCommand(_executionContext.Object, artifactsFile, null);
+                Assert.Single(_global.ArtifactSubjects);
+                Assert.True(_global.ArtifactSubjects.ContainsKey("ghcr.io/x:1.0"));
+            }
+        }
+
+        // ---------- File subjects ----------
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void FileSubject_Absolute_HappyPath()
+        {
+            using (var hostContext = Setup())
+            {
+                var artifactPath = Path.Combine(_rootDirectory, "binary.bin");
+                File.WriteAllBytes(artifactPath, new byte[] { 1, 2, 3, 4 });
+                var artifactsFile = WriteArtifactsFile(artifactPath);
+
+                _command.ProcessCommand(_executionContext.Object, artifactsFile, null);
+
+                Assert.Single(_global.ArtifactSubjects);
+                var subject = _global.ArtifactSubjects["binary.bin"];
+                Assert.Equal(ArtifactSubjectKind.File, subject.Kind);
+                // sha256("\x01\x02\x03\x04") = 9f64a747e1b97f131fabb6b447296c9b6f0201e79fb3c5356e6c77e89b6a806a
+                Assert.Equal("sha256:9f64a747e1b97f131fabb6b447296c9b6f0201e79fb3c5356e6c77e89b6a806a", subject.Digest);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void FileSubject_RelativeToWorkspace()
+        {
+            using (var hostContext = Setup())
+            {
+                Directory.CreateDirectory(Path.Combine(_workspaceDirectory, "dist"));
+                File.WriteAllBytes(Path.Combine(_workspaceDirectory, "dist", "myapp"), new byte[] { 9 });
+                var artifactsFile = WriteArtifactsFile("dist/myapp");
+
+                _command.ProcessCommand(_executionContext.Object, artifactsFile, null);
+
+                Assert.Single(_global.ArtifactSubjects);
+                Assert.True(_global.ArtifactSubjects.ContainsKey("myapp"));
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void FileScheme_TreatsAsFilePathEvenIfLooksLikeOci()
+        {
+            using (var hostContext = Setup())
+            {
+                // File literally named "image@sha256:deadbeef..." — force file path via file:// prefix.
+                var quirkyName = "image@sha256:" + new string('f', 64);
+                var artifactPath = Path.Combine(_rootDirectory, quirkyName);
+                File.WriteAllBytes(artifactPath, new byte[] { 1 });
+                var artifactsFile = WriteArtifactsFile("file://" + artifactPath);
+
+                _command.ProcessCommand(_executionContext.Object, artifactsFile, null);
+
+                Assert.Single(_global.ArtifactSubjects);
+                var subject = _global.ArtifactSubjects[quirkyName];
+                Assert.Equal(ArtifactSubjectKind.File, subject.Kind);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void FileSubject_Missing_Throws()
+        {
+            using (var hostContext = Setup())
+            {
+                var artifactsFile = WriteArtifactsFile(Path.Combine(_rootDirectory, "does-not-exist"));
+                var ex = Assert.Throws<Exception>(() => _command.ProcessCommand(_executionContext.Object, artifactsFile, null));
+                Assert.Contains("line 1", ex.Message);
+                Assert.Contains("does not exist", ex.Message);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void FileSubject_Directory_Throws()
+        {
+            using (var hostContext = Setup())
+            {
+                var dir = Path.Combine(_rootDirectory, "a-directory");
+                Directory.CreateDirectory(dir);
+                var artifactsFile = WriteArtifactsFile(dir);
+                var ex = Assert.Throws<Exception>(() => _command.ProcessCommand(_executionContext.Object, artifactsFile, null));
+                Assert.Contains("line 1", ex.Message);
+                Assert.Contains("not a regular file", ex.Message);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void FileSubject_ContainerAbsolute_InMount_ResolvesToHostFile()
+        {
+            using (var hostContext = Setup())
+            {
+                // The host file lives under a directory that is mounted into
+                // the container. The step declares the file using its
+                // container-namespace path, which must translate back to the
+                // host file so the digest is computed over the right bytes.
+                var hostDirectory = Path.Combine(_rootDirectory, "mounted");
+                Directory.CreateDirectory(hostDirectory);
+                File.WriteAllBytes(Path.Combine(hostDirectory, "app.bin"), new byte[] { 1, 2, 3, 4 });
+
+                var container = new ContainerInfo();
+                var containerDirectory = "/container-workspace";
+                container.AddPathTranslateMapping(hostDirectory, containerDirectory);
+
+                var artifactsFile = WriteArtifactsFile(Path.Combine(containerDirectory, "app.bin"));
+                _command.ProcessCommand(_executionContext.Object, artifactsFile, container);
+
+                Assert.Single(_global.ArtifactSubjects);
+                var subject = _global.ArtifactSubjects["app.bin"];
+                Assert.Equal(ArtifactSubjectKind.File, subject.Kind);
+                // sha256("\x01\x02\x03\x04")
+                Assert.Equal("sha256:9f64a747e1b97f131fabb6b447296c9b6f0201e79fb3c5356e6c77e89b6a806a", subject.Digest);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void FileSubject_ContainerAbsolute_OutsideMount_Throws()
+        {
+            using (var hostContext = Setup())
+            {
+                // An absolute path that does not resolve into any mounted
+                // volume must be rejected rather than silently hashing the
+                // host file that happens to live at that same path.
+                var container = new ContainerInfo();
+                container.AddPathTranslateMapping(Path.Combine(_rootDirectory, "mounted"), "/container-workspace");
+
+                var artifactsFile = WriteArtifactsFile(Path.Combine("/unmapped-container-dir", "secret.bin"));
+                var ex = Assert.Throws<Exception>(() => _command.ProcessCommand(_executionContext.Object, artifactsFile, container));
+                Assert.Contains("line 1", ex.Message);
+                Assert.Contains("not inside a volume mounted", ex.Message);
+            }
+        }
+
+        // ---------- Format / scheme rules ----------
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void EqualsSign_Rejected()
+        {
+            using (var hostContext = Setup())
+            {
+                var artifactsFile = WriteArtifactsFile("name=ghcr.io/x@sha256:" + new string('a', 64));
+                var ex = Assert.Throws<Exception>(() => _command.ProcessCommand(_executionContext.Object, artifactsFile, null));
+                Assert.Contains("line 1", ex.Message);
+                Assert.Contains("'='", ex.Message);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void UnsupportedScheme_Rejected()
+        {
+            using (var hostContext = Setup())
+            {
+                var artifactsFile = WriteArtifactsFile("https://example.com/artifact");
+                var ex = Assert.Throws<Exception>(() => _command.ProcessCommand(_executionContext.Object, artifactsFile, null));
+                Assert.Contains("line 1", ex.Message);
+                Assert.Contains("unsupported URI scheme", ex.Message);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void LineNumberInError_PointsAtRightLine()
+        {
+            using (var hostContext = Setup())
+            {
+                var hex = new string('a', 64);
+                var artifactsFile = WriteArtifactsFile(
+                    "# comment",
+                    $"ghcr.io/ok@sha256:{hex}",
+                    "",
+                    "name=bogus");
+                var ex = Assert.Throws<Exception>(() => _command.ProcessCommand(_executionContext.Object, artifactsFile, null));
+                Assert.Contains("line 4", ex.Message);
+            }
+        }
+
+        // ---------- Size and aggregate limits ----------
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void FileTooLarge_Throws()
+        {
+            using (var hostContext = Setup())
+            {
+                var artifactsFile = Path.Combine(_rootDirectory, "huge");
+                // Slightly larger than 1 MiB.
+                File.WriteAllBytes(artifactsFile, new byte[CreateArtifactsFileCommand.MaxFileSizeBytes + 1]);
+                var ex = Assert.Throws<Exception>(() => _command.ProcessCommand(_executionContext.Object, artifactsFile, null));
+                Assert.Contains("$GITHUB_ARTIFACTS", ex.Message);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void AggregateCap_AllowsDuplicatesAtCap()
+        {
+            using (var hostContext = Setup())
+            {
+                // Pre-fill the aggregate to exactly the cap.
+                var hex = new string('a', 64);
+                for (var i = 0; i < CreateArtifactsFileCommand.MaxAggregateArtifacts; i++)
+                {
+                    var name = $"ghcr.io/x{i}";
+                    _global.ArtifactSubjects[name] = new ArtifactSubject(name, $"sha256:{hex}", ArtifactSubjectKind.OciSubject);
+                }
+
+                // A new step redeclares one of the existing artifacts identically — should NOT throw.
+                var artifactsFile = WriteArtifactsFile($"ghcr.io/x0@sha256:{hex}");
+                _command.ProcessCommand(_executionContext.Object, artifactsFile, null);
+                Assert.Equal(CreateArtifactsFileCommand.MaxAggregateArtifacts, _global.ArtifactSubjects.Count);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void AggregateCap_FailsOnFirstDistinctOverflow()
+        {
+            using (var hostContext = Setup())
+            {
+                var hex = new string('a', 64);
+                for (var i = 0; i < CreateArtifactsFileCommand.MaxAggregateArtifacts; i++)
+                {
+                    var name = $"ghcr.io/x{i}";
+                    _global.ArtifactSubjects[name] = new ArtifactSubject(name, $"sha256:{hex}", ArtifactSubjectKind.OciSubject);
+                }
+
+                var artifactsFile = WriteArtifactsFile($"ghcr.io/new@sha256:{hex}");
+                var ex = Assert.Throws<Exception>(() => _command.ProcessCommand(_executionContext.Object, artifactsFile, null));
+                Assert.Contains("500", ex.Message);
+            }
+        }
+
+        // ---------- Aggregation rules ----------
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Aggregation_DedupsIdentical()
+        {
+            using (var hostContext = Setup())
+            {
+                var hex = new string('a', 64);
+                _global.ArtifactSubjects["ghcr.io/x"] = new ArtifactSubject("ghcr.io/x", $"sha256:{hex}", ArtifactSubjectKind.OciSubject);
+                var artifactsFile = WriteArtifactsFile($"ghcr.io/x@sha256:{hex}");
+                _command.ProcessCommand(_executionContext.Object, artifactsFile, null);
+                Assert.Single(_global.ArtifactSubjects);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Aggregation_FailsOnConflict()
+        {
+            using (var hostContext = Setup())
+            {
+                var hexA = new string('a', 64);
+                var hexB = new string('b', 64);
+                _global.ArtifactSubjects["ghcr.io/x"] = new ArtifactSubject("ghcr.io/x", $"sha256:{hexA}", ArtifactSubjectKind.OciSubject);
+                var artifactsFile = WriteArtifactsFile($"ghcr.io/x@sha256:{hexB}");
+                var ex = Assert.Throws<Exception>(() => _command.ProcessCommand(_executionContext.Object, artifactsFile, null));
+                Assert.Contains("Conflicting digest", ex.Message);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void MultipleEntries_AllProcessed()
+        {
+            using (var hostContext = Setup())
+            {
+                Directory.CreateDirectory(Path.Combine(_workspaceDirectory, "dist"));
+                File.WriteAllBytes(Path.Combine(_workspaceDirectory, "dist", "myapp-linux-amd64"), new byte[] { 7 });
+
+                var hex = new string('a', 64);
+                var artifactsFile = WriteArtifactsFile(
+                    "# Release binary",
+                    "dist/myapp-linux-amd64",
+                    "",
+                    "# Published container image",
+                    $"ghcr.io/octocat/myapp:1.0.0@sha256:{hex}");
+
+                _command.ProcessCommand(_executionContext.Object, artifactsFile, null);
+
+                Assert.Equal(2, _global.ArtifactSubjects.Count);
+                Assert.True(_global.ArtifactSubjects.ContainsKey("myapp-linux-amd64"));
+                Assert.True(_global.ArtifactSubjects.ContainsKey("ghcr.io/octocat/myapp:1.0.0"));
+            }
+        }
+
+        // ---------- Setup helpers ----------
+
+        private string WriteArtifactsFile(params string[] lines)
+        {
+            var path = Path.Combine(_rootDirectory, "artifacts");
+            File.WriteAllText(path, string.Join("\n", lines), new UTF8Encoding(false));
+            return path;
+        }
+
+        private TestHostContext Setup(bool featureFlag = true, string envVarOverride = null, [CallerMemberName] string name = "")
+        {
+            _issues = new List<DTWebApi.Issue>();
+
+            // Ensure no leaked state from prior tests in the same process.
+            Environment.SetEnvironmentVariable(CreateArtifactsFileCommand.EnableEnvVar, envVarOverride);
+
+            var hostContext = new TestHostContext(this, name);
+            _trace = hostContext.GetTrace();
+
+            var workDirectory = hostContext.GetDirectory(WellKnownDirectory.Work);
+            Directory.CreateDirectory(workDirectory);
+            _rootDirectory = Path.Combine(workDirectory, nameof(CreateArtifactsFileCommandL0), name);
+            if (Directory.Exists(_rootDirectory))
+            {
+                Directory.Delete(_rootDirectory, recursive: true);
+            }
+            Directory.CreateDirectory(_rootDirectory);
+
+            _workspaceDirectory = Path.Combine(_rootDirectory, "workspace");
+            Directory.CreateDirectory(_workspaceDirectory);
+
+            var variableValues = new Dictionary<string, VariableValue>(StringComparer.OrdinalIgnoreCase);
+            if (featureFlag)
+            {
+                variableValues[Common.Constants.Runner.Features.AllowArtifactsFile] = new VariableValue(FlagOn);
+            }
+            var variables = new Variables(hostContext, variableValues);
+
+            _global = new GlobalContext
+            {
+                EnvironmentVariables = new Dictionary<string, string>(VarUtil.EnvironmentVariableKeyComparer),
+                Variables = variables,
+                WriteDebug = true,
+                ArtifactSubjects = new Dictionary<string, ArtifactSubject>(StringComparer.Ordinal),
+            };
+
+            _executionContext = new Mock<IExecutionContext>();
+            _executionContext.Setup(x => x.Global).Returns(_global);
+            _executionContext.Setup(x => x.GetGitHubContext("workspace")).Returns(_workspaceDirectory);
+            _executionContext.Setup(x => x.AddIssue(It.IsAny<DTWebApi.Issue>(), It.IsAny<ExecutionContextLogOptions>()))
+                .Callback((DTWebApi.Issue issue, ExecutionContextLogOptions logOptions) =>
+                {
+                    _issues.Add(issue);
+                    _trace.Info($"Issue '{issue.Type}': {issue.Message}");
+                });
+            _executionContext.Setup(x => x.Write(It.IsAny<string>(), It.IsAny<string>()))
+                .Callback((string tag, string message) =>
+                {
+                    _trace.Info($"{tag}{message}");
+                });
+
+            _command = new CreateArtifactsFileCommand();
+            _command.Initialize(hostContext);
+
+            return hostContext;
+        }
+    }
+}

src/Test/L0/Worker/DapDebuggerL0.cs

@@ -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
 {
@@ -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")]
@@ -746,6 +1139,11 @@ namespace GitHub.Runner.Common.Tests.Worker
                 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();
@@ -754,11 +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"
                 });
 
@@ -777,6 +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")]

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/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/FileCommandManagerL0.cs

@@ -0,0 +1,105 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Runtime.CompilerServices;
+using GitHub.Runner.Common;
+using GitHub.Runner.Sdk;
+using GitHub.Runner.Worker;
+using GitHub.Runner.Worker.Container;
+using Moq;
+using Xunit;
+
+namespace GitHub.Runner.Common.Tests.Worker
+{
+    public sealed class FileCommandManagerL0
+    {
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void InitializeFiles_InvokesPopulateInitialContents_OncePerExtension()
+        {
+            using (var hostContext = Setup(out var executionContext, out var ext))
+            {
+                var manager = new FileCommandManager();
+                manager.Initialize(hostContext);
+
+                manager.InitializeFiles(executionContext, null);
+
+                Assert.Equal(1, ext.PopulateCallCount);
+                Assert.True(File.Exists(ext.LastPopulatedPath));
+
+                // A second invocation should populate again with the new
+                // per-step file (file path rotates between calls).
+                var firstPath = ext.LastPopulatedPath;
+                manager.InitializeFiles(executionContext, null);
+                Assert.Equal(2, ext.PopulateCallCount);
+                Assert.NotEqual(firstPath, ext.LastPopulatedPath);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void InitializeFiles_PopulateException_DoesNotAbortInitialization()
+        {
+            using (var hostContext = Setup(out var executionContext, out var ext))
+            {
+                ext.ThrowOnPopulate = true;
+
+                var manager = new FileCommandManager();
+                manager.Initialize(hostContext);
+
+                // Must not throw — failures during populate should be
+                // swallowed so a misbehaving extension cannot block step
+                // setup.
+                manager.InitializeFiles(executionContext, null);
+
+                Assert.Equal(1, ext.PopulateCallCount);
+            }
+        }
+
+        private TestHostContext Setup(out IExecutionContext executionContext, out RecordingFileCommand recordingExtension, [CallerMemberName] string name = "")
+        {
+            var hostContext = new TestHostContext(this, name);
+
+            recordingExtension = new RecordingFileCommand();
+            recordingExtension.Initialize(hostContext);
+
+            var extensionManager = new Mock<IExtensionManager>();
+            extensionManager.Setup(x => x.GetExtensions<IFileCommandExtension>())
+                .Returns(new List<IFileCommandExtension> { recordingExtension });
+            hostContext.SetSingleton<IExtensionManager>(extensionManager.Object);
+
+            var ec = new Mock<IExecutionContext>();
+            ec.Setup(x => x.SetGitHubContext(It.IsAny<string>(), It.IsAny<string>()));
+            executionContext = ec.Object;
+
+            return hostContext;
+        }
+
+        private sealed class RecordingFileCommand : RunnerService, IFileCommandExtension
+        {
+            public string ContextName => "recording";
+            public string FilePrefix => "recording_";
+            public Type ExtensionType => typeof(IFileCommandExtension);
+
+            public int PopulateCallCount { get; private set; }
+            public string LastPopulatedPath { get; private set; }
+            public bool ThrowOnPopulate { get; set; }
+
+            public void PopulateInitialContents(IExecutionContext context, string filePath, ContainerInfo container)
+            {
+                PopulateCallCount++;
+                LastPopulatedPath = filePath;
+                if (ThrowOnPopulate)
+                {
+                    throw new InvalidOperationException("intentional");
+                }
+            }
+
+            public void ProcessCommand(IExecutionContext context, string filePath, ContainerInfo container)
+            {
+            }
+        }
+    }
+}

src/Test/L0/Worker/JobExecutionViewL0.cs

@@ -0,0 +1,130 @@
+using System;
+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 JobExecutionViewL0
+    {
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void RendersPreMainAndPostSections()
+        {
+            var pre = CreateStep("Pre cache", ActionRunStage.Pre);
+            var checkout = CreateStep("Checkout");
+            var post = CreateStep("Post cache", ActionRunStage.Post);
+
+            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 ClaimsPredictedPostStepWithoutChangingLine()
+        {
+            var action = CreateRepositoryActionStep("actions/cache");
+            var checkout = CreateActionRunner("Checkout", ActionRunStage.Main, action);
+            var predicted = new JobExecutionView.PredictedPostStep(
+                "Post Checkout",
+                MatchKeyFor(action.Id));
+
+            var view = new JobExecutionView(
+                "job",
+                new[] { checkout.Object },
+                Array.Empty<IStep>(),
+                new[] { predicted });
+
+            var post = CreateActionRunner("Post Checkout", ActionRunStage.Post, action);
+            var line = view.TryClaimPredictedStep(MatchKeyFor(action.Id), post.Object);
+
+            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 UsesSyntheticCompleteJobLineWhenPostStepSharesName()
+        {
+            var checkout = CreateStep("Checkout");
+            var realPost = CreateStep("Complete job", ActionRunStage.Post);
+
+            var view = new JobExecutionView(
+                "job",
+                new[] { checkout.Object },
+                new[] { realPost.Object });
+
+            Assert.Equal(8, view.TryGetLineForStep(realPost.Object));
+            Assert.Equal(9, view.CompleteJobLine);
+        }
+
+        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, 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 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/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/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/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="8.0.421"
+DOTNETSDK_VERSION="8.0.422"
 DOTNETSDK_INSTALLDIR="$DOTNETSDK_ROOT/$DOTNETSDK_VERSION"
 RUNNER_VERSION=$(cat runnerversion)
 

src/global.json

@@ -1,5 +1,5 @@
 {
   "sdk": {
-    "version": "8.0.421"
+    "version": "8.0.422"
   }
 }

src/runnerversion

@@ -1 +1 @@
-2.334.0
+2.335.0