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"

docs/start/envlinux.md

@@ -25,11 +25,11 @@ The `installdependencies.sh` script should install all required dependencies on
 
 Debian based OS (Debian, Ubuntu, Linux Mint)
 
-- liblttng-ust1 or liblttng-ust0
+- liblttng-ust1t64, liblttng-ust1 or liblttng-ust0
 - libkrb5-3
 - zlib1g
 - libssl3t64, libssl3, libssl1.1, libssl1.0.2 or libssl1.0.0
-- libicu76, libicu75, ..., libicu66, libicu65, libicu63, libicu60, libicu57, libicu55, or libicu52
+- libicu80, libicu79, ..., libicu66, libicu65, libicu63, libicu60, libicu57, libicu55, or libicu52
 
 Fedora based OS (Fedora, Red Hat Enterprise Linux, CentOS, Oracle Linux 7)
 

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/Misc/layoutbin/installdependencies.sh

@@ -94,7 +94,7 @@ then
             fi
         }
 
-        apt_get_with_fallbacks liblttng-ust1 liblttng-ust0
+        apt_get_with_fallbacks liblttng-ust1t64 liblttng-ust1 liblttng-ust0
         if [ $? -ne 0 ]
         then
             echo "'$apt_get' failed with exit code '$?'"
@@ -110,7 +110,7 @@ then
             exit 1
         fi
 
-        apt_get_with_fallbacks libicu76 libicu75 libicu74 libicu73 libicu72 libicu71 libicu70 libicu69 libicu68 libicu67 libicu66 libicu65 libicu63 libicu60 libicu57 libicu55 libicu52
+        apt_get_with_fallbacks libicu80 libicu79 libicu78 libicu77 libicu76 libicu75 libicu74 libicu73 libicu72 libicu71 libicu70 libicu69 libicu68 libicu67 libicu66 libicu65 libicu63 libicu60 libicu57 libicu55 libicu52
         if [ $? -ne 0 ]
         then
             echo "'$apt_get' failed with exit code '$?'"

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

@@ -16,10 +16,19 @@ using Microsoft.DevTunnels.Connections;
 using Microsoft.DevTunnels.Contracts;
 using Microsoft.DevTunnels.Management;
 using Newtonsoft.Json;
-using Pipelines = GitHub.DistributedTask.Pipelines;
 
 namespace GitHub.Runner.Worker.Dap
 {
+    /// <summary>
+    /// Stores information about a completed step for stack trace display.
+    /// </summary>
+    internal sealed class CompletedStepInfo
+    {
+        public string DisplayName { get; set; }
+        public TaskResult? Result { get; set; }
+        public int FrameId { get; set; }
+    }
+
     /// <summary>
     /// Single public facade for the Debug Adapter Protocol subsystem.
     /// Owns the full transport, handshake, step-level pauses, variable
@@ -42,12 +51,8 @@ namespace GitHub.Runner.Worker.Dap
         // Frame ID for the current step (always 1)
         private const int _currentFrameId = 1;
 
-        // Frame ID for the static "job" frame anchored at line 1 of the execution view.
-        private const int _jobFrameId = 2;
-
-        // MVP serves a single synthesized source per session (the job's execution view).
-        // Stable session-scoped ID 1; future sources (composite step-in) will use higher IDs.
-        private const int _executionViewSourceReference = 1;
+        // Frame IDs for completed steps start at 1000
+        private const int _completedFrameIdBase = 1000;
 
         private TcpListener _listener;
         private TcpClient _client;
@@ -58,6 +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;
@@ -87,6 +93,11 @@ namespace GitHub.Runner.Worker.Dap
         // Current execution context
         private IStep _currentStep;
         private IExecutionContext _jobContext;
+        private int _currentStepIndex;
+
+        // Track completed steps for stack trace
+        private readonly List<CompletedStepInfo> _completedSteps = new List<CompletedStepInfo>();
+        private int _nextCompletedFrameId = _completedFrameIdBase;
 
         // Client connection tracking for reconnection support
         private volatile bool _isClientConnected;
@@ -97,8 +108,6 @@ namespace GitHub.Runner.Worker.Dap
         // REPL command executor for run() commands
         private DapReplExecutor _replExecutor;
 
-        private JobExecutionView _executionView;
-
         public bool IsActive =>
             _state == DapSessionState.Ready ||
             _state == DapSessionState.Paused ||
@@ -251,8 +260,7 @@ namespace GitHub.Runner.Worker.Dap
                     }
                     catch (Exception ex)
                     {
-                        Trace.Warning("DAP job-completed pause error.");
-                        Trace.Error(ex);
+                        Trace.Warning($"DAP job-completed pause error: {ex.Message}");
                     }
                 }
 
@@ -262,8 +270,7 @@ namespace GitHub.Runner.Worker.Dap
                 }
                 catch (Exception ex)
                 {
-                    Trace.Warning("DAP OnJobCompleted error.");
-                    Trace.Error(ex);
+                    Trace.Warning($"DAP OnJobCompleted error: {ex.Message}");
                 }
             }
         }
@@ -380,8 +387,7 @@ namespace GitHub.Runner.Worker.Dap
             }
             catch (Exception ex)
             {
-                Trace.Warning("DAP OnStepStarting error.");
-                Trace.Error(ex);
+                Trace.Warning($"DAP OnStepStarting error: {ex.Message}");
             }
         }
 
@@ -394,8 +400,10 @@ namespace GitHub.Runner.Worker.Dap
 
             try
             {
+                var result = step.ExecutionContext?.Result;
                 Trace.Info("Step completed");
-                JobExecutionView view;
+
+                // Add to completed steps list for stack trace
                 lock (_stateLock)
                 {
                     if (_state != DapSessionState.Ready &&
@@ -405,353 +413,20 @@ namespace GitHub.Runner.Worker.Dap
                         return;
                     }
 
-                    // Clear current-step ref if it matches; otherwise leave alone
-                    // (defensive — OnStepStartingAsync may have already advanced it).
-                    if (ReferenceEquals(_currentStep, step))
+                    _completedSteps.Add(new CompletedStepInfo
                     {
-                        _currentStep = null;
-                    }
-                    view = _executionView;
-                }
-
-                // If the skipped step was a Main IActionRunner with a predicted
-                // Post-step placeholder, mark that placeholder as skipped so
-                // the view does not advertise a step that will never run.
-                if (view != null &&
-                    step is IActionRunner actionRunner &&
-                    actionRunner.Stage == ActionRunStage.Main &&
-                    actionRunner.Action != null &&
-                    step.ExecutionContext?.Result == TaskResult.Skipped)
-                {
-                    var matchKey = MatchKeyFor(actionRunner.Action.Id);
-                    if (view.TryMarkSkipped(matchKey))
-                    {
-                        SendLoadedSourceEvent("changed");
-                    }
+                        DisplayName = step.DisplayName,
+                        Result = result,
+                        FrameId = _nextCompletedFrameId++
+                    });
                 }
             }
             catch (Exception ex)
             {
-                Trace.Warning("DAP OnStepCompleted error.");
-                Trace.Error(ex);
+                Trace.Warning($"DAP OnStepCompleted error: {ex.Message}");
             }
         }
 
-        /// <summary>
-        /// Snapshot of the current job execution view, or null if it has not
-        /// been built yet (debugger inactive, or InitializeJob has not yet
-        /// signalled). Phase 2c will consume this for DAP source/stack-trace
-        /// responses.
-        /// </summary>
-        internal JobExecutionView ExecutionView
-        {
-            get
-            {
-                lock (_stateLock)
-                {
-                    return _executionView;
-                }
-            }
-        }
-
-        public async Task OnJobStepsInitializedAsync(IEnumerable<IStep> mainQueue, IEnumerable<IStep> initialPostStack)
-        {
-            if (!IsActive)
-            {
-                return;
-            }
-
-            try
-            {
-                IExecutionContext jobContext;
-                lock (_stateLock)
-                {
-                    jobContext = _jobContext;
-                }
-
-                string jobId = jobContext?.GetGitHubContext("job");
-                if (string.IsNullOrWhiteSpace(jobId))
-                {
-                    jobId = "job";
-                }
-
-                // Materialize mainQueue once so we can iterate it twice
-                // (once for entries, once for the post-step predictor).
-                var mainSteps = mainQueue == null ? new List<IStep>() : new List<IStep>(mainQueue);
-
-                var entries = new List<(JobExecutionViewEntry entry, IStep stepIdentity)>();
-                foreach (var step in mainSteps)
-                {
-                    var entry = StepEntryTranslator.TryTranslate(step);
-                    if (entry != null)
-                    {
-                        entries.Add((entry, step));
-                    }
-                }
-                // Stack<T>.GetEnumerator() yields items in LIFO order — the
-                // same order callers will pop them. We materialize them into
-                // the view in that pop order so post-step entries appear in
-                // execution order.
-                if (initialPostStack != null)
-                {
-                    foreach (var step in initialPostStack)
-                    {
-                        var entry = StepEntryTranslator.TryTranslate(step);
-                        if (entry != null)
-                        {
-                            entries.Add((entry, step));
-                        }
-                    }
-                }
-
-                var view = new JobExecutionView(jobId);
-                if (entries.Count > 0)
-                {
-                    view.AppendRange(entries);
-                }
-
-                // Predict Post-step placeholders for actions that declare
-                // HasPost in their action manifest. Walking Pre+Main runners
-                // in declaration order, then prepending each prediction so
-                // the rendered post section matches the runner's LIFO
-                // post-execution order (the runner's post stack pops in
-                // reverse-registration order). Wrapped in a try/catch so a
-                // missing IActionManager or LoadAction failure cannot
-                // prevent the view from being published.
-                try
-                {
-                    PredictPostPlaceholders(jobContext, mainSteps, view);
-                }
-                catch (Exception ex)
-                {
-                    Trace.Warning("DAP predictor: predicting post placeholders failed; continuing without predictions.");
-                    Trace.Error(ex);
-                }
-
-                lock (_stateLock)
-                {
-                    _executionView = view;
-                }
-
-                Trace.Info($"DAP execution view initialized with {view.EntryCount} entries.");
-                SendLoadedSourceEvent("new");
-            }
-            catch (Exception ex)
-            {
-                Trace.Warning("DAP OnJobStepsInitialized error.");
-                Trace.Error(ex);
-            }
-
-            await Task.CompletedTask;
-        }
-
-        public void OnPostStepRegistered(IStep step)
-        {
-            if (!IsActive || step == null)
-            {
-                return;
-            }
-
-            try
-            {
-                JobExecutionView view;
-                lock (_stateLock)
-                {
-                    view = _executionView;
-                }
-
-                if (view == null)
-                {
-                    return;
-                }
-
-                // Try to claim a previously-predicted placeholder. When
-                // OnJobStepsInitializedAsync ran, we walked the Pre+Main
-                // queue and synthesized a Post placeholder for every action
-                // whose manifest declared HasPost. If this registration
-                // matches one of those placeholders by Action.Id, claim it
-                // in place — no view growth, no `loadedSource changed`
-                // event needed.
-                if (step is IActionRunner postRunner && postRunner.Action != null)
-                {
-                    var matchKey = MatchKeyFor(postRunner.Action.Id);
-                    if (view.TryClaim(matchKey, step).HasValue)
-                    {
-                        return;
-                    }
-                }
-
-                // Unpredicted path: composite-action JIT post discovery,
-                // container hooks, or any other registration we did not
-                // foresee at view-build time. Fall back to append + notify
-                // clients via `loadedSource changed`.
-                var entry = StepEntryTranslator.TryTranslate(step);
-                if (entry == null)
-                {
-                    return;
-                }
-
-                try
-                {
-                    view.Append(entry, step);
-                    SendLoadedSourceEvent("changed");
-                }
-                catch (InvalidOperationException ex)
-                {
-                    // Step already registered — RegisterPostJobStep tolerates
-                    // duplicate registrations in some workflow shapes; mirror
-                    // that semantics here so we don't propagate.
-                    Trace.Info($"DAP OnPostStepRegistered: duplicate step ignored ({ex.Message}).");
-                }
-            }
-            catch (Exception ex)
-            {
-                Trace.Warning("DAP OnPostStepRegistered error.");
-                Trace.Error(ex);
-            }
-        }
-
-        /// <summary>
-        /// Walks <paramref name="mainSteps"/> (the queue of Pre+Main
-        /// IActionRunners produced by JobRunner) and synthesizes a Post
-        /// placeholder entry on <paramref name="view"/> for every action
-        /// whose manifest declares <c>HasPost = true</c>.
-        ///
-        /// Conditions mirror <c>ActionRunner.RunAsync</c> exactly:
-        /// the runner is in Pre or Main stage, the action is a
-        /// <see cref="Pipelines.RepositoryPathReference"/> that is NOT the
-        /// self-repository alias, the action is not a script, and the
-        /// resolved <see cref="Definition"/> reports HasPost.
-        ///
-        /// Predictions are collected in declaration order, then APPENDED
-        /// in reverse so the rendered post section mirrors the runner's
-        /// LIFO post-execution order (the runner's post stack pops in
-        /// reverse-registration order — see ExecutionContext.RegisterPostJobStep).
-        /// </summary>
-        private void PredictPostPlaceholders(IExecutionContext jobContext, IReadOnlyList<IStep> mainSteps, JobExecutionView view)
-        {
-            if (jobContext == null || mainSteps == null || mainSteps.Count == 0 || view == null)
-            {
-                return;
-            }
-
-            IActionManager actionManager;
-            try
-            {
-                actionManager = HostContext.GetService<IActionManager>();
-            }
-            catch (Exception ex)
-            {
-                Trace.Info($"DAP predictor: IActionManager unavailable ({ex.Message}); skipping post-step prediction.");
-                return;
-            }
-
-            var predictions = new List<(JobExecutionViewEntry entry, string matchKey)>();
-            var seenActionIds = new HashSet<Guid>();
-
-            foreach (var step in mainSteps)
-            {
-                if (step is not IActionRunner runner)
-                {
-                    continue;
-                }
-                if (runner.Stage == ActionRunStage.Post)
-                {
-                    // Post entries are already seeded from initialPostStack.
-                    continue;
-                }
-                var action = runner.Action;
-                if (action == null)
-                {
-                    continue;
-                }
-                // ActionRunner.cs:113 — Post only created when the action is
-                // a RepositoryPathReference that is not the self-repository
-                // alias and not a script.
-                if (action.Reference is not Pipelines.RepositoryPathReference repoRef)
-                {
-                    continue;
-                }
-                if (string.Equals(repoRef.RepositoryType, Pipelines.PipelineConstants.SelfAlias, StringComparison.OrdinalIgnoreCase))
-                {
-                    continue;
-                }
-                // (A RepositoryPathReference is never a script — script
-                // run-steps surface as a different ActionStepDefinitionReference
-                // subclass, so the cast above already filtered them out.)
-                // Dedupe by Action.Id: the runtime dedups via
-                // Root.StepsWithPostRegistered.Add(actionRunner.Action.Id),
-                // so two steps referencing the same Action.Id only ever
-                // register one Post. Mirror that here so we don't synthesize
-                // two placeholders for one future registration.
-                if (!seenActionIds.Add(action.Id))
-                {
-                    continue;
-                }
-
-                Definition definition;
-                try
-                {
-                    definition = actionManager.LoadAction(jobContext, action);
-                }
-                catch (Exception ex)
-                {
-                    Trace.Info($"DAP predictor: LoadAction failed for {repoRef.Name} ({ex.Message}); skipping prediction.");
-                    continue;
-                }
-
-                if (definition?.Data?.Execution?.HasPost != true)
-                {
-                    continue;
-                }
-
-                // Compute the Post display name exactly as ActionRunner does
-                // when it constructs the Post IActionRunner (ActionRunner.cs:115-122).
-                var displayName = runner.DisplayName;
-                if (string.IsNullOrEmpty(displayName))
-                {
-                    displayName = "step";
-                }
-                if (runner.Stage == ActionRunStage.Pre &&
-                    displayName.StartsWith("Pre ", StringComparison.OrdinalIgnoreCase))
-                {
-                    displayName = displayName.Substring("Pre ".Length);
-                }
-                var postDisplayName = $"Post {displayName}";
-
-                var entry = new JobExecutionViewEntry(
-                    phase: JobExecutionPhase.Post,
-                    displayName: postDisplayName,
-                    uses: StepEntryTranslator.FormatActionReference(action.Reference));
-
-                predictions.Add((entry, MatchKeyFor(action.Id)));
-            }
-
-            // Reverse declaration order so the rendered post section
-            // matches the LIFO order in which the runner will pop posts.
-            predictions.Reverse();
-
-            foreach (var (entry, key) in predictions)
-            {
-                try
-                {
-                    view.Append(entry, stepIdentity: null, matchKey: key);
-                }
-                catch (Exception ex)
-                {
-                    Trace.Warning("DAP predictor: failed to append Post placeholder; skipping.");
-                    Trace.Error(ex);
-                }
-            }
-        }
-
-        // Stable, opaque key derived from an action's Pipelines.ActionStep.Id.
-        // All IActionRunner instances for the same action (Pre/Main/Post)
-        // share the same Action reference (see ActionRunner.cs:131), so the
-        // Id is constant across phases and is the right join key.
-        private static string MatchKeyFor(Guid actionId) =>
-            $"post:{actionId:N}";
-
         internal async Task HandleMessageAsync(string messageJson, CancellationToken cancellationToken)
         {
             Request request = null;
@@ -793,8 +468,6 @@ namespace GitHub.Runner.Worker.Dap
                         "next" => HandleNext(request),
                         "setBreakpoints" => HandleSetBreakpoints(request),
                         "setExceptionBreakpoints" => HandleSetExceptionBreakpoints(request),
-                        "source" => HandleSource(request),
-                        "loadedSources" => HandleLoadedSources(request),
                         "completions" => HandleCompletions(request),
                         "stepIn" => CreateResponse(request, false, "Step In is not supported. Actions jobs debug at the step level - use 'next' to advance to the next step.", body: null),
                         "stepOut" => CreateResponse(request, false, "Step Out is not supported. Actions jobs debug at the step level - use 'continue' to resume.", body: null),
@@ -818,6 +491,11 @@ namespace GitHub.Runner.Worker.Dap
                     });
                     Trace.Info("Sent initialized event");
                 }
+
+                if (request.Command == "configurationDone")
+                {
+                    SendWelcomeMessage();
+                }
             }
             catch (Exception ex)
             {
@@ -836,6 +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
@@ -1146,9 +825,37 @@ 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 shouldPause;
+            bool pauseOnNextStep;
             CancellationToken cancellationToken;
             lock (_stateLock)
             {
@@ -1160,14 +867,18 @@ namespace GitHub.Runner.Worker.Dap
                 }
 
                 _currentStep = step;
+                _currentStepIndex = _completedSteps.Count;
+                pauseOnNextStep = _pauseOnNextStep;
                 cancellationToken = _jobContext?.CancellationToken ?? CancellationToken.None;
-                shouldPause = ShouldPauseBefore(step, isFirstStep);
             }
 
             // Reset variable references so stale nested refs from the
             // previous step are not served to the client.
             _variableProvider?.Reset();
 
+            // Determine if we should pause
+            bool shouldPause = isFirstStep || pauseOnNextStep;
+
             if (!shouldPause)
             {
                 Trace.Info("Step starting without debugger pause");
@@ -1184,33 +895,13 @@ 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);
         }
 
-        /// <summary>
-        /// Decides whether the debugger should pause before <paramref name="step"/>.
-        /// Today: pause on the first step always; otherwise pause when the user
-        /// has elected step-mode (the 'next' command). Future breakpoint support
-        /// will be a single additional check here against a per-step breakpoint set.
-        /// Caller MUST hold <c>_stateLock</c>.
-        /// </summary>
-        private bool ShouldPauseBefore(IStep step, bool isFirstStep)
-        {
-            if (isFirstStep)
-            {
-                return true;
-            }
-
-            if (_pauseOnNextStep)
-            {
-                return true;
-            }
-
-            // TODO Phase 2c+1: if (_breakpointSet.Contains(step)) return true;
-            return false;
-        }
-
         internal void OnJobCompleted()
         {
             Trace.Info("Job completed, sending terminated event");
@@ -1282,7 +973,7 @@ namespace GitHub.Runner.Worker.Dap
                 SupportsTerminateRequest = false,
                 SupportTerminateDebuggee = false,
                 SupportsDelayedStackTraceLoading = false,
-                SupportsLoadedSourcesRequest = true,
+                SupportsLoadedSourcesRequest = false,
                 SupportsProgressReporting = false,
                 SupportsRunInTerminalRequest = false,
                 SupportsCancelRequest = false,
@@ -1356,155 +1047,72 @@ namespace GitHub.Runner.Worker.Dap
             return CreateResponse(request, true, body: body);
         }
 
-        internal Response HandleStackTrace(Request request)
+        private Response HandleStackTrace(Request request)
         {
             IStep currentStep;
-            JobExecutionView view;
+            int currentStepIndex;
+            CompletedStepInfo[] completedSteps;
             lock (_stateLock)
             {
                 currentStep = _currentStep;
-                view = _executionView;
+                currentStepIndex = _currentStepIndex;
+                completedSteps = _completedSteps.ToArray();
             }
 
             var frames = new List<StackFrame>();
 
-            if (view != null)
+            // Add current step as the top frame
+            if (currentStep != null)
             {
-                var source = BuildExecutionViewSource(view.JobId);
+                var resultIndicator = currentStep.ExecutionContext?.Result != null
+                    ? $" [{currentStep.ExecutionContext.Result}]"
+                    : " [running]";
 
-                // Frame 0: the currently-executing step (only when one is set).
-                if (currentStep != null)
-                {
-                    var stepLine = view.TryGetLineForStep(currentStep) ?? 1;
-                    frames.Add(new StackFrame
-                    {
-                        Id = _currentFrameId,
-                        Name = MaskUserVisibleText(currentStep.DisplayName ?? "step"),
-                        Line = stepLine,
-                        Column = 1,
-                        Source = source,
-                        PresentationHint = "normal",
-                    });
-                }
-
-                // Frame 1: the job (anchors the stack; line 1 = the synthesized header).
-                frames.Add(new StackFrame
-                {
-                    Id = _jobFrameId,
-                    Name = MaskUserVisibleText($"job: {view.JobId}"),
-                    Line = 1,
-                    Column = 1,
-                    Source = source,
-                    PresentationHint = "subtle",
-                });
-            }
-            else if (currentStep != null)
-            {
-                // Defensive: view not yet built but a step is executing.
-                // Still emit a single frame with no Source so the client doesn't choke.
                 frames.Add(new StackFrame
                 {
                     Id = _currentFrameId,
-                    Name = MaskUserVisibleText(currentStep.DisplayName ?? "step"),
+                    Name = MaskUserVisibleText($"{currentStep.DisplayName ?? "Current Step"}{resultIndicator}"),
+                    Line = currentStepIndex + 1,
+                    Column = 1,
+                    PresentationHint = "normal"
+                });
+            }
+            else
+            {
+                frames.Add(new StackFrame
+                {
+                    Id = _currentFrameId,
+                    Name = "(no step executing)",
+                    Line = 0,
+                    Column = 1,
+                    PresentationHint = "subtle"
+                });
+            }
+
+            // Add completed steps as additional frames (most recent first)
+            for (int i = completedSteps.Length - 1; i >= 0; i--)
+            {
+                var completedStep = completedSteps[i];
+                var resultStr = completedStep.Result.HasValue ? $" [{completedStep.Result}]" : "";
+                frames.Add(new StackFrame
+                {
+                    Id = completedStep.FrameId,
+                    Name = MaskUserVisibleText($"{completedStep.DisplayName}{resultStr}"),
                     Line = 1,
                     Column = 1,
-                    PresentationHint = "normal",
+                    PresentationHint = "subtle"
                 });
             }
 
             var body = new StackTraceResponseBody
             {
                 StackFrames = frames,
-                TotalFrames = frames.Count,
+                TotalFrames = frames.Count
             };
 
             return CreateResponse(request, true, body: body);
         }
 
-        /// <summary>
-        /// Builds the synthesized job execution view <see cref="Source"/> descriptor.
-        /// All frames in a session share one Source; the client retrieves its
-        /// content via the DAP <c>source</c> request keyed by <see cref="_executionViewSourceReference"/>.
-        /// </summary>
-        private Source BuildExecutionViewSource(string jobId)
-        {
-            return new Source
-            {
-                Name = MaskUserVisibleText("execution.yml"),
-                Path = MaskUserVisibleText($"{jobId}/execution.yml"),
-                SourceReference = _executionViewSourceReference,
-                PresentationHint = "normal",
-            };
-        }
-
-        internal Response HandleSource(Request request)
-        {
-            SourceArguments args;
-            try
-            {
-                args = request.Arguments?.ToObject<SourceArguments>();
-            }
-            catch (Exception ex)
-            {
-                Trace.Warning($"Failed to parse source arguments: {ex.GetType().Name}");
-                return CreateResponse(request, false, "Invalid source arguments.", body: null);
-            }
-
-            if (args == null)
-            {
-                return CreateResponse(request, false, "Missing source arguments.", body: null);
-            }
-
-            JobExecutionView view;
-            lock (_stateLock)
-            {
-                view = _executionView;
-            }
-
-            if (view == null)
-            {
-                return CreateResponse(request, false, "Execution view not yet available.", body: null);
-            }
-
-            if (args.SourceReference != _executionViewSourceReference)
-            {
-                return CreateResponse(request, false, $"Unknown source reference: {args.SourceReference}.", body: null);
-            }
-
-            var body = new SourceResponseBody
-            {
-                Content = MaskUserVisibleText(view.Yaml),
-                // MimeType intentionally unset: VS Code's debug content provider
-                // short-circuits language detection on the response's mimeType
-                // (exact-match against its registered language mimetypes) and
-                // falls back to plaintext on unknown values. The IANA YAML type
-                // "application/yaml" is not in VS Code's table (it only knows
-                // the legacy "text/x-yaml" synthesized for the built-in YAML
-                // language contribution). By omitting mimeType, clients fall
-                // through to path-extension detection — `.yml` in Source.Path
-                // is the universal mechanism every DAP client honors
-                // consistently (VS Code, nvim-dap, JetBrains).
-            };
-
-            return CreateResponse(request, true, body: body);
-        }
-
-        internal Response HandleLoadedSources(Request request)
-        {
-            JobExecutionView view;
-            lock (_stateLock)
-            {
-                view = _executionView;
-            }
-
-            var body = new LoadedSourcesResponseBody();
-            if (view != null)
-            {
-                body.Sources.Add(BuildExecutionViewSource(view.JobId));
-            }
-            return CreateResponse(request, true, body: body);
-        }
-
         private Response HandleScopes(Request request)
         {
             var args = request.Arguments?.ToObject<ScopesArguments>();
@@ -1625,7 +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
@@ -1720,40 +1333,11 @@ namespace GitHub.Runner.Worker.Dap
             return CreateResponse(request, true, body: null);
         }
 
-        internal Response HandleSetBreakpoints(Request request)
+        private Response HandleSetBreakpoints(Request request)
         {
-            SetBreakpointsArguments args = null;
-            try
-            {
-                args = request.Arguments?.ToObject<SetBreakpointsArguments>();
-            }
-            catch (Exception ex)
-            {
-                Trace.Warning($"Failed to parse setBreakpoints arguments: {ex.GetType().Name}");
-            }
-
-            JobExecutionView view;
-            lock (_stateLock)
-            {
-                view = _executionView;
-            }
-
-            var body = new SetBreakpointsResponseBody();
-            if (args?.Breakpoints != null && view != null)
-            {
-                var source = BuildExecutionViewSource(view.JobId);
-                foreach (var requested in args.Breakpoints)
-                {
-                    body.Breakpoints.Add(new Breakpoint
-                    {
-                        Verified = false,
-                        Line = requested.Line,
-                        Source = source,
-                        Message = "Breakpoint support is coming in a future runner release. The debugger currently pauses at every step boundary; use 'continue' to advance.",
-                    });
-                }
-            }
-            return CreateResponse(request, true, body: body);
+            // MVP: acknowledge but don't process breakpoints
+            // All steps pause automatically via _pauseOnNextStep
+            return CreateResponse(request, true, body: new { breakpoints = Array.Empty<object>() });
         }
 
         private Response HandleSetExceptionBreakpoints(Request request)
@@ -1818,7 +1402,8 @@ namespace GitHub.Runner.Worker.Dap
 
         /// <summary>
         /// Resolves the execution context for a given stack frame ID.
-        /// Frame 1 = current step; frame 2 = job-level (subtle anchor frame).
+        /// Frame 1 = current step; frames 1000+ = completed steps (no
+        /// context available - those steps have already finished).
         /// Falls back to the job-level context when no step is active.
         /// </summary>
         private IExecutionContext GetExecutionContextForFrame(int frameId)
@@ -1828,7 +1413,7 @@ namespace GitHub.Runner.Worker.Dap
                 return GetCurrentExecutionContext();
             }
 
-            // Job/anchor frame — no step-level context.
+            // Completed-step frames don't carry a live execution context.
             return null;
         }
 
@@ -1866,30 +1451,37 @@ namespace GitHub.Runner.Worker.Dap
         }
 
         /// <summary>
-        /// Sends a loadedSource event with the current execution view's source.
-        /// No-op if the view has not been built yet.
+        /// Emits a console output banner telling the user whether REPL
+        /// commands will execute on the host or inside the job container.
         /// </summary>
-        private void SendLoadedSourceEvent(string reason)
+        private void SendExecutionContextBanner()
         {
-            JobExecutionView view;
-            lock (_stateLock)
-            {
-                view = _executionView;
-            }
-            if (view == null)
+            if (!_isClientConnected)
             {
                 return;
             }
 
-            SendEvent(new Event
+            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)))
             {
-                EventType = "loadedSource",
-                Body = new LoadedSourceEventBody
-                {
-                    Reason = reason,
-                    Source = BuildExecutionViewSource(view.JobId),
-                },
-            });
+                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)

src/Runner.Worker/Dap/DapMessages.cs

@@ -537,132 +537,6 @@ namespace GitHub.Runner.Worker.Dap
 
     #endregion
 
-    #region Source Request/Response
-
-    /// <summary>
-    /// Arguments for 'source' request.
-    /// </summary>
-    public class SourceArguments
-    {
-        /// <summary>
-        /// Source descriptor (optional, redundant with sourceReference).
-        /// </summary>
-        [JsonProperty("source", NullValueHandling = NullValueHandling.Ignore)]
-        public Source Source { get; set; }
-
-        /// <summary>
-        /// The reference to the source. Required by DAP spec.
-        /// </summary>
-        [JsonProperty("sourceReference")]
-        public int SourceReference { get; set; }
-    }
-
-    /// <summary>
-    /// Response body for 'source' request.
-    /// </summary>
-    public class SourceResponseBody
-    {
-        /// <summary>
-        /// Content of the source as a string.
-        /// </summary>
-        [JsonProperty("content")]
-        public string Content { get; set; }
-
-        /// <summary>
-        /// Optional content type / mime type of the source.
-        /// </summary>
-        [JsonProperty("mimeType", NullValueHandling = NullValueHandling.Ignore)]
-        public string MimeType { get; set; }
-    }
-
-    #endregion
-
-    #region LoadedSources Request/Response
-
-    /// <summary>
-    /// Response body for 'loadedSources' request.
-    /// </summary>
-    public class LoadedSourcesResponseBody
-    {
-        [JsonProperty("sources")]
-        public List<Source> Sources { get; set; } = new List<Source>();
-    }
-
-    /// <summary>
-    /// Body for 'loadedSource' event.
-    /// </summary>
-    public class LoadedSourceEventBody
-    {
-        /// <summary>
-        /// "new" | "changed" | "removed"
-        /// </summary>
-        [JsonProperty("reason")]
-        public string Reason { get; set; }
-
-        [JsonProperty("source")]
-        public Source Source { get; set; }
-    }
-
-    #endregion
-
-    #region SetBreakpoints Request/Response
-
-    /// <summary>
-    /// Arguments for 'setBreakpoints' request.
-    /// </summary>
-    public class SetBreakpointsArguments
-    {
-        [JsonProperty("source", NullValueHandling = NullValueHandling.Ignore)]
-        public Source Source { get; set; }
-
-        [JsonProperty("breakpoints")]
-        public List<SourceBreakpoint> Breakpoints { get; set; } = new List<SourceBreakpoint>();
-    }
-
-    /// <summary>
-    /// Properties of a breakpoint passed to the setBreakpoints request.
-    /// </summary>
-    public class SourceBreakpoint
-    {
-        [JsonProperty("line")]
-        public int Line { get; set; }
-
-        [JsonProperty("condition", NullValueHandling = NullValueHandling.Ignore)]
-        public string Condition { get; set; }
-
-        [JsonProperty("logMessage", NullValueHandling = NullValueHandling.Ignore)]
-        public string LogMessage { get; set; }
-    }
-
-    /// <summary>
-    /// Response body for 'setBreakpoints' request.
-    /// </summary>
-    public class SetBreakpointsResponseBody
-    {
-        [JsonProperty("breakpoints")]
-        public List<Breakpoint> Breakpoints { get; set; } = new List<Breakpoint>();
-    }
-
-    /// <summary>
-    /// Information about a breakpoint created in setBreakpoints.
-    /// </summary>
-    public class Breakpoint
-    {
-        [JsonProperty("verified")]
-        public bool Verified { get; set; }
-
-        [JsonProperty("line", NullValueHandling = NullValueHandling.Ignore)]
-        public int? Line { get; set; }
-
-        [JsonProperty("source", NullValueHandling = NullValueHandling.Ignore)]
-        public Source Source { get; set; }
-
-        [JsonProperty("message", NullValueHandling = NullValueHandling.Ignore)]
-        public string Message { get; set; }
-    }
-
-    #endregion
-
     #region Scopes Request/Response
 
     /// <summary>

src/Runner.Worker/Dap/DapReplExecutor.cs

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

src/Runner.Worker/Dap/DebuggerConfig.cs

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

src/Runner.Worker/Dap/IDapDebugger.cs

@@ -1,5 +1,4 @@
-using System.Collections.Generic;
-using System.Threading.Tasks;
+using System.Threading.Tasks;
 using GitHub.Runner.Common;
 
 namespace GitHub.Runner.Worker.Dap
@@ -22,23 +21,6 @@ namespace GitHub.Runner.Worker.Dap
         Task WaitUntilReadyAsync();
         Task OnStepStartingAsync(IStep step);
         void OnStepCompleted(IStep step);
-
-        /// <summary>
-        /// Called after JobExtension.InitializeJob has returned and the initial
-        /// step queue + post-step stack have been populated. The debugger uses
-        /// these snapshots to build the synthesized job execution view served
-        /// via the DAP source request.
-        /// </summary>
-        Task OnJobStepsInitializedAsync(IEnumerable<IStep> mainQueue, IEnumerable<IStep> initialPostStack);
-
-        /// <summary>
-        /// Called from ExecutionContext.RegisterPostJobStep after a post-step
-        /// is pushed onto the post-job stack. The debugger appends the step
-        /// to the running execution view so the rendered YAML reflects the
-        /// newly-known post-step.
-        /// </summary>
-        void OnPostStepRegistered(IStep step);
-
         Task OnJobCompletedAsync();
         Task StopAsync();
     }

src/Runner.Worker/Dap/JobExecutionView.cs

@@ -10,9 +10,21 @@ namespace GitHub.Runner.Worker.Dap
     /// and provides O(1) lookup from <see cref="IStep"/> identity to the current line
     /// in the rendered YAML where that step's <c>- step:</c> key appears.
     ///
-    /// Append-only growth model: post-steps are discovered lazily during execution
-    /// and appended. Setup/pre/main entry line numbers are stable across appends —
-    /// only the synthetic Cleanup boundary (which is not tracked here) shifts.
+    /// Each <see cref="Append"/> can register the entry in one of three modes:
+    ///   - With a non-null <c>stepIdentity</c>: registers the IStep→line mapping
+    ///     immediately. Used for entries whose real <see cref="IStep"/> is already
+    ///     known at append time.
+    ///   - With a non-null <c>matchKey</c>: registers an unclaimed placeholder
+    ///     that a later <see cref="TryClaim"/> binds to a real <see cref="IStep"/>.
+    ///     Used for entries whose <see cref="IStep"/> is materialized later. A
+    ///     placeholder that is never claimed simply stays in the view and is never
+    ///     paused on — the IStep→line mapping is only populated on claim.
+    ///   - With neither: a static entry that needs no line lookup.
+    ///
+    /// <see cref="Append"/> and <see cref="AppendRange"/> never remove or reorder
+    /// existing entries. <see cref="TryClaim"/> does not re-render. The IStep→line
+    /// mapping is rebuilt on every render, so lookups stay accurate even if a later
+    /// Append happens to shift previously-emitted entries.
     /// </summary>
     internal sealed class JobExecutionView
     {
@@ -113,20 +125,25 @@ namespace GitHub.Runner.Worker.Dap
         }
 
         /// <summary>
-        /// Append a new entry. If <paramref name="stepIdentity"/> is non-null,
-        /// registers the IStep -> line mapping for later lookup. If
-        /// <paramref name="matchKey"/> is non-null, the entry is registered
-        /// as an unclaimed placeholder that a future
-        /// <see cref="TryClaim(string, IStep)"/> call can bind to a real
-        /// IStep (used by the predictive Post-step path). Re-renders the
-        /// YAML and updates the start-line table.
+        /// Append a new entry. Exactly one of <paramref name="stepIdentity"/>
+        /// or <paramref name="matchKey"/> may be non-null (or both may be
+        /// null for a static entry that needs no line lookup):
+        ///   - <paramref name="stepIdentity"/> non-null: registers the
+        ///     IStep→line mapping immediately. Use when the real
+        ///     <see cref="IStep"/> is known at append time.
+        ///   - <paramref name="matchKey"/> non-null: registers an unclaimed
+        ///     placeholder that a later <see cref="TryClaim"/> binds to a
+        ///     real <see cref="IStep"/>.
+        /// Re-renders the YAML and updates the start-line table.
         /// </summary>
         /// <returns>1-based line number of the newly-appended entry's <c>- step:</c> key.</returns>
         public int Append(JobExecutionViewEntry entry, IStep stepIdentity = null, string matchKey = null)
         {
-            if (entry == null)
+            ArgUtil.NotNull(entry, nameof(entry));
+            if (stepIdentity != null && matchKey != null)
             {
-                throw new ArgumentNullException(nameof(entry));
+                throw new ArgumentException(
+                    "Append cannot register both a step identity and a placeholder match key on the same entry; pass at most one.");
             }
 
             lock (_lock)
@@ -193,46 +210,6 @@ namespace GitHub.Runner.Worker.Dap
             }
         }
 
-        /// <summary>
-        /// Mark a previously-appended unclaimed placeholder as skipped. Used
-        /// when the predicting Main step never runs (skipped by <c>if:</c>),
-        /// so its predicted Post-step placeholder should not appear as a
-        /// step that will execute. Re-renders the view (inline comment only
-        /// — subsequent entry line numbers stay stable).
-        /// </summary>
-        /// <returns>
-        /// true if a matching unclaimed placeholder was marked; false when
-        /// no placeholder exists for <paramref name="matchKey"/>, or the
-        /// placeholder has already been claimed (claim wins).
-        /// </returns>
-        public bool TryMarkSkipped(string matchKey)
-        {
-            ArgUtil.NotNull(matchKey, nameof(matchKey));
-
-            lock (_lock)
-            {
-                if (!_unclaimedByKey.TryGetValue(matchKey, out int index))
-                {
-                    return false;
-                }
-                // Defensive: only mark if it's still an unclaimed placeholder.
-                if (_stepIdentities[index] != null)
-                {
-                    return false;
-                }
-
-                if (_entries[index].IsSkipped)
-                {
-                    // Idempotent — already marked.
-                    return true;
-                }
-                _entries[index].IsSkipped = true;
-                _unclaimedByKey.Remove(matchKey);
-                Render();
-                return true;
-            }
-        }
-
         /// <summary>
         /// Bulk-append for the initial population. Equivalent to calling
         /// <see cref="Append"/> once per pair, but renders only once at the end.

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
 {
@@ -88,15 +85,6 @@ namespace GitHub.Runner.Worker.Dap
         public string WithYaml { get; }
         public string Shell { get; }
         public string WorkingDirectory { get; }
-
-        /// <summary>
-        /// Set when the corresponding step was skipped (e.g. predicted Post
-        /// placeholder for a Main step that never executed because its
-        /// <c>if:</c> evaluated false). Rendered as an inline YAML comment
-        /// on the entry's <c>- step:</c> line so subsequent entry line
-        /// numbers stay stable.
-        /// </summary>
-        public bool IsSkipped { get; internal set; }
     }
 
     /// <summary>
@@ -153,7 +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;
@@ -212,12 +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));
-                if (entry.IsSkipped)
-                {
-                    // Inline comment — keeps following entry line numbers stable.
-                    sb.Append("  # (skipped — main step did not execute)");
-                }
+                sb.Append("  - step: ").Append(YamlScalarFormatter.Format(entry.DisplayName));
                 sb.Append('\n');
                 newlinesEmitted++;
 
@@ -227,7 +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.
@@ -236,19 +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
@@ -260,7 +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))
@@ -289,12 +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)
@@ -349,43 +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);
-            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();
-            if (raw.StartsWith("--- ", 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

@@ -1,238 +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.
-        private static readonly HashSet<string> RunStepInternalKeys = new(StringComparer.Ordinal)
-        {
-            "script",
-            "shell",
-            "working-directory",
-        };
-
-        /// <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, "script", out var scriptTok) && scriptTok != null)
-                        {
-                            run = scriptTok.ToString();
-                        }
-                        if (TryGetMapValue(inputs, "shell", out var shellTok) && shellTok != null)
-                        {
-                            string shellText = shellTok.ToString();
-                            if (!string.IsNullOrEmpty(shellText))
-                            {
-                                shell = shellText;
-                            }
-                        }
-                        if (TryGetMapValue(inputs, "working-directory", 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

@@ -1,148 +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);
-            var emitter = new Emitter(sw);
-            var adapter = new TemplateTokenYamlAdapter(emitter);
-            TemplateWriter.Write(adapter, token);
-
-            string raw = sw.ToString();
-            // Strip YAML document markers ("--- " prefix and "\n..." suffix).
-            if (raw.StartsWith("--- ", 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();
-        }
-    }
-}

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

@@ -338,14 +338,6 @@ namespace GitHub.Runner.Worker
 
             step.ExecutionContext = Root.CreatePostChild(step.DisplayName, IntraActionState, siblingScopeName);
             Root.PostJobSteps.Push(step);
-            // Only consult the DAP debugger when it was actually enabled for this job.
-            // Without this guard, HostContext.GetService<IDapDebugger>() would auto-
-            // instantiate the default singleton for every non-debug job, violating the
-            // "no debugger, no risk" containment property.
-            if (Global.Debugger?.Enabled == true)
-            {
-                HostContext.GetService<Dap.IDapDebugger>().OnPostStepRegistered(step);
-            }
         }
 
         public IExecutionContext CreateChild(
@@ -978,7 +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/Runner.Worker/JobRunner.cs

@@ -230,24 +230,6 @@ namespace GitHub.Runner.Worker
                         jobContext.JobSteps.Enqueue(step);
                     }
 
-                    if (jobContext.Global.Debugger?.Enabled == true)
-                    {
-                        // Only consult the DAP debugger when it was actually enabled for this job.
-                        // Without this guard, HostContext.GetService<IDapDebugger>() would auto-
-                        // instantiate the default singleton for every non-debug job, violating the
-                        // "no debugger, no risk" containment property.
-                        var dapDebugger = HostContext.GetService<Dap.IDapDebugger>();
-                        try
-                        {
-                            await dapDebugger.OnJobStepsInitializedAsync(jobContext.JobSteps, jobContext.PostJobSteps);
-                        }
-                        catch (Exception ex)
-                        {
-                            Trace.Warning("DAP OnJobStepsInitialized error; continuing without DAP view.");
-                            Trace.Error(ex);
-                        }
-                    }
-
                     await stepsRunner.RunAsync(jobContext);
                 }
                 catch (Exception ex)

src/Runner.Worker/StepsRunner.cs

@@ -219,18 +219,12 @@ namespace GitHub.Runner.Worker
                             // Condition is false
                             Trace.Info("Skipping step due to condition evaluation.");
                             CompleteStep(step, TaskResult.Skipped, resultCode: conditionTraceWriter.Trace);
-                            // Notify the DAP debugger so any predicted Post-step
-                            // placeholder for this Main step can be marked as
-                            // skipped — otherwise the rendered view leaves a
-                            // stale "Post X" entry for a step that never ran.
-                            dapDebugger?.OnStepCompleted(step);
                         }
                         else if (conditionEvaluateError != null)
                         {
                             // Condition error
                             step.ExecutionContext.Error(conditionEvaluateError);
                             CompleteStep(step, TaskResult.Failed);
-                            dapDebugger?.OnStepCompleted(step);
                         }
                         else
                         {

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

@@ -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;
 
@@ -868,572 +872,222 @@ namespace GitHub.Runner.Common.Tests.Worker
             }
         }
 
-        // ---------------------------------------------------------------------
-        // Phase 2c: synthesized execution view as DAP source.
-        // ---------------------------------------------------------------------
-
-        private static Mock<GitHub.Runner.Worker.IActionRunner> NewActionRunner(
-            GitHub.Runner.Worker.ActionRunStage stage,
-            string displayName,
-            string actionName = "actions/checkout",
-            string actionRef = "v4")
-        {
-            var mock = new Mock<GitHub.Runner.Worker.IActionRunner>();
-            mock.SetupGet(x => x.Stage).Returns(stage);
-            mock.SetupGet(x => x.DisplayName).Returns(displayName);
-            mock.SetupGet(x => x.Action).Returns(new GitHub.DistributedTask.Pipelines.ActionStep
-            {
-                Reference = new GitHub.DistributedTask.Pipelines.RepositoryPathReference
-                {
-                    Name = actionName,
-                    Ref = actionRef,
-                },
-            });
-            return mock;
-        }
-
-        private async Task DriveDebuggerToReadyAsync(int port)
-        {
-            var waitTask = _debugger.WaitUntilReadyAsync();
-            var client = new TcpClient();
-            await client.ConnectAsync(IPAddress.Loopback, port);
-            var stream = client.GetStream();
-            await SendRequestAsync(stream, new Request
-            {
-                Seq = 1,
-                Type = "request",
-                Command = "configurationDone",
-            });
-            await waitTask;
-            // Hold the client alive via GC root in caller scope through field.
-            _liveDriveClient = client;
-        }
-
-        private TcpClient _liveDriveClient;
-
-        private static Request MakeRequest(string command, object args = null)
-        {
-            return new Request
-            {
-                Seq = 1,
-                Type = "request",
-                Command = command,
-                Arguments = args == null ? null : Newtonsoft.Json.Linq.JObject.FromObject(args),
-            };
-        }
-
         [Fact]
         [Trait("Level", "L0")]
         [Trait("Category", "Worker")]
-        public async Task HandleSource_ReturnsExecutionViewYaml()
+        public async Task WelcomeMessageSendsDefaultHelpWhenOverrideDisabled()
         {
             using (CreateTestContext())
             {
                 var port = GetFreePort();
                 using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
-                var jobContext = CreateJobContextWithTunnel(cts.Token, port, "ci-job");
+                var jobContext = CreateJobContextWithTunnel(cts.Token, port);
                 await _debugger.StartAsync(jobContext.Object);
-                try
-                {
-                    await DriveDebuggerToReadyAsync(port);
-                    var step = NewActionRunner(GitHub.Runner.Worker.ActionRunStage.Main, "Run").Object;
-                    await _debugger.OnJobStepsInitializedAsync(new[] { step }, Array.Empty<IStep>());
 
-                    var response = _debugger.HandleSource(MakeRequest("source", new SourceArguments { SourceReference = 1 }));
-                    Assert.True(response.Success);
-                    var body = Assert.IsType<SourceResponseBody>(response.Body);
-                    Assert.Equal(_debugger.ExecutionView.Yaml, body.Content);
-                    Assert.Null(body.MimeType);
-                }
-                finally
-                {
-                    await _debugger.StopAsync();
-                }
-            }
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task HandleSource_UnknownReference_Fails()
-        {
-            using (CreateTestContext())
-            {
-                var port = GetFreePort();
-                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
-                var jobContext = CreateJobContextWithTunnel(cts.Token, port, "ci-job");
-                await _debugger.StartAsync(jobContext.Object);
-                try
-                {
-                    await DriveDebuggerToReadyAsync(port);
-                    var step = NewActionRunner(GitHub.Runner.Worker.ActionRunStage.Main, "Run").Object;
-                    await _debugger.OnJobStepsInitializedAsync(new[] { step }, Array.Empty<IStep>());
-
-                    var response = _debugger.HandleSource(MakeRequest("source", new SourceArguments { SourceReference = 999 }));
-                    Assert.False(response.Success);
-                    Assert.Contains("Unknown source reference", response.Message);
-                }
-                finally
-                {
-                    await _debugger.StopAsync();
-                }
-            }
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task HandleSource_NoView_Fails()
-        {
-            using (CreateTestContext())
-            {
-                var port = GetFreePort();
-                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
-                var jobContext = CreateJobContextWithTunnel(cts.Token, port, "ci-job");
-                await _debugger.StartAsync(jobContext.Object);
-                try
-                {
-                    await DriveDebuggerToReadyAsync(port);
-                    // No OnJobStepsInitializedAsync call — view is null.
-                    var response = _debugger.HandleSource(MakeRequest("source", new SourceArguments { SourceReference = 1 }));
-                    Assert.False(response.Success);
-                    Assert.Contains("Execution view not yet available", response.Message);
-                }
-                finally
-                {
-                    await _debugger.StopAsync();
-                }
-            }
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task HandleSource_OmitsMimeType()
-        {
-            using (CreateTestContext())
-            {
-                var port = GetFreePort();
-                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
-                var jobContext = CreateJobContextWithTunnel(cts.Token, port, "ci-job");
-                await _debugger.StartAsync(jobContext.Object);
-                try
-                {
-                    await DriveDebuggerToReadyAsync(port);
-                    var step = NewActionRunner(GitHub.Runner.Worker.ActionRunStage.Main, "Run").Object;
-                    await _debugger.OnJobStepsInitializedAsync(new[] { step }, Array.Empty<IStep>());
-
-                    var response = _debugger.HandleSource(MakeRequest("source", new SourceArguments { SourceReference = 1 }));
-                    Assert.True(response.Success);
-                    var body = Assert.IsType<SourceResponseBody>(response.Body);
-                    // MimeType is intentionally omitted so DAP clients fall through
-                    // to path-extension detection (`.yml`) — the most consistently
-                    // honored mechanism across VS Code, nvim-dap, and JetBrains.
-                    Assert.Null(body.MimeType);
-                }
-                finally
-                {
-                    await _debugger.StopAsync();
-                }
-            }
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task HandleSource_MasksSecrets()
-        {
-            const string Secret = "supersecret-shhh-value";
-            using (var hc = CreateTestContext())
-            {
-                hc.SecretMasker.AddValue(Secret);
-
-                var port = GetFreePort();
-                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
-                var jobContext = CreateJobContextWithTunnel(cts.Token, port, "ci-job");
-                await _debugger.StartAsync(jobContext.Object);
-                try
-                {
-                    await DriveDebuggerToReadyAsync(port);
-                    // Embed the secret in a step display name so it ends up in the rendered YAML.
-                    var step = NewActionRunner(GitHub.Runner.Worker.ActionRunStage.Main, $"Run {Secret} step").Object;
-                    await _debugger.OnJobStepsInitializedAsync(new[] { step }, Array.Empty<IStep>());
-
-                    // Sanity-check the raw view actually contains the secret;
-                    // otherwise the masking assertion below would pass vacuously.
-                    Assert.Contains(Secret, _debugger.ExecutionView.Yaml);
-
-                    var response = _debugger.HandleSource(MakeRequest("source", new SourceArguments { SourceReference = 1 }));
-                    Assert.True(response.Success);
-                    var body = Assert.IsType<SourceResponseBody>(response.Body);
-                    Assert.DoesNotContain(Secret, body.Content);
-                    Assert.Contains("***", body.Content);
-                }
-                finally
-                {
-                    await _debugger.StopAsync();
-                }
-            }
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task HandleStackTrace_MasksSecretsInSourcePath()
-        {
-            const string Secret = "secret-job-name-xyz";
-            using (var hc = CreateTestContext())
-            {
-                hc.SecretMasker.AddValue(Secret);
-
-                var port = GetFreePort();
-                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
-                // Job name (== GitHub context "job") embeds the secret.
-                var jobContext = CreateJobContextWithTunnel(cts.Token, port, Secret);
-                await _debugger.StartAsync(jobContext.Object);
-                try
-                {
-                    await DriveDebuggerToReadyAsync(port);
-                    var step = NewActionRunner(GitHub.Runner.Worker.ActionRunStage.Main, "Run").Object;
-                    await _debugger.OnJobStepsInitializedAsync(new[] { step }, Array.Empty<IStep>());
-
-                    _ = _debugger.OnStepStartingAsync(step, isFirstStep: false);
-                    await Task.Delay(50);
-
-                    var response = _debugger.HandleStackTrace(MakeRequest("stackTrace"));
-                    var body = Assert.IsType<StackTraceResponseBody>(response.Body);
-                    Assert.NotEmpty(body.StackFrames);
-                    foreach (var frame in body.StackFrames)
-                    {
-                        if (frame.Source != null)
-                        {
-                            Assert.DoesNotContain(Secret, frame.Source.Path ?? string.Empty);
-                            Assert.DoesNotContain(Secret, frame.Source.Name ?? string.Empty);
-                            Assert.Contains("***", frame.Source.Path ?? string.Empty);
-                        }
-                        Assert.DoesNotContain(Secret, frame.Name ?? string.Empty);
-                    }
-                }
-                finally
-                {
-                    await _debugger.StopAsync();
-                }
-            }
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task HandleStackTrace_TwoFramesWhenStepping()
-        {
-            using (CreateTestContext())
-            {
-                var port = GetFreePort();
-                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
-                var jobContext = CreateJobContextWithTunnel(cts.Token, port, "ci-job");
-                await _debugger.StartAsync(jobContext.Object);
-                try
-                {
-                    await DriveDebuggerToReadyAsync(port);
-                    var step1 = NewActionRunner(GitHub.Runner.Worker.ActionRunStage.Main, "Step One").Object;
-                    var step2 = NewActionRunner(GitHub.Runner.Worker.ActionRunStage.Main, "Step Two", "actions/setup-node", "v3").Object;
-                    await _debugger.OnJobStepsInitializedAsync(new[] { step1, step2 }, Array.Empty<IStep>());
-
-                    // Fire-and-forget: first step pauses, but we just want _currentStep set.
-                    _ = _debugger.OnStepStartingAsync(step1, isFirstStep: false);
-                    await Task.Delay(50);
-
-                    var response = _debugger.HandleStackTrace(MakeRequest("stackTrace"));
-                    var body = Assert.IsType<StackTraceResponseBody>(response.Body);
-                    Assert.Equal(2, body.StackFrames.Count);
-
-                    var frame0 = body.StackFrames[0];
-                    Assert.Equal(1, frame0.Source.SourceReference);
-                    Assert.Equal(_debugger.ExecutionView.TryGetLineForStep(step1), frame0.Line);
-                    Assert.Equal("normal", frame0.PresentationHint);
-
-                    var frame1 = body.StackFrames[1];
-                    Assert.Equal(1, frame1.Line);
-                    Assert.Equal("subtle", frame1.PresentationHint);
-                    Assert.Equal(1, frame1.Source.SourceReference);
-                }
-                finally
-                {
-                    await _debugger.StopAsync();
-                }
-            }
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task HandleStackTrace_OneFrameWhenViewMissing()
-        {
-            using (CreateTestContext())
-            {
-                var port = GetFreePort();
-                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
-                var jobContext = CreateJobContextWithTunnel(cts.Token, port, "ci-job");
-                await _debugger.StartAsync(jobContext.Object);
-                try
-                {
-                    await DriveDebuggerToReadyAsync(port);
-                    var step = NewActionRunner(GitHub.Runner.Worker.ActionRunStage.Main, "Lonely").Object;
-                    // No view built — but pause the step so _currentStep is set.
-                    _ = _debugger.OnStepStartingAsync(step, isFirstStep: false);
-                    await Task.Delay(50);
-
-                    var response = _debugger.HandleStackTrace(MakeRequest("stackTrace"));
-                    var body = Assert.IsType<StackTraceResponseBody>(response.Body);
-                    Assert.Single(body.StackFrames);
-                    Assert.Null(body.StackFrames[0].Source);
-                }
-                finally
-                {
-                    await _debugger.StopAsync();
-                }
-            }
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task HandleStackTrace_NoFramesWhenIdle()
-        {
-            using (CreateTestContext())
-            {
-                var port = GetFreePort();
-                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
-                var jobContext = CreateJobContextWithTunnel(cts.Token, port, "ci-job");
-                await _debugger.StartAsync(jobContext.Object);
-                try
-                {
-                    await DriveDebuggerToReadyAsync(port);
-                    var response = _debugger.HandleStackTrace(MakeRequest("stackTrace"));
-                    var body = Assert.IsType<StackTraceResponseBody>(response.Body);
-                    Assert.Empty(body.StackFrames);
-                    Assert.Equal(0, body.TotalFrames);
-                }
-                finally
-                {
-                    await _debugger.StopAsync();
-                }
-            }
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task HandleLoadedSources_ReturnsExecutionView()
-        {
-            using (CreateTestContext())
-            {
-                var port = GetFreePort();
-                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
-                var jobContext = CreateJobContextWithTunnel(cts.Token, port, "ci-job");
-                await _debugger.StartAsync(jobContext.Object);
-                try
-                {
-                    await DriveDebuggerToReadyAsync(port);
-                    var step = NewActionRunner(GitHub.Runner.Worker.ActionRunStage.Main, "Run").Object;
-                    await _debugger.OnJobStepsInitializedAsync(new[] { step }, Array.Empty<IStep>());
-
-                    var response = _debugger.HandleLoadedSources(MakeRequest("loadedSources"));
-                    Assert.True(response.Success);
-                    var body = Assert.IsType<LoadedSourcesResponseBody>(response.Body);
-                    Assert.Single(body.Sources);
-                    Assert.Equal("execution.yml", body.Sources[0].Name);
-                    Assert.Equal("ci-job/execution.yml", body.Sources[0].Path);
-                    Assert.Equal(1, body.Sources[0].SourceReference);
-                }
-                finally
-                {
-                    await _debugger.StopAsync();
-                }
-            }
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task HandleLoadedSources_NoView_ReturnsEmpty()
-        {
-            using (CreateTestContext())
-            {
-                var port = GetFreePort();
-                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
-                var jobContext = CreateJobContextWithTunnel(cts.Token, port, "ci-job");
-                await _debugger.StartAsync(jobContext.Object);
-                try
-                {
-                    await DriveDebuggerToReadyAsync(port);
-                    var response = _debugger.HandleLoadedSources(MakeRequest("loadedSources"));
-                    Assert.True(response.Success);
-                    var body = Assert.IsType<LoadedSourcesResponseBody>(response.Body);
-                    Assert.Empty(body.Sources);
-                }
-                finally
-                {
-                    await _debugger.StopAsync();
-                }
-            }
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task HandleSetBreakpoints_ReturnsUnverifiedPlaceholders()
-        {
-            using (CreateTestContext())
-            {
-                var port = GetFreePort();
-                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
-                var jobContext = CreateJobContextWithTunnel(cts.Token, port, "ci-job");
-                await _debugger.StartAsync(jobContext.Object);
-                try
-                {
-                    await DriveDebuggerToReadyAsync(port);
-                    var step = NewActionRunner(GitHub.Runner.Worker.ActionRunStage.Main, "Run").Object;
-                    await _debugger.OnJobStepsInitializedAsync(new[] { step }, Array.Empty<IStep>());
-
-                    var args = new SetBreakpointsArguments
-                    {
-                        Source = new Source { SourceReference = 1 },
-                        Breakpoints = new System.Collections.Generic.List<SourceBreakpoint>
-                        {
-                            new SourceBreakpoint { Line = 5 },
-                            new SourceBreakpoint { Line = 10 },
-                            new SourceBreakpoint { Line = 15 },
-                        },
-                    };
-
-                    var response = _debugger.HandleSetBreakpoints(MakeRequest("setBreakpoints", args));
-                    Assert.True(response.Success);
-                    var body = Assert.IsType<SetBreakpointsResponseBody>(response.Body);
-                    Assert.Equal(3, body.Breakpoints.Count);
-                    Assert.All(body.Breakpoints, bp => Assert.False(bp.Verified));
-                    Assert.Equal(new[] { 5, 10, 15 }, body.Breakpoints.ConvertAll(b => b.Line ?? -1));
-                    Assert.All(body.Breakpoints, bp => Assert.False(string.IsNullOrEmpty(bp.Message)));
-                }
-                finally
-                {
-                    await _debugger.StopAsync();
-                }
-            }
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task OnJobStepsInitialized_EmitsLoadedSourceNewEvent()
-        {
-            using (CreateTestContext())
-            {
-                var port = GetFreePort();
-                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
-                var jobContext = CreateJobContextWithTunnel(cts.Token, port, "ci-job");
-                await _debugger.StartAsync(jobContext.Object);
                 using var client = await ConnectClientAsync(port);
-                try
-                {
-                    var stream = client.GetStream();
-                    await SendRequestAsync(stream, new Request { Seq = 1, Type = "request", Command = "configurationDone" });
-                    await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5)); // configurationDone response
-                    await _debugger.WaitUntilReadyAsync();
+                var stream = client.GetStream();
 
-                    var step = NewActionRunner(GitHub.Runner.Worker.ActionRunStage.Main, "Run").Object;
-                    await _debugger.OnJobStepsInitializedAsync(new[] { step }, Array.Empty<IStep>());
-
-                    var msg = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
-                    Assert.Contains("\"event\":\"loadedSource\"", msg);
-                    Assert.Contains("\"reason\":\"new\"", msg);
-                    Assert.Contains("\"sourceReference\":1", msg);
-                }
-                finally
+                await SendRequestAsync(stream, new Request
                 {
-                    await _debugger.StopAsync();
-                }
+                    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 OnPostStepRegistered_EmitsLoadedSourceChangedEvent()
+        public async Task WelcomeMessageShowsCustomMessageWhenOverrideEnabled()
         {
             using (CreateTestContext())
             {
                 var port = GetFreePort();
                 using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
-                var jobContext = CreateJobContextWithTunnel(cts.Token, port, "ci-job");
+                var jobContext = CreateJobContextWithTunnel(cts.Token, port,
+                    overrideWelcomeMessage: true,
+                    welcomeMessage: "Welcome to debugging!");
                 await _debugger.StartAsync(jobContext.Object);
+
                 using var client = await ConnectClientAsync(port);
-                try
+                var stream = client.GetStream();
+
+                await SendRequestAsync(stream, new Request
                 {
-                    var stream = client.GetStream();
-                    await SendRequestAsync(stream, new Request { Seq = 1, Type = "request", Command = "configurationDone" });
-                    await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
-                    await _debugger.WaitUntilReadyAsync();
+                    Seq = 1,
+                    Type = "request",
+                    Command = "configurationDone"
+                });
 
-                    var main = NewActionRunner(GitHub.Runner.Worker.ActionRunStage.Main, "Run").Object;
-                    await _debugger.OnJobStepsInitializedAsync(new[] { main }, Array.Empty<IStep>());
-                    // Drain the "new" loadedSource event.
-                    await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                // First: configurationDone response
+                var configDoneResponse = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"command\":\"configurationDone\"", configDoneResponse);
 
-                    var post = NewActionRunner(GitHub.Runner.Worker.ActionRunStage.Post, "Post Run", "actions/cache", "v3").Object;
-                    _debugger.OnPostStepRegistered(post);
+                // Second: custom welcome message
+                var welcomeMsg = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"event\":\"output\"", welcomeMsg);
+                Assert.Contains("Welcome to debugging!", welcomeMsg);
 
-                    var msg = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
-                    Assert.Contains("\"event\":\"loadedSource\"", msg);
-                    Assert.Contains("\"reason\":\"changed\"", msg);
-                    Assert.Contains("\"sourceReference\":1", msg);
-                }
-                finally
-                {
-                    await _debugger.StopAsync();
-                }
+                await _debugger.StopAsync();
             }
         }
 
         [Fact]
         [Trait("Level", "L0")]
         [Trait("Category", "Worker")]
-        public async Task StackTrace_LineUpdatesAsStepsAdvance()
+        public async Task WelcomeMessageSuppressedWhenOverrideEnabledWithEmptyMessage()
         {
             using (CreateTestContext())
             {
                 var port = GetFreePort();
                 using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
-                var jobContext = CreateJobContextWithTunnel(cts.Token, port, "ci-job");
+                var jobContext = CreateJobContextWithTunnel(cts.Token, port,
+                    overrideWelcomeMessage: true,
+                    welcomeMessage: "");
                 await _debugger.StartAsync(jobContext.Object);
-                try
+
+                using var client = await ConnectClientAsync(port);
+                var stream = client.GetStream();
+
+                await SendRequestAsync(stream, new Request
                 {
-                    await DriveDebuggerToReadyAsync(port);
-                    var s1 = NewActionRunner(GitHub.Runner.Worker.ActionRunStage.Main, "Step 1", "a/b", "v1").Object;
-                    var s2 = NewActionRunner(GitHub.Runner.Worker.ActionRunStage.Main, "Step 2", "c/d", "v2").Object;
-                    await _debugger.OnJobStepsInitializedAsync(new[] { s1, s2 }, Array.Empty<IStep>());
+                    Seq = 1,
+                    Type = "request",
+                    Command = "configurationDone"
+                });
 
-                    _ = _debugger.OnStepStartingAsync(s1, isFirstStep: true);
-                    await Task.Delay(50);
+                // Read configurationDone response
+                var configDoneResponse = await ReadDapMessageAsync(stream, TimeSpan.FromSeconds(5));
+                Assert.Contains("\"command\":\"configurationDone\"", configDoneResponse);
 
-                    var first = _debugger.HandleStackTrace(MakeRequest("stackTrace"));
-                    var firstBody = Assert.IsType<StackTraceResponseBody>(first.Body);
-                    int firstLine = firstBody.StackFrames[0].Line;
-                    Assert.Equal(_debugger.ExecutionView.TryGetLineForStep(s1), firstLine);
-
-                    _ = _debugger.OnStepStartingAsync(s2, isFirstStep: false);
-                    await Task.Delay(50);
-
-                    var second = _debugger.HandleStackTrace(MakeRequest("stackTrace"));
-                    var secondBody = Assert.IsType<StackTraceResponseBody>(second.Body);
-                    int secondLine = secondBody.StackFrames[0].Line;
-                    Assert.Equal(_debugger.ExecutionView.TryGetLineForStep(s2), secondLine);
-                    Assert.NotEqual(firstLine, secondLine);
-                }
-                finally
+                // Send threads request — if welcome message was suppressed, this
+                // should be the next response (no output event in between)
+                await SendRequestAsync(stream, new Request
                 {
-                    await _debugger.StopAsync();
-                }
+                    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/ExecutionContextL0.cs

@@ -7,7 +7,6 @@ using GitHub.DistributedTask.Pipelines.ContextData;
 using GitHub.DistributedTask.WebApi;
 using GitHub.Runner.Worker;
 using GitHub.Runner.Worker.Container;
-using GitHub.Runner.Worker.Dap;
 using GitHub.Runner.Worker.Handlers;
 using Moq;
 using Xunit;
@@ -406,7 +405,6 @@ namespace GitHub.Runner.Common.Tests.Worker
                 hc.EnqueueInstance(pagingLogger5.Object);
                 hc.EnqueueInstance(actionRunner1 as IActionRunner);
                 hc.EnqueueInstance(actionRunner2 as IActionRunner);
-                hc.SetSingleton(new Mock<IDapDebugger>().Object);
                 hc.SetSingleton(jobServerQueue.Object);
 
                 var jobContext = new Runner.Worker.ExecutionContext();
@@ -505,7 +503,6 @@ namespace GitHub.Runner.Common.Tests.Worker
                 hc.EnqueueInstance(pagingLogger5.Object);
                 hc.EnqueueInstance(actionRunner1 as IActionRunner);
                 hc.EnqueueInstance(actionRunner2 as IActionRunner);
-                hc.SetSingleton(new Mock<IDapDebugger>().Object);
                 hc.SetSingleton(jobServerQueue.Object);
 
                 var jobContext = new Runner.Worker.ExecutionContext();
@@ -547,75 +544,6 @@ namespace GitHub.Runner.Common.Tests.Worker
             }
         }
 
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void RegisterPostJobAction_DebuggerDisabled_DoesNotInvokeDapDebugger()
-        {
-            using (TestHostContext hc = CreateTestContext())
-            {
-                // Arrange: Create a job request message with EnableDebugger left at the default (false).
-                TaskOrchestrationPlanReference plan = new();
-                TimelineReference timeline = new();
-                Guid jobId = Guid.NewGuid();
-                string jobName = "some job name";
-                var jobRequest = new Pipelines.AgentJobRequestMessage(plan, timeline, jobId, jobName, jobName, null, null, null, new Dictionary<string, VariableValue>(), new List<MaskHint>(), new Pipelines.JobResources(), new Pipelines.ContextData.DictionaryContextData(), new Pipelines.WorkspaceOptions(), new List<Pipelines.ActionStep>(), null, null, null, null, null);
-                jobRequest.Resources.Repositories.Add(new Pipelines.RepositoryResource()
-                {
-                    Alias = Pipelines.PipelineConstants.SelfAlias,
-                    Id = "github",
-                    Version = "sha1"
-                });
-                jobRequest.ContextData["github"] = new Pipelines.ContextData.DictionaryContextData();
-
-                var pagingLogger = new Mock<IPagingLogger>();
-                var jobServerQueue = new Mock<IJobServerQueue>();
-                jobServerQueue.Setup(x => x.QueueTimelineRecordUpdate(It.IsAny<Guid>(), It.IsAny<TimelineRecord>()));
-                jobServerQueue.Setup(x => x.QueueWebConsoleLine(It.IsAny<Guid>(), It.IsAny<string>(), It.IsAny<long?>()));
-
-                var actionRunner = new ActionRunner();
-                actionRunner.Initialize(hc);
-
-                hc.EnqueueInstance(pagingLogger.Object);
-                hc.EnqueueInstance(pagingLogger.Object);
-                hc.EnqueueInstance(pagingLogger.Object);
-                hc.EnqueueInstance(pagingLogger.Object);
-                hc.EnqueueInstance(pagingLogger.Object);
-                hc.EnqueueInstance(pagingLogger.Object);
-                hc.EnqueueInstance(pagingLogger.Object);
-                hc.EnqueueInstance(actionRunner as IActionRunner);
-
-                // Register a strict mock IDapDebugger. If the production code calls
-                // ANY method on it, the test fails — proving the containment guard
-                // short-circuited before HostContext.GetService<IDapDebugger>().
-                var dapMock = new Mock<IDapDebugger>(MockBehavior.Strict);
-                hc.SetSingleton(dapMock.Object);
-                hc.SetSingleton(jobServerQueue.Object);
-
-                var jobContext = new Runner.Worker.ExecutionContext();
-                jobContext.Initialize(hc);
-                jobContext.InitializeJob(jobRequest, CancellationToken.None);
-
-                var action = jobContext.CreateChild(Guid.NewGuid(), "action_1", "action_1", null, null, 0);
-
-                var postRunner = hc.CreateService<IActionRunner>();
-                postRunner.Action = new Pipelines.ActionStep() { Id = Guid.NewGuid(), Name = "post", DisplayName = "Post", Reference = new Pipelines.RepositoryPathReference() { Name = "actions/action" } };
-                postRunner.Stage = ActionRunStage.Post;
-                postRunner.Condition = "always()";
-                postRunner.DisplayName = "post";
-
-                // Sanity: ensure the production code path actually believes the debugger is disabled.
-                Assert.True(jobContext.Global.Debugger == null || jobContext.Global.Debugger.Enabled == false);
-
-                // Act.
-                action.RegisterPostJobStep(postRunner);
-
-                // Assert: the debugger was never consulted on the non-debug path.
-                dapMock.VerifyNoOtherCalls();
-                Assert.Equal(1, jobContext.PostJobSteps.Count);
-            }
-        }
-
         [Fact]
         [Trait("Level", "L0")]
         [Trait("Category", "Worker")]

src/Test/L0/Worker/JobExecutionViewL0.cs

@@ -426,42 +426,17 @@ namespace GitHub.Runner.Common.Tests.Worker
         [Fact]
         [Trait("Level", "L0")]
         [Trait("Category", "Worker")]
-        public void TryMarkSkipped_MarksUnclaimedPlaceholder()
+        public void Append_RejectsBothStepIdentityAndMatchKey()
         {
+            // Allowing both would orphan the IStep→line mapping the moment
+            // TryClaim overwrites _stepIdentities[index] for a different
+            // step, so the API rejects the combination at append time.
             var view = new JobExecutionView("j");
-            var postEntry = new JobExecutionViewEntry(JobExecutionPhase.Post, "Post X", uses: "actions/x@v1");
-            view.Append(postEntry, stepIdentity: null, matchKey: "k1");
-
-            Assert.True(view.TryMarkSkipped("k1"));
-            Assert.True(postEntry.IsSkipped);
-            Assert.Contains("(skipped — main step did not execute)", view.Yaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void TryMarkSkipped_ReturnsFalseForUnknownKey()
-        {
-            var view = new JobExecutionView("j");
-            Assert.False(view.TryMarkSkipped("nope"));
-            Assert.DoesNotContain("(skipped", view.Yaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void TryMarkSkipped_ReturnsFalseForClaimedPlaceholder()
-        {
-            var view = new JobExecutionView("j");
-            var postEntry = new JobExecutionViewEntry(JobExecutionPhase.Post, "Post X", uses: "actions/x@v1");
-            view.Append(postEntry, stepIdentity: null, matchKey: "k1");
-
-            var step = NewStep("real-post");
-            Assert.NotNull(view.TryClaim("k1", step));
-
-            // Already claimed — must not mark as skipped.
-            Assert.False(view.TryMarkSkipped("k1"));
-            Assert.False(postEntry.IsSkipped);
+            var entry = new JobExecutionViewEntry(JobExecutionPhase.Post, "Post X", uses: "actions/x@v1");
+            Assert.Throws<ArgumentException>(() =>
+                view.Append(entry, stepIdentity: NewStep("real"), matchKey: "k1"));
+            // State unchanged.
+            Assert.Equal(0, view.EntryCount);
         }
     }
 }

src/Test/L0/Worker/JobExecutionViewLifecycleL0.cs

@@ -1,703 +0,0 @@
-using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Net;
-using System.Net.Sockets;
-using System.Runtime.CompilerServices;
-using System.Text;
-using System.Threading;
-using System.Threading.Tasks;
-using GitHub.DistributedTask.Pipelines;
-using GitHub.DistributedTask.WebApi;
-using GitHub.Runner.Worker;
-using GitHub.Runner.Worker.Dap;
-using Moq;
-using Newtonsoft.Json;
-using Xunit;
-
-namespace GitHub.Runner.Common.Tests.Worker
-{
-    public sealed class JobExecutionViewLifecycleL0
-    {
-        private DapDebugger _debugger;
-
-        private TestHostContext CreateTestContext([CallerMemberName] string testName = "")
-        {
-            var hc = new TestHostContext(this, testName);
-            _debugger = new DapDebugger();
-            _debugger.Initialize(hc);
-            _debugger.SkipTunnelRelay = true;
-            _debugger.SkipWebSocketBridge = true;
-            return hc;
-        }
-
-        private static ushort GetFreePort()
-        {
-            using var listener = new TcpListener(IPAddress.Loopback, 0);
-            listener.Start();
-            return (ushort)((IPEndPoint)listener.LocalEndpoint).Port;
-        }
-
-        private static Mock<IExecutionContext> CreateJobContextWithTunnel(CancellationToken cancellationToken, ushort port, string jobName = "ci-job")
-        {
-            var tunnel = new GitHub.DistributedTask.Pipelines.DebuggerTunnelInfo
-            {
-                TunnelId = "test-tunnel",
-                ClusterId = "test-cluster",
-                HostToken = "test-token",
-                Port = port
-            };
-            var debuggerConfig = new DebuggerConfig(true, tunnel);
-            var jobContext = new Mock<IExecutionContext>();
-            jobContext.Setup(x => x.CancellationToken).Returns(cancellationToken);
-            jobContext.Setup(x => x.Global).Returns(new GlobalContext { Debugger = debuggerConfig });
-            jobContext
-                .Setup(x => x.GetGitHubContext(It.IsAny<string>()))
-                .Returns((string contextName) => string.Equals(contextName, "job", StringComparison.Ordinal) ? jobName : null);
-            return jobContext;
-        }
-
-        private static async Task DriveToReadyAsync(DapDebugger debugger, int port)
-        {
-            var waitTask = debugger.WaitUntilReadyAsync();
-            var client = new TcpClient();
-            await client.ConnectAsync(IPAddress.Loopback, port);
-            var stream = client.GetStream();
-            var request = new Request { Seq = 1, Type = "request", Command = "configurationDone" };
-            var json = JsonConvert.SerializeObject(request);
-            var body = Encoding.UTF8.GetBytes(json);
-            var header = Encoding.ASCII.GetBytes($"Content-Length: {body.Length}\r\n\r\n");
-            await stream.WriteAsync(header, 0, header.Length);
-            await stream.WriteAsync(body, 0, body.Length);
-            await stream.FlushAsync();
-            await waitTask;
-            // Keep client alive by holding a reference via GC root in caller scope.
-            // We deliberately don't dispose here; tests dispose the context.
-            _ = client;
-        }
-
-        private static Mock<IActionRunner> NewActionRunner(ActionRunStage stage, string displayName, string actionName = "actions/checkout", string actionRef = "v4", Guid actionId = default)
-        {
-            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(new ActionStep
-            {
-                Id = actionId,
-                Reference = new RepositoryPathReference { Name = actionName, Ref = actionRef },
-            });
-            return mock;
-        }
-
-        private static Mock<IActionRunner> NewSelfActionRunner(ActionRunStage stage, string displayName, Guid actionId = default)
-        {
-            // RepositoryType = "self" — the predictor must skip these.
-            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(new ActionStep
-            {
-                Id = actionId,
-                Reference = new RepositoryPathReference
-                {
-                    RepositoryType = GitHub.DistributedTask.Pipelines.PipelineConstants.SelfAlias,
-                    Path = "./.github/actions/local",
-                },
-            });
-            return mock;
-        }
-
-        private static Mock<IActionRunner> NewScriptActionRunner(ActionRunStage stage, string displayName, Guid actionId = default)
-        {
-            // ScriptReference — a `run:` step. Not a RepositoryPathReference,
-            // so the predictor's pattern match falls through.
-            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(new ActionStep
-            {
-                Id = actionId,
-                Reference = new ScriptReference(),
-            });
-            return mock;
-        }
-
-        // IActionManager mock that returns specific Definitions per action by
-        // matching on the action's reference Name. Actions whose name is not
-        // in the map get a Definition with HasPost = false.
-        private static Mock<IActionManager> NewActionManagerWithPost(params string[] actionNamesWithPost)
-        {
-            var withPost = new HashSet<string>(actionNamesWithPost, StringComparer.Ordinal);
-            var mock = new Mock<IActionManager>();
-            mock.Setup(x => x.LoadAction(It.IsAny<IExecutionContext>(), It.IsAny<ActionStep>()))
-                .Returns((IExecutionContext _, ActionStep step) =>
-                {
-                    var name = (step.Reference as RepositoryPathReference)?.Name ?? "";
-                    return new Definition
-                    {
-                        Data = new ActionDefinitionData
-                        {
-                            Execution = withPost.Contains(name)
-                                ? new NodeJSActionExecutionData { Post = "post.js" }
-                                : new NodeJSActionExecutionData(),
-                        },
-                    };
-                });
-            return mock;
-        }
-
-        private static IStep NewJobExtensionRunner(string displayName)
-        {
-            return new JobExtensionRunner(
-                runAsync: (_, __) => Task.CompletedTask,
-                condition: null,
-                displayName: displayName,
-                data: null);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task OnJobStepsInitialized_NotActive_NoOps()
-        {
-            using (CreateTestContext())
-            {
-                var step = NewActionRunner(ActionRunStage.Main, "Run").Object;
-
-                await _debugger.OnJobStepsInitializedAsync(new[] { step }, Array.Empty<IStep>());
-
-                Assert.Null(_debugger.ExecutionView);
-            }
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task OnPostStepRegistered_NotActive_NoOps()
-        {
-            using (CreateTestContext())
-            {
-                var step = NewActionRunner(ActionRunStage.Post, "Post Run").Object;
-                _debugger.OnPostStepRegistered(step); // must not throw
-                Assert.Null(_debugger.ExecutionView);
-                await Task.CompletedTask;
-            }
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task OnJobStepsInitialized_Active_BuildsView()
-        {
-            using (CreateTestContext())
-            {
-                var port = GetFreePort();
-                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
-                var jobContext = CreateJobContextWithTunnel(cts.Token, port);
-                await _debugger.StartAsync(jobContext.Object);
-                try
-                {
-                    await DriveToReadyAsync(_debugger, port);
-
-                    var main1 = NewActionRunner(ActionRunStage.Main, "Run actions/checkout@v4").Object;
-                    var main2 = NewActionRunner(ActionRunStage.Main, "Run actions/setup-node@v3", "actions/setup-node", "v3").Object;
-                    var jobExt = NewJobExtensionRunner("Set up job");
-                    var post1 = NewActionRunner(ActionRunStage.Post, "Post Run actions/checkout@v4").Object;
-
-                    await _debugger.OnJobStepsInitializedAsync(
-                        new IStep[] { main1, jobExt, main2 },
-                        new IStep[] { post1 });
-
-                    var view = _debugger.ExecutionView;
-                    Assert.NotNull(view);
-                    Assert.Equal(3, view.EntryCount); // jobExt filtered out
-                    Assert.Contains("Run actions/checkout@v4", view.Yaml);
-                    Assert.Contains("Run actions/setup-node@v3", view.Yaml);
-                    Assert.Contains("Post Run actions/checkout@v4", view.Yaml);
-                    Assert.NotNull(view.TryGetLineForStep(main1));
-                    Assert.NotNull(view.TryGetLineForStep(main2));
-                    Assert.NotNull(view.TryGetLineForStep(post1));
-                }
-                finally
-                {
-                    await _debugger.StopAsync();
-                }
-            }
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task OnJobStepsInitialized_PreservesQueueOrder()
-        {
-            using (CreateTestContext())
-            {
-                var port = GetFreePort();
-                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
-                var jobContext = CreateJobContextWithTunnel(cts.Token, port);
-                await _debugger.StartAsync(jobContext.Object);
-                try
-                {
-                    await DriveToReadyAsync(_debugger, port);
-
-                    var s1 = NewActionRunner(ActionRunStage.Main, "Step 1", "a/b", "v1").Object;
-                    var s2 = NewActionRunner(ActionRunStage.Main, "Step 2", "c/d", "v2").Object;
-                    var s3 = NewActionRunner(ActionRunStage.Main, "Step 3", "e/f", "v3").Object;
-
-                    await _debugger.OnJobStepsInitializedAsync(new[] { s1, s2, s3 }, Array.Empty<IStep>());
-
-                    var view = _debugger.ExecutionView;
-                    Assert.Equal(3, view.EntryCount);
-                    var l1 = view.TryGetLineForStep(s1);
-                    var l2 = view.TryGetLineForStep(s2);
-                    var l3 = view.TryGetLineForStep(s3);
-                    Assert.NotNull(l1);
-                    Assert.NotNull(l2);
-                    Assert.NotNull(l3);
-                    Assert.True(l1 < l2);
-                    Assert.True(l2 < l3);
-                    Assert.Equal(view.GetLine(0), l1);
-                    Assert.Equal(view.GetLine(1), l2);
-                    Assert.Equal(view.GetLine(2), l3);
-                }
-                finally
-                {
-                    await _debugger.StopAsync();
-                }
-            }
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task OnPostStepRegistered_AppendsToView()
-        {
-            using (CreateTestContext())
-            {
-                var port = GetFreePort();
-                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
-                var jobContext = CreateJobContextWithTunnel(cts.Token, port);
-                await _debugger.StartAsync(jobContext.Object);
-                try
-                {
-                    await DriveToReadyAsync(_debugger, port);
-
-                    var main1 = NewActionRunner(ActionRunStage.Main, "Run actions/checkout@v4").Object;
-                    await _debugger.OnJobStepsInitializedAsync(new[] { main1 }, Array.Empty<IStep>());
-                    Assert.Equal(1, _debugger.ExecutionView.EntryCount);
-
-                    var post1 = NewActionRunner(ActionRunStage.Post, "Post Run actions/cache@v3", "actions/cache", "v3").Object;
-                    _debugger.OnPostStepRegistered(post1);
-
-                    var view = _debugger.ExecutionView;
-                    Assert.Equal(2, view.EntryCount);
-                    Assert.Contains("Post Run actions/cache@v3", view.Yaml);
-                    Assert.NotNull(view.TryGetLineForStep(post1));
-                }
-                finally
-                {
-                    await _debugger.StopAsync();
-                }
-            }
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task OnPostStepRegistered_BeforeViewBuilt_NoOps()
-        {
-            using (CreateTestContext())
-            {
-                var port = GetFreePort();
-                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
-                var jobContext = CreateJobContextWithTunnel(cts.Token, port);
-                await _debugger.StartAsync(jobContext.Object);
-                try
-                {
-                    await DriveToReadyAsync(_debugger, port);
-
-                    var post = NewActionRunner(ActionRunStage.Post, "Post Run").Object;
-                    _debugger.OnPostStepRegistered(post); // must not throw
-
-                    Assert.Null(_debugger.ExecutionView);
-                }
-                finally
-                {
-                    await _debugger.StopAsync();
-                }
-            }
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task OnPostStepRegistered_DuplicateStep_DoesNotThrow()
-        {
-            using (CreateTestContext())
-            {
-                var port = GetFreePort();
-                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
-                var jobContext = CreateJobContextWithTunnel(cts.Token, port);
-                await _debugger.StartAsync(jobContext.Object);
-                try
-                {
-                    await DriveToReadyAsync(_debugger, port);
-                    await _debugger.OnJobStepsInitializedAsync(Array.Empty<IStep>(), Array.Empty<IStep>());
-
-                    var post = NewActionRunner(ActionRunStage.Post, "Post Run").Object;
-                    _debugger.OnPostStepRegistered(post);
-                    _debugger.OnPostStepRegistered(post); // duplicate, must be silently ignored
-
-                    Assert.Equal(1, _debugger.ExecutionView.EntryCount);
-                }
-                finally
-                {
-                    await _debugger.StopAsync();
-                }
-            }
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task OnPostStepRegistered_FilteredStep_NoOps()
-        {
-            using (CreateTestContext())
-            {
-                var port = GetFreePort();
-                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
-                var jobContext = CreateJobContextWithTunnel(cts.Token, port);
-                await _debugger.StartAsync(jobContext.Object);
-                try
-                {
-                    await DriveToReadyAsync(_debugger, port);
-                    await _debugger.OnJobStepsInitializedAsync(Array.Empty<IStep>(), Array.Empty<IStep>());
-
-                    var before = _debugger.ExecutionView.EntryCount;
-                    _debugger.OnPostStepRegistered(NewJobExtensionRunner("Cleanup"));
-                    Assert.Equal(before, _debugger.ExecutionView.EntryCount);
-                }
-                finally
-                {
-                    await _debugger.StopAsync();
-                }
-            }
-        }
-
-        // ---- Predictive Post-step synthesis ----
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task OnJobStepsInitialized_PredictsPostForActionsWithHasPost()
-        {
-            using (var hc = CreateTestContext())
-            {
-                hc.SetSingleton<IActionManager>(NewActionManagerWithPost("actions/has-post").Object);
-
-                var port = GetFreePort();
-                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
-                var jobContext = CreateJobContextWithTunnel(cts.Token, port);
-                await _debugger.StartAsync(jobContext.Object);
-                try
-                {
-                    await DriveToReadyAsync(_debugger, port);
-
-                    var withPost = NewActionRunner(ActionRunStage.Main, "Run actions/has-post@v1", "actions/has-post", "v1", actionId: Guid.NewGuid()).Object;
-                    var noPost = NewActionRunner(ActionRunStage.Main, "Run actions/no-post@v1", "actions/no-post", "v1", actionId: Guid.NewGuid()).Object;
-
-                    await _debugger.OnJobStepsInitializedAsync(new[] { withPost, noPost }, Array.Empty<IStep>());
-
-                    var view = _debugger.ExecutionView;
-                    Assert.NotNull(view);
-                    // 2 main entries + 1 predicted post placeholder.
-                    Assert.Equal(3, view.EntryCount);
-                    Assert.Contains("post:\n", view.Yaml);
-                    Assert.Contains("Post Run actions/has-post@v1", view.Yaml);
-                    Assert.DoesNotContain("Post Run actions/no-post@v1", view.Yaml);
-                }
-                finally
-                {
-                    await _debugger.StopAsync();
-                }
-            }
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task OnJobStepsInitialized_PostPredictionsInReverseOrder()
-        {
-            using (var hc = CreateTestContext())
-            {
-                // Both actions have post — predictions must render in
-                // reverse declaration order to mirror the runner's LIFO
-                // post-execution order.
-                hc.SetSingleton<IActionManager>(NewActionManagerWithPost("actions/a", "actions/b").Object);
-
-                var port = GetFreePort();
-                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
-                var jobContext = CreateJobContextWithTunnel(cts.Token, port);
-                await _debugger.StartAsync(jobContext.Object);
-                try
-                {
-                    await DriveToReadyAsync(_debugger, port);
-
-                    var aMain = NewActionRunner(ActionRunStage.Main, "Run actions/a@v1", "actions/a", "v1", actionId: Guid.NewGuid()).Object;
-                    var bMain = NewActionRunner(ActionRunStage.Main, "Run actions/b@v1", "actions/b", "v1", actionId: Guid.NewGuid()).Object;
-
-                    await _debugger.OnJobStepsInitializedAsync(new[] { aMain, bMain }, Array.Empty<IStep>());
-
-                    string yaml = _debugger.ExecutionView.Yaml;
-                    int idxPostB = yaml.IndexOf("Post Run actions/b@v1", StringComparison.Ordinal);
-                    int idxPostA = yaml.IndexOf("Post Run actions/a@v1", StringComparison.Ordinal);
-                    Assert.True(idxPostB > 0 && idxPostA > 0, "both post placeholders expected");
-                    // Reverse declaration order: Post B appears BEFORE Post A.
-                    Assert.True(idxPostB < idxPostA, $"expected Post B before Post A (b={idxPostB} a={idxPostA})");
-                }
-                finally
-                {
-                    await _debugger.StopAsync();
-                }
-            }
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task OnJobStepsInitialized_SkipsScriptSteps()
-        {
-            using (var hc = CreateTestContext())
-            {
-                // Even if the action manager would say HasPost, the predictor
-                // must skip script run-steps because their reference is not
-                // a RepositoryPathReference.
-                hc.SetSingleton<IActionManager>(NewActionManagerWithPost(/* nothing */).Object);
-
-                var port = GetFreePort();
-                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
-                var jobContext = CreateJobContextWithTunnel(cts.Token, port);
-                await _debugger.StartAsync(jobContext.Object);
-                try
-                {
-                    await DriveToReadyAsync(_debugger, port);
-
-                    var script = NewScriptActionRunner(ActionRunStage.Main, "Run script", Guid.NewGuid()).Object;
-                    await _debugger.OnJobStepsInitializedAsync(new[] { script }, Array.Empty<IStep>());
-
-                    var view = _debugger.ExecutionView;
-                    Assert.NotNull(view);
-                    Assert.DoesNotContain("post:\n", view.Yaml);
-                    Assert.DoesNotContain("Post ", view.Yaml);
-                }
-                finally
-                {
-                    await _debugger.StopAsync();
-                }
-            }
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task OnJobStepsInitialized_SkipsSelfActions()
-        {
-            using (var hc = CreateTestContext())
-            {
-                // Self-action: ActionRunner.cs:106 guards against creating a
-                // Post for self-repository references. The predictor mirrors
-                // that, regardless of what the manifest reports.
-                hc.SetSingleton<IActionManager>(NewActionManagerWithPost("anything").Object);
-
-                var port = GetFreePort();
-                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
-                var jobContext = CreateJobContextWithTunnel(cts.Token, port);
-                await _debugger.StartAsync(jobContext.Object);
-                try
-                {
-                    await DriveToReadyAsync(_debugger, port);
-
-                    var selfRunner = NewSelfActionRunner(ActionRunStage.Main, "Run ./local-action", Guid.NewGuid()).Object;
-                    await _debugger.OnJobStepsInitializedAsync(new[] { selfRunner }, Array.Empty<IStep>());
-
-                    Assert.DoesNotContain("post:\n", _debugger.ExecutionView.Yaml);
-                }
-                finally
-                {
-                    await _debugger.StopAsync();
-                }
-            }
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task OnPostStepRegistered_ClaimsExistingPlaceholder()
-        {
-            using (var hc = CreateTestContext())
-            {
-                hc.SetSingleton<IActionManager>(NewActionManagerWithPost("actions/has-post").Object);
-
-                var port = GetFreePort();
-                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
-                var jobContext = CreateJobContextWithTunnel(cts.Token, port);
-                await _debugger.StartAsync(jobContext.Object);
-                try
-                {
-                    await DriveToReadyAsync(_debugger, port);
-
-                    var actionId = Guid.NewGuid();
-                    var mainRunner = NewActionRunner(ActionRunStage.Main, "Run actions/has-post@v1", "actions/has-post", "v1", actionId: actionId).Object;
-                    await _debugger.OnJobStepsInitializedAsync(new[] { mainRunner }, Array.Empty<IStep>());
-
-                    var view = _debugger.ExecutionView;
-                    int before = view.EntryCount;
-                    Assert.Equal(2, before); // main + predicted post placeholder
-
-                    // The real Post IActionRunner shares the same Action.Id
-                    // as the Main runner (ActionRunner.cs:131).
-                    var postRunner = NewActionRunner(ActionRunStage.Post, "Post actions/has-post@v1", "actions/has-post", "v1", actionId: actionId).Object;
-                    _debugger.OnPostStepRegistered(postRunner);
-
-                    // No new entry: the placeholder was claimed.
-                    Assert.Equal(before, view.EntryCount);
-                    Assert.NotNull(view.TryGetLineForStep(postRunner));
-                }
-                finally
-                {
-                    await _debugger.StopAsync();
-                }
-            }
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task OnPostStepRegistered_UnpredictedFallsBackToAppend()
-        {
-            using (var hc = CreateTestContext())
-            {
-                // Manager returns no HasPost — no predictions made.
-                hc.SetSingleton<IActionManager>(NewActionManagerWithPost(/* nothing */).Object);
-
-                var port = GetFreePort();
-                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
-                var jobContext = CreateJobContextWithTunnel(cts.Token, port);
-                await _debugger.StartAsync(jobContext.Object);
-                try
-                {
-                    await DriveToReadyAsync(_debugger, port);
-
-                    var mainRunner = NewActionRunner(ActionRunStage.Main, "Run actions/a@v1", "actions/a", "v1", actionId: Guid.NewGuid()).Object;
-                    await _debugger.OnJobStepsInitializedAsync(new[] { mainRunner }, Array.Empty<IStep>());
-
-                    var view = _debugger.ExecutionView;
-                    int before = view.EntryCount;
-                    Assert.Equal(1, before); // just main, no predicted post
-
-                    var unpredictedPost = NewActionRunner(ActionRunStage.Post, "Post Surprise", "actions/surprise", "v1", actionId: Guid.NewGuid()).Object;
-                    _debugger.OnPostStepRegistered(unpredictedPost);
-
-                    // Falls back to Append.
-                    Assert.Equal(before + 1, view.EntryCount);
-                    Assert.NotNull(view.TryGetLineForStep(unpredictedPost));
-                    Assert.Contains("Post Surprise", view.Yaml);
-                }
-                finally
-                {
-                    await _debugger.StopAsync();
-                }
-            }
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task OnPostStepRegistered_DuplicateClaim_NoDoubleEntry()
-        {
-            using (var hc = CreateTestContext())
-            {
-                hc.SetSingleton<IActionManager>(NewActionManagerWithPost("actions/has-post").Object);
-
-                var port = GetFreePort();
-                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
-                var jobContext = CreateJobContextWithTunnel(cts.Token, port);
-                await _debugger.StartAsync(jobContext.Object);
-                try
-                {
-                    await DriveToReadyAsync(_debugger, port);
-
-                    var actionId = Guid.NewGuid();
-                    var mainRunner = NewActionRunner(ActionRunStage.Main, "Run actions/has-post@v1", "actions/has-post", "v1", actionId: actionId).Object;
-                    await _debugger.OnJobStepsInitializedAsync(new[] { mainRunner }, Array.Empty<IStep>());
-                    Assert.Equal(2, _debugger.ExecutionView.EntryCount);
-
-                    // First registration claims the placeholder.
-                    var post1 = NewActionRunner(ActionRunStage.Post, "Post actions/has-post@v1", "actions/has-post", "v1", actionId: actionId).Object;
-                    _debugger.OnPostStepRegistered(post1);
-                    Assert.Equal(2, _debugger.ExecutionView.EntryCount);
-
-                    // Second registration with the same Action.Id but a
-                    // different IStep: TryClaim returns null (already
-                    // claimed). Falls through to Append. But the entry
-                    // it builds matches no existing step, so a new entry
-                    // would be added — UNLESS we constructed the second
-                    // post as a duplicate IStep registration of the same
-                    // step. Here we intentionally pass the same `post1`
-                    // step a second time — Append will reject the
-                    // already-registered step, the handler swallows it.
-                    _debugger.OnPostStepRegistered(post1);
-
-                    Assert.Equal(2, _debugger.ExecutionView.EntryCount);
-                }
-                finally
-                {
-                    await _debugger.StopAsync();
-                }
-            }
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task OnStepCompleted_SkippedMainStep_MarksPostPlaceholder()
-        {
-            using (var hc = CreateTestContext())
-            {
-                hc.SetSingleton<IActionManager>(NewActionManagerWithPost("actions/has-post").Object);
-
-                var port = GetFreePort();
-                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
-                var jobContext = CreateJobContextWithTunnel(cts.Token, port);
-                await _debugger.StartAsync(jobContext.Object);
-                try
-                {
-                    await DriveToReadyAsync(_debugger, port);
-
-                    var actionId = Guid.NewGuid();
-                    var mainMock = NewActionRunner(ActionRunStage.Main, "Run actions/has-post@v1", "actions/has-post", "v1", actionId: actionId);
-                    var execCtx = new Mock<IExecutionContext>();
-                    execCtx.SetupGet(x => x.Result).Returns(TaskResult.Skipped);
-                    mainMock.SetupGet(x => x.ExecutionContext).Returns(execCtx.Object);
-
-                    await _debugger.OnJobStepsInitializedAsync(new[] { mainMock.Object }, Array.Empty<IStep>());
-
-                    var view = _debugger.ExecutionView;
-                    Assert.Equal(2, view.EntryCount); // main + predicted post placeholder
-                    Assert.DoesNotContain("(skipped", view.Yaml);
-
-                    _debugger.OnStepCompleted(mainMock.Object);
-
-                    Assert.Equal(2, _debugger.ExecutionView.EntryCount);
-                    Assert.Contains("(skipped — main step did not execute)", _debugger.ExecutionView.Yaml);
-                    // Inline annotation must not have introduced a new line.
-                    Assert.Equal(view.Yaml.Split('\n').Length, _debugger.ExecutionView.Yaml.Split('\n').Length);
-                }
-                finally
-                {
-                    await _debugger.StopAsync();
-                }
-            }
-        }
-    }
-}

src/Test/L0/Worker/JobExecutionViewRendererL0.cs

@@ -1,6 +1,5 @@
 using System;
 using System.Collections.Generic;
-using System.Linq;
 using GitHub.Runner.Worker.Dap;
 using Xunit;
 
@@ -585,33 +584,15 @@ namespace GitHub.Runner.Common.Tests.Worker
         [Fact]
         [Trait("Level", "L0")]
         [Trait("Category", "Worker")]
-        public void Render_EmitsSkippedAnnotationForMarkedEntry()
+        public void Render_AlwaysUsesLfLineBreaks()
         {
-            var entry = new JobExecutionViewEntry(JobExecutionPhase.Post, "Post X", uses: "actions/x@v1");
-            entry.IsSkipped = true;
-
-            var result = JobExecutionViewRenderer.Render("j", new[] { entry });
-
-            // Annotation is inline on the `- step:` line so subsequent
-            // entry line numbers stay stable.
-            Assert.Contains("- step: Post X  # (skipped — main step did not execute)\n", result.Yaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Render_SkippedAnnotation_DoesNotShiftSubsequentLines()
-        {
-            var skipped = new JobExecutionViewEntry(JobExecutionPhase.Post, "Post A", uses: "actions/a@v1");
-            var following = new JobExecutionViewEntry(JobExecutionPhase.Post, "Post B", uses: "actions/b@v1");
-
-            var unmarked = JobExecutionViewRenderer.Render("j", new[] { skipped, following });
-            skipped.IsSkipped = true;
-            var marked = JobExecutionViewRenderer.Render("j", new[] { skipped, following });
-
-            // Following entry's start line must not move when the prior
-            // entry gets an inline skipped annotation.
-            Assert.Equal(unmarked.EntryStartLines[1], marked.EntryStartLines[1]);
+            // Regression: YamlDotNet's Emitter calls WriteLine, which on
+            // Windows produces CRLF (the host's Environment.NewLine).
+            // 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);
         }
     }
 }

src/Test/L0/Worker/JobExtensionL0.cs

@@ -141,7 +141,6 @@ namespace GitHub.Runner.Common.Tests.Worker
             hc.SetSingleton(_diagnosticLogManager.Object);
             hc.SetSingleton(_jobHookProvider.Object);
             hc.SetSingleton(_snapshotOperationProvider.Object);
-            hc.SetSingleton(new Mock<IDapDebugger>().Object);
             hc.EnqueueInstance<IPagingLogger>(_logger.Object); // JobExecutionContext
             hc.EnqueueInstance<IPagingLogger>(_logger.Object); // job start hook
             hc.EnqueueInstance<IPagingLogger>(_logger.Object); // Initial Job

src/Test/L0/Worker/JobRunnerL0.cs

@@ -1,6 +1,5 @@
 using GitHub.DistributedTask.WebApi;
 using GitHub.Runner.Worker;
-using GitHub.Runner.Worker.Dap;
 using Moq;
 using System;
 using System.Collections.Generic;
@@ -84,7 +83,6 @@ namespace GitHub.Runner.Common.Tests.Worker
             hc.SetSingleton(_extensions.Object);
             hc.SetSingleton(_temp.Object);
             hc.SetSingleton(_diagnosticLogManager.Object);
-            hc.SetSingleton(new Mock<IDapDebugger>().Object);
             hc.EnqueueInstance<IExecutionContext>(_jobEc);
             hc.EnqueueInstance<IPagingLogger>(_logger.Object);
             hc.EnqueueInstance<IJobExtension>(_jobExtension.Object);
@@ -177,29 +175,5 @@ namespace GitHub.Runner.Common.Tests.Worker
                 Assert.Equal(TaskResult.Succeeded, _jobEc.Result);
             }
         }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public async Task DebuggerDisabled_DoesNotInvokeDapDebugger()
-        {
-            using (TestHostContext hc = CreateTestContext())
-            {
-                // Override the lenient IDapDebugger singleton from CreateTestContext
-                // with a strict mock. If the containment guard fails, the production
-                // code will call OnJobStepsInitializedAsync and the strict mock will throw.
-                var dapMock = new Mock<IDapDebugger>(MockBehavior.Strict);
-                hc.SetSingleton(dapMock.Object);
-
-                var message = GetMessage();
-                // EnableDebugger defaults to false on AgentJobRequestMessage.
-                Assert.False(message.EnableDebugger);
-
-                await _jobRunner.RunAsync(message, _tokenSource.Token);
-
-                Assert.Equal(TaskResult.Succeeded, _jobEc.Result);
-                dapMock.VerifyNoOtherCalls();
-            }
-        }
     }
 }

src/Test/L0/Worker/StepEntryTranslatorL0.cs

@@ -1,425 +0,0 @@
-using System;
-using System.Collections.Generic;
-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()
-        {
-            var action = new ActionStep
-            {
-                Reference = new ScriptReference(),
-                Inputs = Map(
-                    ("script", Str("npm test")),
-                    ("shell", Str("bash")),
-                    ("working-directory", 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")),
-                    ("script", Str("leak")),
-                    ("shell", Str("leak")),
-                    ("working-directory", 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("script", entry.WithYaml);
-            Assert.DoesNotContain("shell", entry.WithYaml);
-            Assert.DoesNotContain("working-directory", 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

@@ -1,155 +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("${{ runner.os }}", yaml);
-            Assert.DoesNotContain("Linux", yaml);
-        }
-
-        [Fact]
-        [Trait("Level", "L0")]
-        [Trait("Category", "Worker")]
-        public void Serialize_PreservesCompositeExpressionInStringToken()
-        {
-            // Composite strings like `${{ runner.os }}-primes` are parsed
-            // as a StringToken whose value is exactly that literal. The
-            // adapter must round-trip the literal unchanged.
-            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_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"));
-        }
-    }
-}

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"
   }
 }