Add JobExecutionView state container

The DAP debugger needs to map a runtime IStep back to a source line
when answering `stackTrace` requests. The renderer (#PR1b) produces
the YAML and per-entry start lines from an immutable list, but the
debugger's view grows over the job's lifetime: post steps register
lazily, and the integration layer needs O(1) IStep -> line lookup
at every pause.

This commit adds JobExecutionView, a stateful append-only wrapper
around the renderer. It maintains:
  - the current entry list,
  - the most recent rendered YAML,
  - a Dictionary<IStep, int> for fast line lookup.

Each Append can register an entry in one of three modes:
  - with a stepIdentity: registers the IStep -> line mapping
    immediately;
  - with a matchKey: registers an unclaimed placeholder that a
    later TryClaim binds to a real IStep (used when an entry is
    predicted before the runner materializes its IStep, e.g. a
    Post-step placeholder synthesized at job-init from an action's
    metadata);
  - with neither: a static informational entry that needs no line
    lookup.

This is part 3 of 5. The DAP-integration PR that consumes this
container is the final follow-up.

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

.devcontainer/devcontainer.json

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

images/Dockerfile

@@ -5,8 +5,8 @@ ARG TARGETOS
 ARG TARGETARCH
 ARG RUNNER_VERSION
 ARG RUNNER_CONTAINER_HOOKS_VERSION=0.7.0
-ARG DOCKER_VERSION=29.4.0
-ARG BUILDX_VERSION=0.33.0
+ARG DOCKER_VERSION=29.5.0
+ARG BUILDX_VERSION=0.34.0
 
 RUN apt update -y && apt install curl unzip -y
 

src/Runner.Common/Constants.cs

@@ -179,6 +179,7 @@ namespace GitHub.Runner.Common
                 public static readonly string EmitCompositeMarkers = "actions_runner_emit_composite_markers";
                 public static readonly string BatchActionResolution = "actions_batch_action_resolution";
                 public static readonly string UseBearerTokenForCodeload = "actions_use_bearer_token_for_codeload";
+                public static readonly string OverrideDebuggerWelcomeMessage = "actions_runner_override_debugger_welcome_message";
             }
 
             // Node version migration related constants

src/Runner.Worker/Dap/DapDebugger.cs

@@ -63,6 +63,7 @@ namespace GitHub.Runner.Worker.Dap
         private volatile DapSessionState _state = DapSessionState.NotStarted;
         private CancellationTokenRegistration? _cancellationRegistration;
         private bool _isFirstStep = true;
+        private bool _welcomeMessageSent;
 
         // Dev Tunnel relay host for remote debugging
         private TunnelRelayTunnelHost _tunnelRelayHost;
@@ -490,6 +491,11 @@ namespace GitHub.Runner.Worker.Dap
                     });
                     Trace.Info("Sent initialized event");
                 }
+
+                if (request.Command == "configurationDone")
+                {
+                    SendWelcomeMessage();
+                }
             }
             catch (Exception ex)
             {
@@ -508,6 +514,7 @@ namespace GitHub.Runner.Worker.Dap
         internal void HandleClientConnected()
         {
             _isClientConnected = true;
+            _welcomeMessageSent = false;
             Trace.Info("Client connected to debug session");
 
             // If we're paused, re-send the stopped event so the new client
@@ -818,6 +825,34 @@ namespace GitHub.Runner.Worker.Dap
             });
         }
 
+        internal void SendWelcomeMessage()
+        {
+            if (_welcomeMessageSent)
+            {
+                return;
+            }
+            _welcomeMessageSent = true;
+
+            var debuggerConfig = _jobContext?.Global?.Debugger;
+            if (debuggerConfig?.OverrideWelcomeMessage == true)
+            {
+                if (!string.IsNullOrEmpty(debuggerConfig.WelcomeMessage))
+                {
+                    SendOutput("console", debuggerConfig.WelcomeMessage);
+                    Trace.Info("Sent custom welcome message");
+                }
+                else
+                {
+                    Trace.Info("Welcome message suppressed by override");
+                }
+            }
+            else
+            {
+                SendOutput("console", DapReplParser.GetGeneralHelp());
+                Trace.Info("Sent default welcome message");
+            }
+        }
+
         internal async Task OnStepStartingAsync(IStep step, bool isFirstStep)
         {
             bool pauseOnNextStep;
@@ -860,6 +895,9 @@ namespace GitHub.Runner.Worker.Dap
             // Send stopped event to debugger (only if client is connected)
             SendStoppedEvent(reason, description);
 
+            // Emit a banner so the user knows where REPL commands will execute
+            SendExecutionContextBanner();
+
             // Wait for debugger command
             await WaitForCommandAsync(cancellationToken);
         }
@@ -1195,7 +1233,12 @@ namespace GitHub.Runner.Worker.Dap
 
                 case RunCommand run:
                     var context = GetExecutionContextForFrame(frameId);
-                    return await _replExecutor.ExecuteRunCommandAsync(run, context, cancellationToken);
+                    bool isActionStep;
+                    lock (_stateLock)
+                    {
+                        isActionStep = _currentStep is IActionRunner;
+                    }
+                    return await _replExecutor.ExecuteRunCommandAsync(run, context, isActionStep, cancellationToken);
 
                 default:
                     return new EvaluateResponseBody
@@ -1407,6 +1450,40 @@ namespace GitHub.Runner.Worker.Dap
             });
         }
 
+        /// <summary>
+        /// Emits a console output banner telling the user whether REPL
+        /// commands will execute on the host or inside the job container.
+        /// </summary>
+        private void SendExecutionContextBanner()
+        {
+            if (!_isClientConnected)
+            {
+                return;
+            }
+
+            bool isActionStep = _currentStep is IActionRunner;
+            var container = _jobContext?.Global?.Container;
+
+            string target;
+            if (isActionStep && container != null &&
+                (!string.IsNullOrEmpty(container.ContainerId) ||
+                 FeatureManager.IsContainerHooksEnabled(_jobContext?.Global?.Variables)))
+            {
+                var image = container.ContainerImage ?? "container";
+                var shortId = !string.IsNullOrEmpty(container.ContainerId) && container.ContainerId.Length >= 12
+                    ? container.ContainerId.Substring(0, 12)
+                    : container.ContainerId ?? "";
+                var idSuffix = !string.IsNullOrEmpty(shortId) ? $" ({shortId})" : "";
+                target = $"job container: {image}{idSuffix}";
+            }
+            else
+            {
+                target = "runner host";
+            }
+
+            SendOutput("console", $"\nCommands will run on {target}\n");
+        }
+
         private string MaskUserVisibleText(string value)
         {
             if (string.IsNullOrEmpty(value))

src/Runner.Worker/Dap/DapReplExecutor.cs

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

src/Runner.Worker/Dap/DebuggerConfig.cs

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

src/Runner.Worker/Dap/JobExecutionViewRenderer.cs

@@ -1,11 +1,8 @@
 using System;
 using System.Collections.Generic;
 using System.Globalization;
-using System.IO;
 using System.Text;
 using GitHub.Runner.Sdk;
-using YamlDotNet.Core;
-using YamlDotNet.Core.Events;
 
 namespace GitHub.Runner.Worker.Dap
 {
@@ -144,7 +141,7 @@ namespace GitHub.Runner.Worker.Dap
             int newlinesEmitted = 0;
 
             // Header (3 lines).
-            sb.Append("# Job: ").Append(FormatScalar(jobId)).Append('\n');
+            sb.Append("# Job: ").Append(YamlScalarFormatter.Format(jobId)).Append('\n');
             sb.Append("# Runner execution plan — read-only.\n");
             sb.Append('\n');
             newlinesEmitted += 3;
@@ -203,7 +200,7 @@ namespace GitHub.Runner.Worker.Dap
                 // 1-based line of the `- step:` key for this entry.
                 startLines[i] = newlinesEmitted + 1;
 
-                sb.Append("  - step: ").Append(FormatScalar(entry.DisplayName));
+                sb.Append("  - step: ").Append(YamlScalarFormatter.Format(entry.DisplayName));
                 sb.Append('\n');
                 newlinesEmitted++;
 
@@ -213,7 +210,7 @@ namespace GitHub.Runner.Worker.Dap
                     case JobExecutionPhase.Post:
                         if (!string.IsNullOrEmpty(entry.Uses))
                         {
-                            sb.Append("    action: ").Append(FormatScalar(entry.Uses)).Append('\n');
+                            sb.Append("    action: ").Append(YamlScalarFormatter.Format(entry.Uses)).Append('\n');
                             newlinesEmitted++;
                         }
                         // No source: annotation for pre/post.
@@ -222,19 +219,19 @@ namespace GitHub.Runner.Worker.Dap
                     case JobExecutionPhase.Main:
                         if (!string.IsNullOrEmpty(entry.Id))
                         {
-                            sb.Append("    id: ").Append(FormatScalar(entry.Id)).Append('\n');
+                            sb.Append("    id: ").Append(YamlScalarFormatter.Format(entry.Id)).Append('\n');
                             newlinesEmitted++;
                         }
                         if (!string.IsNullOrEmpty(entry.Uses))
                         {
-                            sb.Append("    uses: ").Append(FormatScalar(entry.Uses)).Append('\n');
+                            sb.Append("    uses: ").Append(YamlScalarFormatter.Format(entry.Uses)).Append('\n');
                             newlinesEmitted++;
                         }
                         if (!string.IsNullOrEmpty(entry.Run))
                         {
                             if (entry.Run.IndexOf('\n') < 0)
                             {
-                                sb.Append("    run: ").Append(FormatScalar(entry.Run)).Append('\n');
+                                sb.Append("    run: ").Append(YamlScalarFormatter.Format(entry.Run)).Append('\n');
                                 newlinesEmitted++;
                             }
                             else
@@ -246,7 +243,7 @@ namespace GitHub.Runner.Worker.Dap
                         }
                         if (!string.IsNullOrEmpty(entry.If))
                         {
-                            sb.Append("    if: ").Append(FormatScalar(entry.If)).Append('\n');
+                            sb.Append("    if: ").Append(YamlScalarFormatter.Format(entry.If)).Append('\n');
                             newlinesEmitted++;
                         }
                         if (!string.IsNullOrEmpty(entry.ContinueOnError))
@@ -275,12 +272,12 @@ namespace GitHub.Runner.Worker.Dap
                         }
                         if (!string.IsNullOrEmpty(entry.Shell))
                         {
-                            sb.Append("    shell: ").Append(FormatScalar(entry.Shell)).Append('\n');
+                            sb.Append("    shell: ").Append(YamlScalarFormatter.Format(entry.Shell)).Append('\n');
                             newlinesEmitted++;
                         }
                         if (!string.IsNullOrEmpty(entry.WorkingDirectory))
                         {
-                            sb.Append("    working-directory: ").Append(FormatScalar(entry.WorkingDirectory)).Append('\n');
+                            sb.Append("    working-directory: ").Append(YamlScalarFormatter.Format(entry.WorkingDirectory)).Append('\n');
                             newlinesEmitted++;
                         }
                         if (entry.SourcePath != null)
@@ -335,55 +332,5 @@ namespace GitHub.Runner.Worker.Dap
             }
             return n;
         }
-
-        /// <summary>
-        /// Formats a single string as a YAML 1.x flow scalar, delegating
-        /// quoting/escaping decisions to YamlDotNet. This avoids maintaining
-        /// our own escape table for every YAML-significant character: we
-        /// just emit the value through the YAML library and use whichever
-        /// scalar style (plain, single-quoted, double-quoted) it picks.
-        /// A new <see cref="Emitter"/> is created per call, so the helper
-        /// is safe to invoke concurrently.
-        /// </summary>
-        internal static string FormatScalar(string value)
-        {
-            if (value == null)
-            {
-                throw new ArgumentNullException(nameof(value));
-            }
-
-            using var sw = new StringWriter(CultureInfo.InvariantCulture);
-            // Force LF line breaks; YamlDotNet's Emitter calls WriteLine,
-            // which would otherwise produce CRLF on Windows and break
-            // both our document-end stripping below and downstream
-            // consumers that assume a single line-break convention.
-            sw.NewLine = "\n";
-            var emitter = new Emitter(sw);
-            emitter.Emit(new StreamStart());
-            emitter.Emit(new DocumentStart(null, null, true));
-            emitter.Emit(new Scalar(null, null, value, ScalarStyle.Any, true, true));
-            emitter.Emit(new DocumentEnd(true));
-            emitter.Emit(new StreamEnd());
-
-            string raw = sw.ToString();
-            // Strip YAML document markers. Emitter elides these for most
-            // scalars but emits "--- " (with space) for some edge cases
-            // (e.g. empty strings). Defensively handle "---\n" too.
-            if (raw.StartsWith("--- ", StringComparison.Ordinal))
-            {
-                raw = raw.Substring(4);
-            }
-            else if (raw.StartsWith("---\n", StringComparison.Ordinal))
-            {
-                raw = raw.Substring(4);
-            }
-            raw = raw.TrimEnd('\n');
-            const string DocEndMarker = "\n...";
-            if (raw.EndsWith(DocEndMarker, StringComparison.Ordinal))
-            {
-                raw = raw.Substring(0, raw.Length - DocEndMarker.Length);
-            }
-            return raw.TrimEnd('\n');
-        }
     }
 }

src/Runner.Worker/Dap/StepEntryTranslator.cs

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

src/Runner.Worker/Dap/TemplateTokenYamlAdapter.cs

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

src/Runner.Worker/Dap/YamlScalarFormatter.cs

@@ -0,0 +1,63 @@
+using System;
+using System.Globalization;
+using System.IO;
+using GitHub.Runner.Sdk;
+using YamlDotNet.Core;
+using YamlDotNet.Core.Events;
+
+namespace GitHub.Runner.Worker.Dap
+{
+    /// <summary>
+    /// Formats a single string as a quote-safe YAML scalar by routing it
+    /// through YamlDotNet's <see cref="Emitter"/>. The returned text is
+    /// safe to splice into a hand-emitted YAML document fragment.
+    ///
+    /// Caller responsibility: this only handles the scalar value; it does
+    /// not emit a key, indent, or trailing newline.
+    /// </summary>
+    internal static class YamlScalarFormatter
+    {
+        /// <summary>
+        /// Return <paramref name="value"/> formatted as a YAML scalar:
+        /// plain, single-quoted, or double-quoted as the emitter chooses,
+        /// with no surrounding document markers or trailing newline.
+        /// </summary>
+        public static string Format(string value)
+        {
+            ArgUtil.NotNull(value, nameof(value));
+
+            using var sw = new StringWriter(CultureInfo.InvariantCulture);
+            // Force LF line breaks; YamlDotNet's Emitter calls WriteLine,
+            // which would otherwise produce CRLF on Windows and break
+            // both our document-end stripping below and downstream
+            // consumers that assume a single line-break convention.
+            sw.NewLine = "\n";
+            var emitter = new Emitter(sw);
+            emitter.Emit(new StreamStart());
+            emitter.Emit(new DocumentStart(null, null, true));
+            emitter.Emit(new Scalar(null, null, value, ScalarStyle.Any, true, true));
+            emitter.Emit(new DocumentEnd(true));
+            emitter.Emit(new StreamEnd());
+
+            string raw = sw.ToString();
+            // Strip YAML document markers. Emitter elides these for most
+            // scalars but emits "--- " (with space) for some edge cases
+            // (e.g. empty strings). Defensively handle "---\n" too.
+            if (raw.StartsWith("--- ", StringComparison.Ordinal))
+            {
+                raw = raw.Substring(4);
+            }
+            else if (raw.StartsWith("---\n", StringComparison.Ordinal))
+            {
+                raw = raw.Substring(4);
+            }
+            raw = raw.TrimEnd('\n');
+            const string DocEndMarker = "\n...";
+            if (raw.EndsWith(DocEndMarker, StringComparison.Ordinal))
+            {
+                raw = raw.Substring(0, raw.Length - DocEndMarker.Length);
+            }
+            return raw;
+        }
+    }
+}

src/Runner.Worker/ExecutionContext.cs

@@ -970,7 +970,8 @@ namespace GitHub.Runner.Worker
             Global.WriteDebug = Global.Variables.Step_Debug ?? false;
 
             // Debugger enabled flag (from acquire response).
-            Global.Debugger = new Dap.DebuggerConfig(message.EnableDebugger, message.DebuggerTunnel);
+            var overrideDebuggerWelcomeMessage = Global.Variables.GetBoolean(Constants.Runner.Features.OverrideDebuggerWelcomeMessage) ?? false;
+            Global.Debugger = new Dap.DebuggerConfig(message.EnableDebugger, message.DebuggerTunnel, overrideDebuggerWelcomeMessage, message.DebuggerWelcomeMessage);
 
             // Hook up JobServerQueueThrottling event, we will log warning on server tarpit.
             _jobServerQueue.JobServerQueueThrottling += JobServerQueueThrottling_EventReceived;

src/Sdk/DTPipelines/Pipelines/AgentJobRequestMessage.cs

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

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

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

src/Test/L0/Worker/DapDebuggerL0.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.IO;
 using System.Net;
 using System.Net.Sockets;
@@ -236,7 +236,7 @@ namespace GitHub.Runner.Common.Tests.Worker
             }
         }
 
-        private static Mock<IExecutionContext> CreateJobContextWithTunnel(CancellationToken cancellationToken, ushort port, string jobName = null)
+        private static Mock<IExecutionContext> CreateJobContextWithTunnel(CancellationToken cancellationToken, ushort port, string jobName = null, bool overrideWelcomeMessage = false, string welcomeMessage = null)
         {
             var tunnel = new GitHub.DistributedTask.Pipelines.DebuggerTunnelInfo
             {
@@ -245,7 +245,7 @@ namespace GitHub.Runner.Common.Tests.Worker
                 HostToken = "test-token",
                 Port = port
             };
-            var debuggerConfig = new DebuggerConfig(true, tunnel);
+            var debuggerConfig = new DebuggerConfig(true, tunnel, overrideWelcomeMessage, welcomeMessage);
             var jobContext = new Mock<IExecutionContext>();
             jobContext.Setup(x => x.CancellationToken).Returns(cancellationToken);
             jobContext.Setup(x => x.Global).Returns(new GlobalContext { Debugger = debuggerConfig });
@@ -742,6 +742,8 @@ namespace GitHub.Runner.Common.Tests.Worker
 
                 // Read the configurationDone response
                 await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                // Read the welcome message output event
+                await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
                 await waitTask;
 
                 // Complete the job — OnJobCompletedAsync pauses when stepping,
@@ -849,6 +851,8 @@ namespace GitHub.Runner.Common.Tests.Worker
                     Command = "configurationDone"
                 });
 
+                await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                // Read the welcome message output event
                 await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
                 await waitTask;
 
@@ -867,5 +871,224 @@ namespace GitHub.Runner.Common.Tests.Worker
                 Assert.Equal(completedTask, finished);
             }
         }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task WelcomeMessageSendsDefaultHelpWhenOverrideDisabled()
+        {
+            using (CreateTestContext())
+            {
+                var port = GetFreePort();
+                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
+                var jobContext = CreateJobContextWithTunnel(cts.Token, port);
+                await _debugger.StartAsync(jobContext.Object);
+
+                using var client = await ConnectClientAsync(port);
+                var stream = client.GetStream();
+
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 1,
+                    Type = "request",
+                    Command = "configurationDone"
+                });
+
+                // First message: configurationDone response
+                var configDoneResponse = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"command\":\"configurationDone\"", configDoneResponse);
+
+                // Second message: welcome output event with default help text
+                var welcomeMsg = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"event\":\"output\"", welcomeMsg);
+                Assert.Contains("\"category\":\"console\"", welcomeMsg);
+                Assert.Contains("Actions Debug Console", welcomeMsg);
+                Assert.Contains("help", welcomeMsg);
+
+                await _debugger.StopAsync();
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task WelcomeMessageShowsCustomMessageWhenOverrideEnabled()
+        {
+            using (CreateTestContext())
+            {
+                var port = GetFreePort();
+                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
+                var jobContext = CreateJobContextWithTunnel(cts.Token, port,
+                    overrideWelcomeMessage: true,
+                    welcomeMessage: "Welcome to debugging!");
+                await _debugger.StartAsync(jobContext.Object);
+
+                using var client = await ConnectClientAsync(port);
+                var stream = client.GetStream();
+
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 1,
+                    Type = "request",
+                    Command = "configurationDone"
+                });
+
+                // First: configurationDone response
+                var configDoneResponse = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"command\":\"configurationDone\"", configDoneResponse);
+
+                // Second: custom welcome message
+                var welcomeMsg = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"event\":\"output\"", welcomeMsg);
+                Assert.Contains("Welcome to debugging!", welcomeMsg);
+
+                await _debugger.StopAsync();
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task WelcomeMessageSuppressedWhenOverrideEnabledWithEmptyMessage()
+        {
+            using (CreateTestContext())
+            {
+                var port = GetFreePort();
+                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
+                var jobContext = CreateJobContextWithTunnel(cts.Token, port,
+                    overrideWelcomeMessage: true,
+                    welcomeMessage: "");
+                await _debugger.StartAsync(jobContext.Object);
+
+                using var client = await ConnectClientAsync(port);
+                var stream = client.GetStream();
+
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 1,
+                    Type = "request",
+                    Command = "configurationDone"
+                });
+
+                // Read configurationDone response
+                var configDoneResponse = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"command\":\"configurationDone\"", configDoneResponse);
+
+                // Send threads request — if welcome message was suppressed, this
+                // should be the next response (no output event in between)
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 2,
+                    Type = "request",
+                    Command = "threads"
+                });
+
+                var threadsResponse = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"command\":\"threads\"", threadsResponse);
+
+                await _debugger.StopAsync();
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task WelcomeMessageSuppressedWhenOverrideEnabledWithNullMessage()
+        {
+            using (CreateTestContext())
+            {
+                var port = GetFreePort();
+                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
+                var jobContext = CreateJobContextWithTunnel(cts.Token, port,
+                    overrideWelcomeMessage: true,
+                    welcomeMessage: null);
+                await _debugger.StartAsync(jobContext.Object);
+
+                using var client = await ConnectClientAsync(port);
+                var stream = client.GetStream();
+
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 1,
+                    Type = "request",
+                    Command = "configurationDone"
+                });
+
+                // Read configurationDone response
+                var configDoneResponse = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"command\":\"configurationDone\"", configDoneResponse);
+
+                // Send threads request — if welcome message was suppressed, this
+                // should be the next response (no output event in between)
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 2,
+                    Type = "request",
+                    Command = "threads"
+                });
+
+                var threadsResponse = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"command\":\"threads\"", threadsResponse);
+
+                await _debugger.StopAsync();
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task WelcomeMessageSentOnlyOnce()
+        {
+            using (CreateTestContext())
+            {
+                var port = GetFreePort();
+                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
+                var jobContext = CreateJobContextWithTunnel(cts.Token, port);
+                await _debugger.StartAsync(jobContext.Object);
+
+                using var client = await ConnectClientAsync(port);
+                var stream = client.GetStream();
+
+                // First configurationDone
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 1,
+                    Type = "request",
+                    Command = "configurationDone"
+                });
+
+                var configDoneResponse = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"command\":\"configurationDone\"", configDoneResponse);
+
+                // Welcome message should appear
+                var welcomeMsg = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"event\":\"output\"", welcomeMsg);
+                Assert.Contains("Actions Debug Console", welcomeMsg);
+
+                // Second configurationDone — should NOT produce another welcome message
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 2,
+                    Type = "request",
+                    Command = "configurationDone"
+                });
+
+                var secondResponse = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"command\":\"configurationDone\"", secondResponse);
+
+                // Next message should be threads response, not another welcome output
+                await SendRequestAsync(stream, new Request
+                {
+                    Seq = 3,
+                    Type = "request",
+                    Command = "threads"
+                });
+
+                var threadsResponse = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"command\":\"threads\"", threadsResponse);
+
+                await _debugger.StopAsync();
+            }
+        }
     }
 }

src/Test/L0/Worker/DapReplExecutorL0.cs

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

src/Test/L0/Worker/JobExecutionViewRendererL0.cs

@@ -588,23 +588,11 @@ namespace GitHub.Runner.Common.Tests.Worker
         {
             // Regression: YamlDotNet's Emitter calls WriteLine, which on
             // Windows produces CRLF (the host's Environment.NewLine).
-            // FormatScalar / TemplateTokenYamlAdapter.Serialize must force
-            // LF so the rendered view round-trips regardless of platform.
+            // The renderer's hand-emitted skeleton always uses '\n'; this
+            // test asserts the scalar formatter doesn't sneak CRLF in.
             var entry = new JobExecutionViewEntry(JobExecutionPhase.Main, "with: colon", id: "step-1", uses: "actions/checkout@v4");
             var result = JobExecutionViewRenderer.Render("job-1", new[] { entry });
             Assert.DoesNotContain("\r", result.Yaml);
         }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void FormatScalar_AlwaysUsesLfLineBreaks()
-        {
-            // Direct check on FormatScalar to guard against future refactors
-            // that bypass the full Render path but still emit through
-            // YamlDotNet.
-            Assert.DoesNotContain("\r", JobExecutionViewRenderer.FormatScalar("with: colon"));
-            Assert.DoesNotContain("\r", JobExecutionViewRenderer.FormatScalar("hello"));
-        }
     }
 }

src/Test/L0/Worker/StepEntryTranslatorL0.cs

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

src/Test/L0/Worker/TemplateTokenYamlAdapterL0.cs

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

src/Test/L0/Worker/YamlScalarFormatterL0.cs

@@ -0,0 +1,119 @@
+using System;
+using System.Collections.Generic;
+using GitHub.Runner.Worker.Dap;
+using Xunit;
+using YamlDotNet.Serialization;
+
+namespace GitHub.Runner.Common.Tests.Worker
+{
+    public sealed class YamlScalarFormatterL0
+    {
+        private static readonly IDeserializer Deserializer = new DeserializerBuilder().Build();
+
+        // Embed the formatter output inside a minimal YAML mapping and
+        // round-trip through YamlDotNet, asserting the parsed value equals
+        // the original input. Decouples assertions from the emitter's
+        // quoting choices (plain vs single- vs double-quoted).
+        private static void AssertRoundTrips(string value)
+        {
+            string scalar = YamlScalarFormatter.Format(value);
+            string yaml = $"k: {scalar}\n";
+
+            Dictionary<string, object> doc;
+            try
+            {
+                doc = Deserializer.Deserialize<Dictionary<string, object>>(yaml);
+            }
+            catch (Exception ex)
+            {
+                throw new Xunit.Sdk.XunitException(
+                    $"Formatted scalar did not round-trip as valid YAML.\nInput: '{value}'\nFormatted: '{scalar}'\nFull YAML:\n{yaml}\nError: {ex}");
+            }
+            Assert.NotNull(doc);
+            Assert.True(doc.ContainsKey("k"), $"missing key in parsed doc. Formatted: '{scalar}'");
+            Assert.Equal(value, doc["k"] as string);
+        }
+
+        [Theory]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        [InlineData("hello")]
+        [InlineData("with: colon")]
+        [InlineData("with#hash")]
+        [InlineData(" leading")]
+        [InlineData("trailing ")]
+        [InlineData("a\"b")]
+        [InlineData("a\\b")]
+        [InlineData("@at")]
+        [InlineData("*star")]
+        [InlineData("&amp")]
+        [InlineData("?question")]
+        [InlineData("!exclaim")]
+        [InlineData("- dash")]
+        [InlineData("{brace}")]
+        [InlineData("[bracket]")]
+        public void Format_RoundTripsThroughYamlDeserializer(string value)
+        {
+            // The formatter must produce output that, embedded under a key,
+            // parses back to exactly the input. The emitter is free to
+            // pick plain, single-quoted, or double-quoted style.
+            AssertRoundTrips(value);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Format_PlainAscii_NoQuotingNeeded()
+        {
+            // Sanity check that the simple case stays plain.
+            Assert.Equal("hello", YamlScalarFormatter.Format("hello"));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Format_NoTrailingNewline()
+        {
+            Assert.False(YamlScalarFormatter.Format("hello").EndsWith("\n"));
+            Assert.False(YamlScalarFormatter.Format("with: colon").EndsWith("\n"));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Format_NoDocumentMarkers()
+        {
+            // The emitter wraps the scalar in a document; the formatter
+            // must strip both `--- ` (with space) and `---\n` (on its
+            // own line) prefixes plus the `\n...` suffix.
+            Assert.DoesNotContain("---", YamlScalarFormatter.Format("hello"));
+            Assert.DoesNotContain("...", YamlScalarFormatter.Format("hello"));
+            // Empty string is one of the cases where the emitter does
+            // produce a document marker by default.
+            Assert.DoesNotContain("---", YamlScalarFormatter.Format(""));
+            Assert.DoesNotContain("...", YamlScalarFormatter.Format(""));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Format_AlwaysUsesLfLineBreaks()
+        {
+            // Regression: YamlDotNet's Emitter calls WriteLine, which on
+            // Windows produces CRLF (the host's Environment.NewLine).
+            // Format must force LF so the output round-trips regardless
+            // of platform.
+            Assert.DoesNotContain('\r', YamlScalarFormatter.Format("hello"));
+            Assert.DoesNotContain('\r', YamlScalarFormatter.Format("with: colon"));
+            Assert.DoesNotContain('\r', YamlScalarFormatter.Format(""));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Format_NullValue_Throws()
+        {
+            Assert.Throws<ArgumentNullException>(() => YamlScalarFormatter.Format(null));
+        }
+    }
+}

src/dev.sh

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

src/global.json

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