Merge branch 'main' into dap-execution-view-foundation

- Remove unused `using System.Linq;` from JobExecutionViewRendererL0
    and `using System.Collections.Generic;` from StepEntryTranslatorL0.
  - StepEntryTranslator: read run-step inputs through
    `PipelineConstants.ScriptStepInputs.*` (`script`, `shell`,
    `workingDirectory`). The runner stores these keys in their
    constants-defined spelling — see ActionManifestManagerWrapper:244 —
    not their kebab-case workflow-YAML form, so the previous
    `working-directory` lookup never matched in practice. Tests
    updated to use the same constants.
  - TemplateTokenYamlAdapter / JobExecutionViewRenderer.FormatScalar:
    defensively also strip a `---\n` prefix on top of the existing
    `--- ` prefix. Not observed in any input I exercised, but cheap
    insurance against an Emitter quirk on some YAML configurations.
  - JobExecutionView.Append: reject passing both `stepIdentity` and
    `matchKey`. The combination would orphan the original step's
    line mapping the moment TryClaim overwrites `_stepIdentities`
    for a different step. Add an L0 test covering the precondition.

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": "8.0.421"
+      "version": "8.0.420"
     },
     "ghcr.io/devcontainers/features/node:1": {
       "version": "20"

images/Dockerfile

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

src/Runner.Common/Constants.cs

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

src/Runner.Worker/Dap/DapDebugger.cs

@@ -63,7 +63,6 @@ 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;
@@ -491,11 +490,6 @@ namespace GitHub.Runner.Worker.Dap
                     });
                     Trace.Info("Sent initialized event");
                 }
-
-                if (request.Command == "configurationDone")
-                {
-                    SendWelcomeMessage();
-                }
             }
             catch (Exception ex)
             {
@@ -514,7 +508,6 @@ 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
@@ -825,34 +818,6 @@ namespace GitHub.Runner.Worker.Dap
             });
         }
 
-        internal void SendWelcomeMessage()
-        {
-            if (_welcomeMessageSent)
-            {
-                return;
-            }
-            _welcomeMessageSent = true;
-
-            var debuggerConfig = _jobContext?.Global?.Debugger;
-            if (debuggerConfig?.OverrideWelcomeMessage == true)
-            {
-                if (!string.IsNullOrEmpty(debuggerConfig.WelcomeMessage))
-                {
-                    SendOutput("console", debuggerConfig.WelcomeMessage);
-                    Trace.Info("Sent custom welcome message");
-                }
-                else
-                {
-                    Trace.Info("Welcome message suppressed by override");
-                }
-            }
-            else
-            {
-                SendOutput("console", DapReplParser.GetGeneralHelp());
-                Trace.Info("Sent default welcome message");
-            }
-        }
-
         internal async Task OnStepStartingAsync(IStep step, bool isFirstStep)
         {
             bool pauseOnNextStep;
@@ -895,9 +860,6 @@ 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);
         }
@@ -1233,12 +1195,7 @@ namespace GitHub.Runner.Worker.Dap
 
                 case RunCommand run:
                     var context = GetExecutionContextForFrame(frameId);
-                    bool isActionStep;
-                    lock (_stateLock)
-                    {
-                        isActionStep = _currentStep is IActionRunner;
-                    }
-                    return await _replExecutor.ExecuteRunCommandAsync(run, context, isActionStep, cancellationToken);
+                    return await _replExecutor.ExecuteRunCommandAsync(run, context, cancellationToken);
 
                 default:
                     return new EvaluateResponseBody
@@ -1450,40 +1407,6 @@ namespace GitHub.Runner.Worker.Dap
             });
         }
 
-        /// <summary>
-        /// Emits a console output banner telling the user whether REPL
-        /// commands will execute on the host or inside the job container.
-        /// </summary>
-        private void SendExecutionContextBanner()
-        {
-            if (!_isClientConnected)
-            {
-                return;
-            }
-
-            bool isActionStep = _currentStep is IActionRunner;
-            var container = _jobContext?.Global?.Container;
-
-            string target;
-            if (isActionStep && container != null &&
-                (!string.IsNullOrEmpty(container.ContainerId) ||
-                 FeatureManager.IsContainerHooksEnabled(_jobContext?.Global?.Variables)))
-            {
-                var image = container.ContainerImage ?? "container";
-                var shortId = !string.IsNullOrEmpty(container.ContainerId) && container.ContainerId.Length >= 12
-                    ? container.ContainerId.Substring(0, 12)
-                    : container.ContainerId ?? "";
-                var idSuffix = !string.IsNullOrEmpty(shortId) ? $" ({shortId})" : "";
-                target = $"job container: {image}{idSuffix}";
-            }
-            else
-            {
-                target = "runner host";
-            }
-
-            SendOutput("console", $"\nCommands will run on {target}\n");
-        }
-
         private string MaskUserVisibleText(string value)
         {
             if (string.IsNullOrEmpty(value))

src/Runner.Worker/Dap/DapReplExecutor.cs

@@ -9,7 +9,6 @@ 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
@@ -44,7 +43,6 @@ namespace GitHub.Runner.Worker.Dap
         public async Task<EvaluateResponseBody> ExecuteRunCommandAsync(
             RunCommand command,
             IExecutionContext context,
-            bool isActionStep,
             CancellationToken cancellationToken)
         {
             if (context == null)
@@ -54,7 +52,7 @@ namespace GitHub.Runner.Worker.Dap
 
             try
             {
-                return await ExecuteScriptAsync(command, context, isActionStep, cancellationToken);
+                return await ExecuteScriptAsync(command, context, cancellationToken);
             }
             catch (Exception ex)
             {
@@ -67,17 +65,9 @@ namespace GitHub.Runner.Worker.Dap
         private async Task<EvaluateResponseBody> ExecuteScriptAsync(
             RunCommand command,
             IExecutionContext context,
-            bool isActionStep,
             CancellationToken cancellationToken)
         {
-            // 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
+            // 1. Resolve shell — same logic as ScriptHandler
             string shellCommand;
             string argFormat;
 
@@ -97,9 +87,9 @@ namespace GitHub.Runner.Worker.Dap
                 argFormat = ScriptHandlerHelpers.GetScriptArgumentsFormat(shellCommand);
             }
 
-            _trace.Info($"Resolved REPL shell (container={isContainerStepHost})");
+            _trace.Info("Resolved REPL shell");
 
-            // 3. Expand ${{ }} expressions in the script body, just like
+            // 2. 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);
@@ -121,47 +111,25 @@ namespace GitHub.Runner.Worker.Dap
 
             try
             {
-                // 4. Resolve script path — translate for container if needed
-                var resolvedPath = stepHost.ResolvePathForStepHost(context, scriptFilePath).Replace("\"", "\\\"");
+                // 3. Format arguments with script path
+                var resolvedPath = 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);
 
-                // 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.
+                // 4. Resolve shell command path
                 string prependPath = string.Join(
                     Path.PathSeparator.ToString(),
                     Enumerable.Reverse(context.Global.PrependPath));
-                var fileName = isContainerStepHost
-                    ? shellCommand
-                    : WhichUtil.Which(shellCommand, false, _trace, prependPath) ?? shellCommand;
+                var commandPath = WhichUtil.Which(shellCommand, false, _trace, prependPath)
+                    ?? shellCommand;
 
-                // 6. Build environment — merge from execution context like a real step
+                // 5. Build environment — merge from execution context like a real step
                 var environment = BuildEnvironment(context, command.Env);
 
-                // 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
+                // 6. Resolve working directory
                 var workingDirectory = command.WorkingDirectory;
                 if (string.IsNullOrEmpty(workingDirectory))
                 {
@@ -173,60 +141,48 @@ 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");
 
-                // 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))
+                // 7. Execute via IProcessInvoker (same as DefaultStepHost)
+                int exitCode;
+                using (var processInvoker = _hostContext.CreateService<IProcessInvoker>())
                 {
-                    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");
+                    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);
                 }
 
-                // 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}");
 
-                // 10. Return only the exit code summary (output was already streamed)
+                // 8. 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}.",
@@ -242,43 +198,6 @@ 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
 {
@@ -8,12 +8,10 @@ namespace GitHub.Runner.Worker.Dap
     /// </summary>
     public sealed class DebuggerConfig
     {
-        public DebuggerConfig(bool enabled, DebuggerTunnelInfo tunnel, bool overrideWelcomeMessage = false, string welcomeMessage = null)
+        public DebuggerConfig(bool enabled, DebuggerTunnelInfo tunnel)
         {
             Enabled = enabled;
             Tunnel = tunnel;
-            OverrideWelcomeMessage = overrideWelcomeMessage;
-            WelcomeMessage = welcomeMessage;
         }
 
         /// <summary>Whether the debugger is enabled for this job.</summary>
@@ -25,19 +23,6 @@ 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/JobExecutionView.cs

@@ -0,0 +1,276 @@
+using System;
+using System.Collections.Generic;
+using GitHub.Runner.Sdk;
+
+namespace GitHub.Runner.Worker.Dap
+{
+    /// <summary>
+    /// Stateful, append-only container that wraps <see cref="JobExecutionViewRenderer"/>
+    /// for runtime use. Maintains a mutable list of entries, caches the rendered YAML,
+    /// and provides O(1) lookup from <see cref="IStep"/> identity to the current line
+    /// in the rendered YAML where that step's <c>- step:</c> key appears.
+    ///
+    /// Each <see cref="Append"/> can register the entry in one of three modes:
+    ///   - With a non-null <c>stepIdentity</c>: registers the IStep→line mapping
+    ///     immediately. Used for entries whose real <see cref="IStep"/> is already
+    ///     known at append time.
+    ///   - With a non-null <c>matchKey</c>: registers an unclaimed placeholder
+    ///     that a later <see cref="TryClaim"/> binds to a real <see cref="IStep"/>.
+    ///     Used for entries whose <see cref="IStep"/> is materialized later. A
+    ///     placeholder that is never claimed simply stays in the view and is never
+    ///     paused on — the IStep→line mapping is only populated on claim.
+    ///   - With neither: a static entry that needs no line lookup.
+    ///
+    /// <see cref="Append"/> and <see cref="AppendRange"/> never remove or reorder
+    /// existing entries. <see cref="TryClaim"/> does not re-render. The IStep→line
+    /// mapping is rebuilt on every render, so lookups stay accurate even if a later
+    /// Append happens to shift previously-emitted entries.
+    /// </summary>
+    internal sealed class JobExecutionView
+    {
+        private readonly object _lock = new();
+        private readonly string _jobId;
+        private readonly List<JobExecutionViewEntry> _entries = new();
+        private readonly List<IStep> _stepIdentities = new();
+        private readonly Dictionary<IStep, int> _lineByStep =
+            new(ReferenceEqualityComparer.Instance);
+        // Map matchKey -> entry index for placeholders awaiting a future
+        // TryClaim. Removed when claimed.
+        private readonly Dictionary<string, int> _unclaimedByKey =
+            new(StringComparer.Ordinal);
+        private string _yaml;
+        private IReadOnlyList<int> _entryStartLines = Array.Empty<int>();
+
+        public JobExecutionView(string jobId)
+        {
+            if (string.IsNullOrWhiteSpace(jobId))
+            {
+                throw new ArgumentException("jobId must not be null or whitespace.", nameof(jobId));
+            }
+
+            _jobId = jobId;
+            Render();
+        }
+
+        public string JobId
+        {
+            get { return _jobId; }
+        }
+
+        /// <summary>
+        /// Currently rendered YAML. Always reflects all entries appended so far,
+        /// plus the synthetic Setup header and Cleanup footer emitted by the renderer.
+        /// </summary>
+        public string Yaml
+        {
+            get
+            {
+                lock (_lock)
+                {
+                    return _yaml;
+                }
+            }
+        }
+
+        /// <summary>Number of entries (excludes synthetic Setup/Cleanup boundaries).</summary>
+        public int EntryCount
+        {
+            get
+            {
+                lock (_lock)
+                {
+                    return _entries.Count;
+                }
+            }
+        }
+
+        /// <summary>
+        /// 1-based line where entry <paramref name="entryIndex"/>'s <c>- step:</c> key
+        /// currently appears in <see cref="Yaml"/>.
+        /// </summary>
+        public int GetLine(int entryIndex)
+        {
+            lock (_lock)
+            {
+                if (entryIndex < 0 || entryIndex >= _entries.Count)
+                {
+                    throw new ArgumentOutOfRangeException(nameof(entryIndex));
+                }
+
+                return _entryStartLines[entryIndex];
+            }
+        }
+
+        /// <summary>
+        /// 1-based line for the entry whose <see cref="IStep"/> reference identity
+        /// matches <paramref name="step"/>. Returns null if <paramref name="step"/>
+        /// is null or has not been registered.
+        /// </summary>
+        public int? TryGetLineForStep(IStep step)
+        {
+            if (step == null)
+            {
+                return null;
+            }
+
+            lock (_lock)
+            {
+                if (_lineByStep.TryGetValue(step, out var line))
+                {
+                    return line;
+                }
+
+                return null;
+            }
+        }
+
+        /// <summary>
+        /// Append a new entry. Exactly one of <paramref name="stepIdentity"/>
+        /// or <paramref name="matchKey"/> may be non-null (or both may be
+        /// null for a static entry that needs no line lookup):
+        ///   - <paramref name="stepIdentity"/> non-null: registers the
+        ///     IStep→line mapping immediately. Use when the real
+        ///     <see cref="IStep"/> is known at append time.
+        ///   - <paramref name="matchKey"/> non-null: registers an unclaimed
+        ///     placeholder that a later <see cref="TryClaim"/> binds to a
+        ///     real <see cref="IStep"/>.
+        /// Re-renders the YAML and updates the start-line table.
+        /// </summary>
+        /// <returns>1-based line number of the newly-appended entry's <c>- step:</c> key.</returns>
+        public int Append(JobExecutionViewEntry entry, IStep stepIdentity = null, string matchKey = null)
+        {
+            ArgUtil.NotNull(entry, nameof(entry));
+            if (stepIdentity != null && matchKey != null)
+            {
+                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)
+            {
+                if (stepIdentity != null && _lineByStep.ContainsKey(stepIdentity))
+                {
+                    throw new InvalidOperationException("step already registered in execution view");
+                }
+                if (matchKey != null && _unclaimedByKey.ContainsKey(matchKey))
+                {
+                    throw new InvalidOperationException($"matchKey already registered: {matchKey}");
+                }
+
+                _entries.Add(entry);
+                _stepIdentities.Add(stepIdentity);
+                Render();
+
+                int index = _entries.Count - 1;
+                if (matchKey != null)
+                {
+                    _unclaimedByKey[matchKey] = index;
+                }
+                return _entryStartLines[index];
+            }
+        }
+
+        /// <summary>
+        /// Bind a previously-appended placeholder entry (registered via
+        /// <see cref="Append(JobExecutionViewEntry, IStep, string)"/> with
+        /// a non-null <c>matchKey</c>) to a real <see cref="IStep"/>.
+        /// Returns the 1-based line of the now-claimed entry on success.
+        /// Returns null when no unclaimed placeholder exists for
+        /// <paramref name="matchKey"/>, OR when <paramref name="stepIdentity"/>
+        /// is already registered for a different entry (defensive).
+        /// Does not re-render: claim only updates the IStep -> line index.
+        /// </summary>
+        public int? TryClaim(string matchKey, IStep stepIdentity)
+        {
+            if (matchKey == null)
+            {
+                throw new ArgumentNullException(nameof(matchKey));
+            }
+            if (stepIdentity == null)
+            {
+                throw new ArgumentNullException(nameof(stepIdentity));
+            }
+
+            lock (_lock)
+            {
+                if (!_unclaimedByKey.TryGetValue(matchKey, out int index))
+                {
+                    return null;
+                }
+                if (_lineByStep.ContainsKey(stepIdentity))
+                {
+                    // Bail rather than double-register the step.
+                    return null;
+                }
+
+                _unclaimedByKey.Remove(matchKey);
+                _stepIdentities[index] = stepIdentity;
+                _lineByStep[stepIdentity] = _entryStartLines[index];
+                return _entryStartLines[index];
+            }
+        }
+
+        /// <summary>
+        /// Bulk-append for the initial population. Equivalent to calling
+        /// <see cref="Append"/> once per pair, but renders only once at the end.
+        /// State is left unchanged if any input is invalid.
+        /// </summary>
+        public void AppendRange(IEnumerable<(JobExecutionViewEntry entry, IStep stepIdentity)> items)
+        {
+            ArgUtil.NotNull(items, nameof(items));
+
+            // Materialize first so we don't enumerate twice.
+            var materialized = new List<(JobExecutionViewEntry entry, IStep stepIdentity)>(items);
+            for (int i = 0; i < materialized.Count; i++)
+            {
+                if (materialized[i].entry == null)
+                {
+                    throw new ArgumentException($"items[{i}].entry is null.", nameof(items));
+                }
+            }
+
+            lock (_lock)
+            {
+                // Validate no duplicates within the input or with existing identities,
+                // before mutating state.
+                var seen = new HashSet<IStep>(ReferenceEqualityComparer.Instance);
+                foreach (var (_, stepIdentity) in materialized)
+                {
+                    if (stepIdentity == null)
+                    {
+                        continue;
+                    }
+                    if (_lineByStep.ContainsKey(stepIdentity) || !seen.Add(stepIdentity))
+                    {
+                        throw new InvalidOperationException("step already registered in execution view");
+                    }
+                }
+
+                foreach (var (entry, stepIdentity) in materialized)
+                {
+                    _entries.Add(entry);
+                    _stepIdentities.Add(stepIdentity);
+                }
+                Render();
+            }
+        }
+
+        // Caller MUST hold _lock (constructor's call is safe — no concurrent access yet).
+        private void Render()
+        {
+            var result = JobExecutionViewRenderer.Render(_jobId, _entries.AsReadOnly());
+            _yaml = result.Yaml;
+            _entryStartLines = result.EntryStartLines;
+
+            _lineByStep.Clear();
+            for (int i = 0; i < _stepIdentities.Count; i++)
+            {
+                var step = _stepIdentities[i];
+                if (step != null)
+                {
+                    _lineByStep[step] = _entryStartLines[i];
+                }
+            }
+        }
+    }
+}

src/Runner.Worker/Dap/JobExecutionViewRenderer.cs

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

src/Runner.Worker/Dap/StepEntryTranslator.cs

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

src/Runner.Worker/Dap/TemplateTokenYamlAdapter.cs

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

src/Runner.Worker/Dap/YamlScalarFormatter.cs

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

src/Runner.Worker/ExecutionContext.cs

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

src/Sdk/DTPipelines/Pipelines/AgentJobRequestMessage.cs

@@ -267,21 +267,6 @@ 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");
@@ -161,26 +161,6 @@ public sealed class AgentJobRequestMessageL0
         Assert.Empty(recoveredMessage.ActionsDependencies);
     }
 
-    [Fact]
-    [Trait("Level", "L0")]
-    [Trait("Category", "Common")]
-    public void VerifyDebuggerWelcomeMessageRoundTrips()
-    {
-        // Arrange
-        var serializer = new DataContractJsonSerializer(typeof(AgentJobRequestMessage));
-        string json = DoubleQuotify("{'DebuggerWelcomeMessage': 'Welcome to debugging!'}");
-
-        // Act
-        using var stream = new MemoryStream();
-        stream.Write(Encoding.UTF8.GetBytes(json));
-        stream.Position = 0;
-        var recoveredMessage = serializer.ReadObject(stream) as AgentJobRequestMessage;
-
-        // Assert
-        Assert.NotNull(recoveredMessage);
-        Assert.Equal("Welcome to debugging!", recoveredMessage.DebuggerWelcomeMessage);
-    }
-
     private static string DoubleQuotify(string text)
     {
         return text.Replace('\'', '"');

src/Test/L0/Worker/DapDebuggerL0.cs

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

src/Test/L0/Worker/DapReplExecutorL0.cs

@@ -5,12 +5,9 @@ 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;
 
@@ -43,8 +40,7 @@ namespace GitHub.Runner.Common.Tests.Worker
 
         private Mock<IExecutionContext> CreateMockContext(
             DictionaryContextData exprValues = null,
-            IDictionary<string, IDictionary<string, string>> jobDefaults = null,
-            ContainerInfo container = null)
+            IDictionary<string, IDictionary<string, string>> jobDefaults = null)
         {
             var mock = new Mock<IExecutionContext>();
             mock.Setup(x => x.ExpressionValues).Returns(exprValues ?? new DictionaryContextData());
@@ -55,7 +51,6 @@ 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);
 
@@ -70,7 +65,7 @@ namespace GitHub.Runner.Common.Tests.Worker
             using (CreateTestContext())
             {
                 var command = new RunCommand { Script = "echo hello" };
-                var result = await _executor.ExecuteRunCommandAsync(command, null, false, CancellationToken.None);
+                var result = await _executor.ExecuteRunCommandAsync(command, null, CancellationToken.None);
 
                 Assert.Equal("error", result.Type);
                 Assert.Contains("No execution context available", result.Result);
@@ -238,101 +233,5 @@ 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/JobExecutionViewL0.cs

@@ -0,0 +1,442 @@
+using System;
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+using GitHub.Runner.Worker;
+using GitHub.Runner.Worker.Dap;
+using Moq;
+using Xunit;
+
+namespace GitHub.Runner.Common.Tests.Worker
+{
+    public sealed class JobExecutionViewL0
+    {
+        private static JobExecutionViewEntry MainEntry(string name)
+        {
+            return new JobExecutionViewEntry(JobExecutionPhase.Main, name, run: name);
+        }
+
+        private static IStep NewStep(string displayName = "step")
+        {
+            var mock = new Mock<IStep>();
+            mock.Setup(s => s.DisplayName).Returns(displayName);
+            return mock.Object;
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Constructor_RendersEmptyView()
+        {
+            var view = new JobExecutionView("my-job");
+
+            Assert.Equal(0, view.EntryCount);
+            Assert.Contains("# Job: my-job", view.Yaml);
+            Assert.Contains("- step: Setup job", view.Yaml);
+            Assert.Contains("- step: Complete job", view.Yaml);
+
+            // Only the two synthetic boundaries appear.
+            int stepCount = view.Yaml.Split("- step: ").Length - 1;
+            Assert.Equal(2, stepCount);
+        }
+
+        [Theory]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        [InlineData(null)]
+        [InlineData("")]
+        [InlineData("   ")]
+        public void Constructor_ThrowsOnInvalidJobId(string jobId)
+        {
+            Assert.Throws<ArgumentException>(() => new JobExecutionView(jobId));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Append_IncrementsEntryCount()
+        {
+            var view = new JobExecutionView("j");
+
+            int line0 = view.Append(MainEntry("a"));
+            int line1 = view.Append(MainEntry("b"));
+            int line2 = view.Append(MainEntry("c"));
+
+            Assert.Equal(3, view.EntryCount);
+            Assert.True(line0 < line1);
+            Assert.True(line1 < line2);
+            Assert.Equal(line0, view.GetLine(0));
+            Assert.Equal(line1, view.GetLine(1));
+            Assert.Equal(line2, view.GetLine(2));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Append_PreservesPriorEntryLines()
+        {
+            var view = new JobExecutionView("j");
+
+            int l0 = view.Append(MainEntry("a"));
+            int l1 = view.Append(MainEntry("b"));
+            int l2 = view.Append(MainEntry("c"));
+
+            view.Append(MainEntry("d"));
+            Assert.Equal(l0, view.GetLine(0));
+            Assert.Equal(l1, view.GetLine(1));
+            Assert.Equal(l2, view.GetLine(2));
+
+            view.Append(MainEntry("e"));
+            Assert.Equal(l0, view.GetLine(0));
+            Assert.Equal(l1, view.GetLine(1));
+            Assert.Equal(l2, view.GetLine(2));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Append_RegistersStepIdentity()
+        {
+            var view = new JobExecutionView("j");
+            var step = NewStep();
+
+            int line = view.Append(MainEntry("a"), step);
+
+            Assert.Equal(line, view.GetLine(0));
+            Assert.Equal(line, view.TryGetLineForStep(step));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Append_NullStepIdentity_StillAppends()
+        {
+            var view = new JobExecutionView("j");
+
+            view.Append(MainEntry("a"), stepIdentity: null);
+
+            Assert.Equal(1, view.EntryCount);
+            Assert.Null(view.TryGetLineForStep(null));
+            Assert.Contains("- step: a", view.Yaml);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Append_DuplicateStepIdentity_Throws()
+        {
+            var view = new JobExecutionView("j");
+            var step = NewStep();
+
+            view.Append(MainEntry("a"), step);
+            Assert.Throws<InvalidOperationException>(() => view.Append(MainEntry("b"), step));
+
+            // State preserved: only the first entry is present.
+            Assert.Equal(1, view.EntryCount);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Append_NullEntry_Throws()
+        {
+            var view = new JobExecutionView("j");
+            Assert.Throws<ArgumentNullException>(() => view.Append(null));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void AppendRange_AppendsAllAndRendersOnce()
+        {
+            var view = new JobExecutionView("j");
+            var steps = Enumerable.Range(0, 5).Select(i => NewStep("s" + i)).ToList();
+            var items = steps
+                .Select((s, i) => (entry: MainEntry("e" + i), stepIdentity: s))
+                .ToList();
+
+            view.AppendRange(items);
+
+            Assert.Equal(5, view.EntryCount);
+            for (int i = 0; i < 5; i++)
+            {
+                int line = view.GetLine(i);
+                Assert.Equal(line, view.TryGetLineForStep(steps[i]));
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void AppendRange_RejectsDuplicateInInput()
+        {
+            var view = new JobExecutionView("j");
+            var dup = NewStep();
+            var items = new List<(JobExecutionViewEntry, IStep)>
+            {
+                (MainEntry("a"), dup),
+                (MainEntry("b"), dup),
+            };
+
+            Assert.Throws<InvalidOperationException>(() => view.AppendRange(items));
+            Assert.Equal(0, view.EntryCount);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void AppendRange_RejectsOverlapWithExisting()
+        {
+            var view = new JobExecutionView("j");
+            var step = NewStep();
+            view.Append(MainEntry("a"), step);
+
+            var items = new List<(JobExecutionViewEntry, IStep)>
+            {
+                (MainEntry("b"), step),
+            };
+
+            Assert.Throws<InvalidOperationException>(() => view.AppendRange(items));
+            Assert.Equal(1, view.EntryCount);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void AppendRange_NullItems_Throws()
+        {
+            var view = new JobExecutionView("j");
+            Assert.Throws<ArgumentNullException>(() => view.AppendRange(null));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void TryGetLineForStep_NullStep_ReturnsNull()
+        {
+            var view = new JobExecutionView("j");
+            Assert.Null(view.TryGetLineForStep(null));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void TryGetLineForStep_UnknownStep_ReturnsNull()
+        {
+            var view = new JobExecutionView("j");
+            var step = NewStep();
+            Assert.Null(view.TryGetLineForStep(step));
+        }
+
+        [Theory]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        [InlineData(-1)]
+        [InlineData(2)]
+        public void GetLine_OutOfRange_Throws(int index)
+        {
+            var view = new JobExecutionView("j");
+            view.Append(MainEntry("a"));
+            view.Append(MainEntry("b"));
+
+            Assert.Throws<ArgumentOutOfRangeException>(() => view.GetLine(index));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Yaml_UpdatesAfterAppend()
+        {
+            var view = new JobExecutionView("j");
+            view.Append(MainEntry("first"));
+            string before = view.Yaml;
+            Assert.Contains("- step: first", before);
+
+            view.Append(MainEntry("second"));
+            string after = view.Yaml;
+
+            Assert.Contains("- step: first", after);
+            Assert.Contains("- step: second", after);
+            Assert.NotEqual(before, after);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Yaml_AlwaysEndsWithCleanupBoundary()
+        {
+            var view = new JobExecutionView("j");
+            Assert.EndsWith("cleanup:\n  - step: Complete job\n", view.Yaml);
+
+            view.Append(MainEntry("a"));
+            Assert.EndsWith("cleanup:\n  - step: Complete job\n", view.Yaml);
+
+            view.Append(MainEntry("b"));
+            view.Append(MainEntry("c"));
+            Assert.EndsWith("cleanup:\n  - step: Complete job\n", view.Yaml);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Append_WithMatchKey_TracksUnclaimed()
+        {
+            var view = new JobExecutionView("j");
+
+            int line = view.Append(MainEntry("placeholder"), stepIdentity: null, matchKey: "k1");
+
+            var step = NewStep("real");
+            int? claimed = view.TryClaim("k1", step);
+            Assert.Equal(line, claimed);
+            Assert.Equal(line, view.TryGetLineForStep(step));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void TryClaim_UnknownKey_ReturnsNull()
+        {
+            var view = new JobExecutionView("j");
+            view.Append(MainEntry("a"), stepIdentity: null, matchKey: "k1");
+
+            Assert.Null(view.TryClaim("nope", NewStep()));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void TryClaim_AlreadyClaimed_ReturnsNull()
+        {
+            var view = new JobExecutionView("j");
+            view.Append(MainEntry("a"), stepIdentity: null, matchKey: "k1");
+
+            var first = NewStep("first");
+            Assert.NotNull(view.TryClaim("k1", first));
+
+            var second = NewStep("second");
+            Assert.Null(view.TryClaim("k1", second));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void TryClaim_StepAlreadyRegistered_ReturnsNull()
+        {
+            var view = new JobExecutionView("j");
+            var step = NewStep();
+            // Step is registered for the first entry.
+            view.Append(MainEntry("a"), step);
+            // A placeholder is registered for the second entry.
+            view.Append(MainEntry("b"), stepIdentity: null, matchKey: "k1");
+
+            // Trying to claim the placeholder with the already-registered
+            // step must return null (defensive — would otherwise double-bind).
+            Assert.Null(view.TryClaim("k1", step));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Append_DuplicateMatchKey_Throws()
+        {
+            var view = new JobExecutionView("j");
+            view.Append(MainEntry("a"), stepIdentity: null, matchKey: "k1");
+
+            Assert.Throws<InvalidOperationException>(
+                () => view.Append(MainEntry("b"), stepIdentity: null, matchKey: "k1"));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Append_MatchKeyNull_BehavesLikeOldOverload()
+        {
+            var view = new JobExecutionView("j");
+            var step = NewStep();
+
+            int line = view.Append(MainEntry("a"), step);
+
+            Assert.Equal(line, view.GetLine(0));
+            Assert.Equal(line, view.TryGetLineForStep(step));
+            // TryClaim with any key must return null since no matchKey was registered.
+            Assert.Null(view.TryClaim("anything", NewStep()));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void TryClaim_AfterClaim_TryGetLineForStepResolves()
+        {
+            var view = new JobExecutionView("j");
+            int line = view.Append(MainEntry("placeholder"), stepIdentity: null, matchKey: "k1");
+
+            var step = NewStep();
+            Assert.Equal(line, view.TryClaim("k1", step));
+            Assert.Equal(line, view.TryGetLineForStep(step));
+
+            // And a later Append doesn't lose the claim (Render rebuilds
+            // the IStep -> line map from the persisted identities).
+            view.Append(MainEntry("b"));
+            Assert.Equal(line, view.TryGetLineForStep(step));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void TryClaim_NullArgs_Throws()
+        {
+            var view = new JobExecutionView("j");
+            Assert.Throws<ArgumentNullException>(() => view.TryClaim(null, NewStep()));
+            Assert.Throws<ArgumentNullException>(() => view.TryClaim("k", null));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task ConcurrentAppends_DontCorruptState()
+        {
+            var view = new JobExecutionView("j");
+            const int N = 50;
+            var steps = Enumerable.Range(0, N).Select(i => NewStep("s" + i)).ToList();
+            var returnedLines = new ConcurrentBag<int>();
+
+            var tasks = Enumerable.Range(0, N).Select(i => Task.Run(() =>
+            {
+                int line = view.Append(MainEntry("e" + i), steps[i]);
+                returnedLines.Add(line);
+            })).ToArray();
+
+            await Task.WhenAll(tasks);
+
+            Assert.Equal(N, view.EntryCount);
+            Assert.Equal(N, returnedLines.Distinct().Count());
+
+            // Every step identity resolves to some line in [0, N).
+            var entryLines = Enumerable.Range(0, N).Select(view.GetLine).ToHashSet();
+            Assert.Equal(N, entryLines.Count);
+            foreach (var step in steps)
+            {
+                int? line = view.TryGetLineForStep(step);
+                Assert.NotNull(line);
+                Assert.Contains(line.Value, entryLines);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        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 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/JobExecutionViewRendererL0.cs

@@ -588,11 +588,23 @@ namespace GitHub.Runner.Common.Tests.Worker
         {
             // 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.
+            // FormatScalar / TemplateTokenYamlAdapter.Serialize must force
+            // LF so the rendered view round-trips regardless of platform.
             var entry = new JobExecutionViewEntry(JobExecutionPhase.Main, "with: colon", id: "step-1", uses: "actions/checkout@v4");
             var result = JobExecutionViewRenderer.Render("job-1", new[] { entry });
             Assert.DoesNotContain("\r", result.Yaml);
         }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void FormatScalar_AlwaysUsesLfLineBreaks()
+        {
+            // Direct check on FormatScalar to guard against future refactors
+            // that bypass the full Render path but still emit through
+            // YamlDotNet.
+            Assert.DoesNotContain("\r", JobExecutionViewRenderer.FormatScalar("with: colon"));
+            Assert.DoesNotContain("\r", JobExecutionViewRenderer.FormatScalar("hello"));
+        }
     }
 }

src/Test/L0/Worker/StepEntryTranslatorL0.cs

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

src/Test/L0/Worker/TemplateTokenYamlAdapterL0.cs

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

src/Test/L0/Worker/YamlScalarFormatterL0.cs

@@ -1,119 +0,0 @@
-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="8.0.421"
+DOTNETSDK_VERSION="8.0.420"
 DOTNETSDK_INSTALLDIR="$DOTNETSDK_ROOT/$DOTNETSDK_VERSION"
 RUNNER_VERSION=$(cat runnerversion)
 

src/global.json

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