Add JobExecutionViewRenderer for DAP execution view

The DAP debugger serves a synthesized YAML document as the job's
`source`. That document is a 1:1 representation of how the runner
sees the job — not the workflow file — so pre and post action steps
appear as their own 'lines' that the user can pause on (and
eventually breakpoint, set in a follow-up PR).

This commit adds the core rendering algorithm: given a list of
phase-tagged entries (`JobExecutionViewEntry`), produce the
phase-keyed YAML plus a parallel array of 1-based line numbers
pointing at each entry's `- step:` key. The line numbers are what
later powers the DAP `stackTrace` handler.

Why hand-emit the skeleton instead of serializing a DTO?
Per-entry line offsets must be tracked at emission time. Using a
generic YAML serializer would force a second pass to scan the
output for `- step:` lines, which is fragile and breaks the moment
indentation conventions shift. Scalar values still go through the
library (via YamlScalarFormatter from #PR1a), so we don't carry
quoting rules.

Example output for a typical job (build, build, post step):

    # Job: build
    # Runner execution plan — read-only.

    setup:
      - step: Setup job

    main:
      - step: Run actions/checkout@v6
        uses: actions/checkout@v6
        if: success()
      - step: Cache Primes
        id: cache-primes
        uses: actions/cache@v5
        if: success()
        with:
          path: prime-numbers
          key: ${{ runner.os }}-primes

    post:
      - step: Post Cache Primes
        action: actions/cache@v5

    cleanup:
      - step: Complete job

This is part 2 of 5 splitting the previously-monolithic foundation
for review tractability. The wiring that turns runner state into
these entries lives in the next PRs.

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

.devcontainer/devcontainer.json

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

docs/start/envlinux.md

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

images/Dockerfile

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

src/Misc/layoutbin/installdependencies.sh

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

src/Runner.Common/Constants.cs

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

src/Runner.Worker/Dap/DapDebugger.cs

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

src/Runner.Worker/Dap/DapReplExecutor.cs

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

src/Runner.Worker/Dap/DebuggerConfig.cs

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

src/Runner.Worker/Dap/JobExecutionViewRenderer.cs

@@ -0,0 +1,336 @@
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using System.Text;
+using GitHub.Runner.Sdk;
+
+namespace GitHub.Runner.Worker.Dap
+{
+    /// <summary>
+    /// Phase a step occupies in the runner's flat execution sequence.
+    /// Setup and Cleanup are NOT modeled here — they are synthetic
+    /// boundaries hard-coded by <see cref="JobExecutionViewRenderer"/>
+    /// and cannot be constructed by callers.
+    /// </summary>
+    internal enum JobExecutionPhase
+    {
+        Pre,
+        Main,
+        Post,
+    }
+
+    /// <summary>
+    /// One step in the rendered execution view. Pure data; no link to
+    /// any worker type. Phase 2 will translate runner step objects
+    /// into instances of this record.
+    /// </summary>
+    internal sealed class JobExecutionViewEntry
+    {
+        public JobExecutionViewEntry(
+            JobExecutionPhase phase,
+            string displayName,
+            string uses = null,
+            string run = null,
+            string sourcePath = null,
+            int sourceLine = 0,
+            string id = null,
+            string @if = null,
+            string continueOnError = null,
+            string timeoutMinutes = null,
+            string envYaml = null,
+            string withYaml = null,
+            string shell = null,
+            string workingDirectory = null)
+        {
+            if (string.IsNullOrWhiteSpace(displayName))
+            {
+                throw new ArgumentException("displayName must not be null or whitespace.", nameof(displayName));
+            }
+            if (sourcePath != null && sourceLine < 1)
+            {
+                throw new ArgumentException(
+                    "sourceLine must be >= 1 when sourcePath is provided.",
+                    nameof(sourceLine));
+            }
+
+            Phase = phase;
+            DisplayName = displayName;
+            Uses = uses;
+            Run = run;
+            SourcePath = sourcePath;
+            SourceLine = sourceLine;
+            Id = id;
+            If = @if;
+            ContinueOnError = continueOnError;
+            TimeoutMinutes = timeoutMinutes;
+            EnvYaml = envYaml;
+            WithYaml = withYaml;
+            Shell = shell;
+            WorkingDirectory = workingDirectory;
+        }
+
+        public JobExecutionPhase Phase { get; }
+        public string DisplayName { get; }
+        public string Uses { get; }
+        public string Run { get; }
+        public string SourcePath { get; }
+        public int SourceLine { get; }
+        public string Id { get; }
+        public string If { get; }
+        public string ContinueOnError { get; }
+        public string TimeoutMinutes { get; }
+        // Pre-serialized YAML fragment, already indented for embedding
+        // under the entry's `env:` key (6-space child indent).
+        public string EnvYaml { get; }
+        public string WithYaml { get; }
+        public string Shell { get; }
+        public string WorkingDirectory { get; }
+    }
+
+    /// <summary>
+    /// Output of <see cref="JobExecutionViewRenderer.Render"/>: the YAML
+    /// document plus a parallel array of 1-based line numbers, one per
+    /// input entry, where each entry's <c>- step:</c> key appears.
+    /// Synthetic Setup/Cleanup boundaries are not tracked here.
+    /// </summary>
+    internal readonly struct RenderResult
+    {
+        public RenderResult(string yaml, IReadOnlyList<int> entryStartLines)
+        {
+            Yaml = yaml;
+            EntryStartLines = entryStartLines;
+        }
+
+        public string Yaml { get; }
+        public IReadOnlyList<int> EntryStartLines { get; }
+    }
+
+    /// <summary>
+    /// Renders a job's execution-view YAML. Pure function; no I/O,
+    /// no logging, no static state. Output format and Setup/Cleanup
+    /// boundaries are fixed; callers cannot influence them.
+    ///
+    /// Output is structured as phase-keyed top-level sections:
+    /// <c>setup:</c>, <c>pre:</c>, <c>main:</c>, <c>post:</c>, <c>cleanup:</c>.
+    /// <c>setup:</c> and <c>cleanup:</c> always render; <c>pre:</c>,
+    /// <c>main:</c>, <c>post:</c> only render when they contain at least
+    /// one entry.
+    /// </summary>
+    internal static class JobExecutionViewRenderer
+    {
+        public static RenderResult Render(string jobId, IReadOnlyList<JobExecutionViewEntry> entries)
+        {
+            if (string.IsNullOrWhiteSpace(jobId))
+            {
+                throw new ArgumentException("jobId must not be null or whitespace.", nameof(jobId));
+            }
+            ArgUtil.NotNull(entries, nameof(entries));
+
+            // Pre-validate non-null entries before any output, so partial
+            // state is never observed by callers.
+            for (int i = 0; i < entries.Count; i++)
+            {
+                if (entries[i] == null)
+                {
+                    throw new ArgumentException($"entries[{i}] is null.", nameof(entries));
+                }
+            }
+
+            var sb = new StringBuilder();
+            var startLines = new int[entries.Count];
+            int newlinesEmitted = 0;
+
+            // Header (3 lines).
+            sb.Append("# Job: ").Append(YamlScalarFormatter.Format(jobId)).Append('\n');
+            sb.Append("# Runner execution plan — read-only.\n");
+            sb.Append('\n');
+            newlinesEmitted += 3;
+
+            // setup: section — always present.
+            sb.Append("setup:\n");
+            sb.Append("  - step: Setup job\n");
+            newlinesEmitted += 2;
+
+            // Render phase sections in fixed order. Each emits a leading
+            // blank line separator before its header.
+            EmitPhaseSection(sb, "pre", JobExecutionPhase.Pre, entries, startLines, ref newlinesEmitted);
+            EmitPhaseSection(sb, "main", JobExecutionPhase.Main, entries, startLines, ref newlinesEmitted);
+            EmitPhaseSection(sb, "post", JobExecutionPhase.Post, entries, startLines, ref newlinesEmitted);
+
+            // cleanup: section — always present, preceded by a blank line.
+            sb.Append('\n');
+            sb.Append("cleanup:\n");
+            sb.Append("  - step: Complete job\n");
+
+            return new RenderResult(sb.ToString(), Array.AsReadOnly(startLines));
+        }
+
+        private static void EmitPhaseSection(
+            StringBuilder sb,
+            string sectionName,
+            JobExecutionPhase phase,
+            IReadOnlyList<JobExecutionViewEntry> entries,
+            int[] startLines,
+            ref int newlinesEmitted)
+        {
+            // Skip the section entirely if no entries belong to this phase.
+            bool any = false;
+            for (int i = 0; i < entries.Count; i++)
+            {
+                if (entries[i].Phase == phase) { any = true; break; }
+            }
+            if (!any)
+            {
+                return;
+            }
+
+            // Blank line separator + section header.
+            sb.Append('\n');
+            sb.Append(sectionName).Append(":\n");
+            newlinesEmitted += 2;
+
+            for (int i = 0; i < entries.Count; i++)
+            {
+                var entry = entries[i];
+                if (entry.Phase != phase)
+                {
+                    continue;
+                }
+
+                // 1-based line of the `- step:` key for this entry.
+                startLines[i] = newlinesEmitted + 1;
+
+                sb.Append("  - step: ").Append(YamlScalarFormatter.Format(entry.DisplayName));
+                sb.Append('\n');
+                newlinesEmitted++;
+
+                switch (phase)
+                {
+                    case JobExecutionPhase.Pre:
+                    case JobExecutionPhase.Post:
+                        if (!string.IsNullOrEmpty(entry.Uses))
+                        {
+                            sb.Append("    action: ").Append(YamlScalarFormatter.Format(entry.Uses)).Append('\n');
+                            newlinesEmitted++;
+                        }
+                        // No source: annotation for pre/post.
+                        break;
+
+                    case JobExecutionPhase.Main:
+                        if (!string.IsNullOrEmpty(entry.Id))
+                        {
+                            sb.Append("    id: ").Append(YamlScalarFormatter.Format(entry.Id)).Append('\n');
+                            newlinesEmitted++;
+                        }
+                        if (!string.IsNullOrEmpty(entry.Uses))
+                        {
+                            sb.Append("    uses: ").Append(YamlScalarFormatter.Format(entry.Uses)).Append('\n');
+                            newlinesEmitted++;
+                        }
+                        if (!string.IsNullOrEmpty(entry.Run))
+                        {
+                            if (entry.Run.IndexOf('\n') < 0)
+                            {
+                                sb.Append("    run: ").Append(YamlScalarFormatter.Format(entry.Run)).Append('\n');
+                                newlinesEmitted++;
+                            }
+                            else
+                            {
+                                sb.Append("    run: |\n");
+                                newlinesEmitted++;
+                                newlinesEmitted += AppendIndentedBlock(sb, entry.Run, "      ");
+                            }
+                        }
+                        if (!string.IsNullOrEmpty(entry.If))
+                        {
+                            sb.Append("    if: ").Append(YamlScalarFormatter.Format(entry.If)).Append('\n');
+                            newlinesEmitted++;
+                        }
+                        if (!string.IsNullOrEmpty(entry.ContinueOnError))
+                        {
+                            sb.Append("    continue-on-error: ").Append(entry.ContinueOnError).Append('\n');
+                            newlinesEmitted++;
+                        }
+                        if (!string.IsNullOrEmpty(entry.TimeoutMinutes))
+                        {
+                            sb.Append("    timeout-minutes: ").Append(entry.TimeoutMinutes).Append('\n');
+                            newlinesEmitted++;
+                        }
+                        if (!string.IsNullOrEmpty(entry.EnvYaml))
+                        {
+                            sb.Append("    env:\n");
+                            newlinesEmitted++;
+                            sb.Append(entry.EnvYaml).Append('\n');
+                            newlinesEmitted += CountChar(entry.EnvYaml, '\n') + 1;
+                        }
+                        if (!string.IsNullOrEmpty(entry.WithYaml))
+                        {
+                            sb.Append("    with:\n");
+                            newlinesEmitted++;
+                            sb.Append(entry.WithYaml).Append('\n');
+                            newlinesEmitted += CountChar(entry.WithYaml, '\n') + 1;
+                        }
+                        if (!string.IsNullOrEmpty(entry.Shell))
+                        {
+                            sb.Append("    shell: ").Append(YamlScalarFormatter.Format(entry.Shell)).Append('\n');
+                            newlinesEmitted++;
+                        }
+                        if (!string.IsNullOrEmpty(entry.WorkingDirectory))
+                        {
+                            sb.Append("    working-directory: ").Append(YamlScalarFormatter.Format(entry.WorkingDirectory)).Append('\n');
+                            newlinesEmitted++;
+                        }
+                        if (entry.SourcePath != null)
+                        {
+                            sb.Append("    source: ")
+                              .Append(entry.SourcePath)
+                              .Append(':')
+                              .Append(entry.SourceLine.ToString(CultureInfo.InvariantCulture))
+                              .Append('\n');
+                            newlinesEmitted++;
+                        }
+                        break;
+                }
+            }
+        }
+
+        private static int AppendIndentedBlock(StringBuilder sb, string text, string indent)
+        {
+            int newlines = 0;
+            int i = 0;
+            while (i < text.Length)
+            {
+                int end = text.IndexOf('\n', i);
+                int lineEnd = end < 0 ? text.Length : end;
+                int trimEnd = lineEnd;
+                if (trimEnd > i && text[trimEnd - 1] == '\r')
+                {
+                    trimEnd--;
+                }
+                if (trimEnd > i)
+                {
+                    sb.Append(indent);
+                    sb.Append(text, i, trimEnd - i);
+                }
+                sb.Append('\n');
+                newlines++;
+                if (end < 0)
+                {
+                    break;
+                }
+                i = end + 1;
+            }
+            return newlines;
+        }
+
+        private static int CountChar(string s, char c)
+        {
+            int n = 0;
+            for (int i = 0; i < s.Length; i++)
+            {
+                if (s[i] == c) n++;
+            }
+            return n;
+        }
+    }
+}

src/Runner.Worker/Dap/YamlScalarFormatter.cs

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

src/Runner.Worker/ExecutionContext.cs

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

src/Sdk/DTPipelines/Pipelines/AgentJobRequestMessage.cs

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

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

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

src/Test/L0/Worker/DapDebuggerL0.cs

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

src/Test/L0/Worker/DapReplExecutorL0.cs

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

src/Test/L0/Worker/JobExecutionViewRendererL0.cs

@@ -0,0 +1,598 @@
+using System;
+using System.Collections.Generic;
+using GitHub.Runner.Worker.Dap;
+using Xunit;
+
+namespace GitHub.Runner.Common.Tests.Worker
+{
+    public sealed class JobExecutionViewRendererL0
+    {
+        // Verbatim expected YAML for the design doc's "Worked example".
+        // The render output is structured as phase-keyed top-level sections;
+        // there is no per-entry `phase:` field. The setup: and cleanup:
+        // sections always render; pre:/main:/post: render only when
+        // they contain at least one entry. The Main entries surface
+        // user-authored step parameters pre-evaluation (no expression
+        // substitution); Pre/Post entries stay minimal.
+        private const string ExpectedWorkedExampleYaml =
+            "# Job: build\n" +
+            "# Runner execution plan — read-only.\n" +
+            "\n" +
+            "setup:\n" +
+            "  - step: Setup job\n" +
+            "\n" +
+            "pre:\n" +
+            "  - step: Pre actions/checkout@v4\n" +
+            "    action: actions/checkout@v4\n" +
+            "  - step: Pre actions/cache@v5\n" +
+            "    action: actions/cache@v5\n" +
+            "\n" +
+            "main:\n" +
+            "  - step: actions/checkout@v4\n" +
+            "    uses: actions/checkout@v4\n" +
+            "    source: .github/workflows/ci.yml:10\n" +
+            "  - step: Cache Primes\n" +
+            "    id: cache-primes\n" +
+            "    uses: actions/cache@v5\n" +
+            "    with:\n" +
+            "      path: prime-numbers\n" +
+            "      key: ${{ runner.os }}-primes\n" +
+            "    source: .github/workflows/ci.yml:12\n" +
+            "  - step: Run tests\n" +
+            "    id: test\n" +
+            "    run: |\n" +
+            "      echo starting\n" +
+            "      npm test\n" +
+            "    if: ${{ github.event_name == 'push' }}\n" +
+            "    env:\n" +
+            "      NODE_ENV: production\n" +
+            "    shell: bash\n" +
+            "    working-directory: ./api\n" +
+            "    source: .github/workflows/ci.yml:18\n" +
+            "  - step: npm ci\n" +
+            "    run: npm ci\n" +
+            "    source: .github/workflows/ci.yml:28\n" +
+            "\n" +
+            "post:\n" +
+            "  - step: Post actions/cache@v5\n" +
+            "    action: actions/cache@v5\n" +
+            "  - step: Post actions/checkout@v4\n" +
+            "    action: actions/checkout@v4\n" +
+            "\n" +
+            "cleanup:\n" +
+            "  - step: Complete job\n";
+
+        private static List<JobExecutionViewEntry> WorkedExampleEntries()
+        {
+            return new List<JobExecutionViewEntry>
+            {
+                new JobExecutionViewEntry(JobExecutionPhase.Pre, "Pre actions/checkout@v4", uses: "actions/checkout@v4"),
+                new JobExecutionViewEntry(JobExecutionPhase.Pre, "Pre actions/cache@v5", uses: "actions/cache@v5"),
+                new JobExecutionViewEntry(JobExecutionPhase.Main, "actions/checkout@v4", uses: "actions/checkout@v4", sourcePath: ".github/workflows/ci.yml", sourceLine: 10),
+                new JobExecutionViewEntry(
+                    JobExecutionPhase.Main,
+                    "Cache Primes",
+                    uses: "actions/cache@v5",
+                    id: "cache-primes",
+                    withYaml: "      path: prime-numbers\n      key: ${{ runner.os }}-primes",
+                    sourcePath: ".github/workflows/ci.yml",
+                    sourceLine: 12),
+                new JobExecutionViewEntry(
+                    JobExecutionPhase.Main,
+                    "Run tests",
+                    run: "echo starting\nnpm test",
+                    id: "test",
+                    @if: "${{ github.event_name == 'push' }}",
+                    envYaml: "      NODE_ENV: production",
+                    shell: "bash",
+                    workingDirectory: "./api",
+                    sourcePath: ".github/workflows/ci.yml",
+                    sourceLine: 18),
+                new JobExecutionViewEntry(JobExecutionPhase.Main, "npm ci", run: "npm ci", sourcePath: ".github/workflows/ci.yml", sourceLine: 28),
+                new JobExecutionViewEntry(JobExecutionPhase.Post, "Post actions/cache@v5", uses: "actions/cache@v5"),
+                new JobExecutionViewEntry(JobExecutionPhase.Post, "Post actions/checkout@v4", uses: "actions/checkout@v4"),
+            };
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_MatchesDesignDocWorkedExample()
+        {
+            var entries = WorkedExampleEntries();
+
+            var result = JobExecutionViewRenderer.Render("build", entries);
+
+            Assert.Equal(ExpectedWorkedExampleYaml, result.Yaml);
+            Assert.Equal(8, result.EntryStartLines.Count);
+            var lines = result.Yaml.Split('\n');
+            for (int i = 0; i < entries.Count; i++)
+            {
+                Assert.StartsWith("  - step: ", lines[result.EntryStartLines[i] - 1]);
+                Assert.Contains(entries[i].DisplayName, lines[result.EntryStartLines[i] - 1]);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_AlwaysEmitsSetupAndCleanup()
+        {
+            var result = JobExecutionViewRenderer.Render("job-1", new List<JobExecutionViewEntry>());
+
+            const string expected =
+                "# Job: job-1\n" +
+                "# Runner execution plan — read-only.\n" +
+                "\n" +
+                "setup:\n" +
+                "  - step: Setup job\n" +
+                "\n" +
+                "cleanup:\n" +
+                "  - step: Complete job\n";
+            Assert.Equal(expected, result.Yaml);
+            Assert.Empty(result.EntryStartLines);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_OmitsEmptyOptionalSections()
+        {
+            // Only a Main entry — pre:/post: must not appear.
+            var result = JobExecutionViewRenderer.Render("j", new[]
+            {
+                new JobExecutionViewEntry(JobExecutionPhase.Main, "echo", run: "echo hello"),
+            });
+
+            Assert.Contains("setup:\n", result.Yaml);
+            Assert.Contains("main:\n", result.Yaml);
+            Assert.Contains("cleanup:\n", result.Yaml);
+            Assert.DoesNotContain("\npre:\n", result.Yaml);
+            Assert.DoesNotContain("\npost:\n", result.Yaml);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_EmitsPhaseSectionsInFixedOrder()
+        {
+            // Input order [Post, Pre, Main] should still render as setup → pre → main → post → cleanup.
+            var entries = new[]
+            {
+                new JobExecutionViewEntry(JobExecutionPhase.Post, "post-a", uses: "a/b@v1"),
+                new JobExecutionViewEntry(JobExecutionPhase.Pre, "pre-a", uses: "a/b@v1"),
+                new JobExecutionViewEntry(JobExecutionPhase.Main, "main-a", uses: "a/b@v1"),
+            };
+
+            var result = JobExecutionViewRenderer.Render("j", entries);
+            string yaml = result.Yaml;
+
+            int setupIdx = yaml.IndexOf("setup:\n", StringComparison.Ordinal);
+            int preIdx = yaml.IndexOf("\npre:\n", StringComparison.Ordinal);
+            int mainIdx = yaml.IndexOf("\nmain:\n", StringComparison.Ordinal);
+            int postIdx = yaml.IndexOf("\npost:\n", StringComparison.Ordinal);
+            int cleanupIdx = yaml.IndexOf("\ncleanup:\n", StringComparison.Ordinal);
+            Assert.True(setupIdx >= 0 && preIdx > setupIdx && mainIdx > preIdx && postIdx > mainIdx && cleanupIdx > postIdx,
+                $"section ordering wrong: setup={setupIdx} pre={preIdx} main={mainIdx} post={postIdx} cleanup={cleanupIdx}");
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_StartLinesAlignWithInputOrder()
+        {
+            // Input order is [Pre, Main, Post]; output order is also pre/main/post,
+            // but startLines must be indexed by INPUT position, not by section.
+            var entries = new[]
+            {
+                new JobExecutionViewEntry(JobExecutionPhase.Pre, "pre-x", uses: "x/y@v1"),     // index 0
+                new JobExecutionViewEntry(JobExecutionPhase.Main, "main-x", uses: "x/y@v1"),    // index 1
+                new JobExecutionViewEntry(JobExecutionPhase.Post, "post-x", uses: "x/y@v1"),    // index 2
+            };
+
+            var result = JobExecutionViewRenderer.Render("j", entries);
+            var lines = result.Yaml.Split('\n');
+
+            Assert.StartsWith("  - step: pre-x", lines[result.EntryStartLines[0] - 1]);
+            Assert.StartsWith("  - step: main-x", lines[result.EntryStartLines[1] - 1]);
+            Assert.StartsWith("  - step: post-x", lines[result.EntryStartLines[2] - 1]);
+            // And input-order ordering of start lines is strictly increasing
+            // when phases are in declaration order matching the section order.
+            Assert.True(result.EntryStartLines[0] < result.EntryStartLines[1]);
+            Assert.True(result.EntryStartLines[1] < result.EntryStartLines[2]);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_StartLinesFollowInputOrderEvenWhenPhasesAreInterleaved()
+        {
+            // Input order is [Main A, Pre B, Main C]: pre section will render
+            // first (Pre B) and main second (Main A then Main C). startLines
+            // must still be indexed by input order.
+            var entries = new[]
+            {
+                new JobExecutionViewEntry(JobExecutionPhase.Main, "main-a", uses: "a@v1"),  // index 0 — renders in main section
+                new JobExecutionViewEntry(JobExecutionPhase.Pre, "pre-b", uses: "b@v1"),    // index 1 — renders in pre section
+                new JobExecutionViewEntry(JobExecutionPhase.Main, "main-c", uses: "c@v1"),  // index 2 — renders in main section
+            };
+
+            var result = JobExecutionViewRenderer.Render("j", entries);
+            var lines = result.Yaml.Split('\n');
+
+            Assert.StartsWith("  - step: main-a", lines[result.EntryStartLines[0] - 1]);
+            Assert.StartsWith("  - step: pre-b", lines[result.EntryStartLines[1] - 1]);
+            Assert.StartsWith("  - step: main-c", lines[result.EntryStartLines[2] - 1]);
+            // The pre section comes before main: input-index-1 entry's line is
+            // before input-index-0 entry's line.
+            Assert.True(result.EntryStartLines[1] < result.EntryStartLines[0]);
+            Assert.True(result.EntryStartLines[0] < result.EntryStartLines[2]);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_EntryStartLinesPointAtStepKeys()
+        {
+            var entries = WorkedExampleEntries();
+            var result = JobExecutionViewRenderer.Render("build", entries);
+            var lines = result.Yaml.Split('\n');
+
+            for (int i = 0; i < result.EntryStartLines.Count; i++)
+            {
+                int oneBased = result.EntryStartLines[i];
+                Assert.True(oneBased >= 1 && oneBased <= lines.Length, $"start line {oneBased} out of range");
+                Assert.StartsWith("  - step: ", lines[oneBased - 1]);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_EntryStartLinesExcludeSetupAndCleanup()
+        {
+            var entries = WorkedExampleEntries();
+            var result = JobExecutionViewRenderer.Render("build", entries);
+            var lines = result.Yaml.Split('\n');
+
+            int setupLine = -1, cleanupLine = -1;
+            for (int i = 0; i < lines.Length; i++)
+            {
+                if (lines[i] == "  - step: Setup job") setupLine = i + 1;
+                if (lines[i] == "  - step: Complete job") cleanupLine = i + 1;
+            }
+            Assert.True(setupLine > 0 && cleanupLine > 0, "Setup/Cleanup lines must exist");
+            Assert.DoesNotContain(setupLine, result.EntryStartLines);
+            Assert.DoesNotContain(cleanupLine, result.EntryStartLines);
+        }
+
+        [Theory]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        [InlineData("hello")]
+        [InlineData("with: colon")]
+        [InlineData("with#hash")]
+        [InlineData(" leading")]
+        [InlineData("trailing ")]
+        [InlineData("a\"b")]
+        [InlineData("a\\b")]
+        [InlineData("@at")]
+        [InlineData("*star")]
+        public void Render_QuotesSpecialChars(string displayName)
+        {
+            // Round-trip the rendered YAML through YamlDotNet's deserializer
+            // and assert the parsed step's display name matches the input.
+            // This decouples the test from any specific quoting style.
+            var entry = new JobExecutionViewEntry(JobExecutionPhase.Main, displayName);
+            var result = JobExecutionViewRenderer.Render("j", new[] { entry });
+
+            var deserializer = new YamlDotNet.Serialization.DeserializerBuilder().Build();
+            var doc = deserializer.Deserialize<Dictionary<string, List<Dictionary<string, object>>>>(result.Yaml);
+            Assert.NotNull(doc);
+            Assert.True(doc.ContainsKey("main"), "rendered YAML missing top-level 'main' key");
+            var mainSteps = doc["main"];
+            Assert.Single(mainSteps);
+            Assert.Equal(displayName, mainSteps[0]["step"] as string);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_EmitsSourceAnnotationForMainStep()
+        {
+            var entry = new JobExecutionViewEntry(
+                JobExecutionPhase.Main,
+                "npm ci",
+                run: "npm ci",
+                sourcePath: ".github/workflows/ci.yml",
+                sourceLine: 42);
+
+            var result = JobExecutionViewRenderer.Render("j", new[] { entry });
+
+            Assert.Contains("    source: .github/workflows/ci.yml:42\n", result.Yaml);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_OmitsSourceAnnotationForPreAndPost()
+        {
+            var pre = new JobExecutionViewEntry(
+                JobExecutionPhase.Pre,
+                "Pre actions/checkout@v4",
+                uses: "actions/checkout@v4",
+                sourcePath: ".github/workflows/ci.yml",
+                sourceLine: 9);
+            var post = new JobExecutionViewEntry(
+                JobExecutionPhase.Post,
+                "Post actions/checkout@v4",
+                uses: "actions/checkout@v4",
+                sourcePath: ".github/workflows/ci.yml",
+                sourceLine: 9);
+
+            var result = JobExecutionViewRenderer.Render("j", new[] { pre, post });
+
+            Assert.DoesNotContain("source:", result.Yaml);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_EmitsMultilineRunAsBlockScalar()
+        {
+            var entry = new JobExecutionViewEntry(
+                JobExecutionPhase.Main,
+                "multi",
+                run: "echo a\necho b\necho c");
+
+            var result = JobExecutionViewRenderer.Render("j", new[] { entry });
+
+            Assert.Contains("    run: |\n", result.Yaml);
+            Assert.Contains("      echo a\n", result.Yaml);
+            Assert.Contains("      echo b\n", result.Yaml);
+            Assert.Contains("      echo c\n", result.Yaml);
+            Assert.DoesNotContain("truncated", result.Yaml);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_EmitsAllUserAuthoredParamsForActionStep()
+        {
+            var entry = new JobExecutionViewEntry(
+                JobExecutionPhase.Main,
+                "Run action",
+                uses: "actions/cache@v5",
+                id: "cache-primes",
+                @if: "${{ github.event_name == 'push' }}",
+                continueOnError: "true",
+                timeoutMinutes: "10",
+                envYaml: "      NODE_ENV: production",
+                withYaml: "      path: prime-numbers\n      key: ${{ runner.os }}-primes",
+                sourcePath: "ci.yml",
+                sourceLine: 5);
+
+            var result = JobExecutionViewRenderer.Render("j", new[] { entry });
+
+            Assert.Contains("    id: cache-primes\n", result.Yaml);
+            Assert.Contains("    uses: actions/cache@v5\n", result.Yaml);
+            Assert.Contains("    continue-on-error: true\n", result.Yaml);
+            Assert.Contains("    timeout-minutes: 10\n", result.Yaml);
+            Assert.Contains("    env:\n      NODE_ENV: production\n", result.Yaml);
+            Assert.Contains("    with:\n      path: prime-numbers\n      key: ${{ runner.os }}-primes\n", result.Yaml);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_EmitsRunStepWithShellAndWorkingDirectory()
+        {
+            var entry = new JobExecutionViewEntry(
+                JobExecutionPhase.Main,
+                "Run tests",
+                run: "echo starting\nnpm test",
+                id: "test",
+                shell: "bash",
+                workingDirectory: "./api");
+
+            var result = JobExecutionViewRenderer.Render("j", new[] { entry });
+
+            Assert.Contains("    run: |\n      echo starting\n      npm test\n", result.Yaml);
+            Assert.Contains("    shell: bash\n", result.Yaml);
+            Assert.Contains("    working-directory: ./api\n", result.Yaml);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_PreservesExpressionsInRenderedYaml()
+        {
+            var entry = new JobExecutionViewEntry(
+                JobExecutionPhase.Main,
+                "Cache",
+                uses: "actions/cache@v5",
+                withYaml: "      key: ${{ runner.os }}-primes");
+
+            var result = JobExecutionViewRenderer.Render("j", new[] { entry });
+
+            // Expressions render exactly as authored — no evaluation.
+            Assert.Contains("${{ runner.os }}-primes", result.Yaml);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_PrePostStepsRemainMinimal()
+        {
+            // Even if a pre/post entry carries user-param fields (it shouldn't
+            // in production, but the renderer must defensively drop them),
+            // only step: + action: render for these phases.
+            var pre = new JobExecutionViewEntry(
+                JobExecutionPhase.Pre,
+                "Pre actions/cache@v5",
+                uses: "actions/cache@v5",
+                id: "should-not-appear",
+                envYaml: "      X: y",
+                withYaml: "      key: nope");
+            var post = new JobExecutionViewEntry(
+                JobExecutionPhase.Post,
+                "Post actions/cache@v5",
+                uses: "actions/cache@v5",
+                id: "should-not-appear",
+                envYaml: "      X: y");
+
+            var result = JobExecutionViewRenderer.Render("j", new[] { pre, post });
+
+            Assert.DoesNotContain("id:", result.Yaml);
+            Assert.DoesNotContain("env:", result.Yaml);
+            Assert.DoesNotContain("with:", result.Yaml);
+            Assert.DoesNotContain("should-not-appear", result.Yaml);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_FieldOrderIsStable()
+        {
+            var entry = new JobExecutionViewEntry(
+                JobExecutionPhase.Main,
+                "Everything",
+                uses: "actions/cache@v5",
+                id: "x",
+                @if: "always()",
+                continueOnError: "false",
+                timeoutMinutes: "5",
+                envYaml: "      A: 1",
+                withYaml: "      key: k",
+                sourcePath: "ci.yml",
+                sourceLine: 1);
+
+            var result = JobExecutionViewRenderer.Render("j", new[] { entry });
+            var y = result.Yaml;
+            int iStep = y.IndexOf("    - step: ", StringComparison.Ordinal) >= 0
+                ? y.IndexOf("- step:", StringComparison.Ordinal) : y.IndexOf("- step:", StringComparison.Ordinal);
+            int iId = y.IndexOf("    id:", StringComparison.Ordinal);
+            int iUses = y.IndexOf("    uses:", StringComparison.Ordinal);
+            int iIf = y.IndexOf("    if:", StringComparison.Ordinal);
+            int iCoe = y.IndexOf("    continue-on-error:", StringComparison.Ordinal);
+            int iTm = y.IndexOf("    timeout-minutes:", StringComparison.Ordinal);
+            int iEnv = y.IndexOf("    env:", StringComparison.Ordinal);
+            int iWith = y.IndexOf("    with:", StringComparison.Ordinal);
+            int iSrc = y.IndexOf("    source:", StringComparison.Ordinal);
+            Assert.True(iId < iUses && iUses < iIf && iIf < iCoe && iCoe < iTm && iTm < iEnv && iEnv < iWith && iWith < iSrc,
+                $"order wrong: id={iId} uses={iUses} if={iIf} coe={iCoe} tm={iTm} env={iEnv} with={iWith} src={iSrc}");
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_OmitsEmptyOptionalFields()
+        {
+            var entry = new JobExecutionViewEntry(
+                JobExecutionPhase.Main,
+                "bare",
+                uses: "a/b@v1");
+
+            var result = JobExecutionViewRenderer.Render("j", new[] { entry });
+            Assert.DoesNotContain("    id:", result.Yaml);
+            Assert.DoesNotContain("    if:", result.Yaml);
+            Assert.DoesNotContain("    continue-on-error:", result.Yaml);
+            Assert.DoesNotContain("    timeout-minutes:", result.Yaml);
+            Assert.DoesNotContain("    env:", result.Yaml);
+            Assert.DoesNotContain("    with:", result.Yaml);
+            Assert.DoesNotContain("    shell:", result.Yaml);
+            Assert.DoesNotContain("    working-directory:", result.Yaml);
+            Assert.DoesNotContain("    source:", result.Yaml);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_HandlesEmptyEntries()
+        {
+            var result = JobExecutionViewRenderer.Render("j", new List<JobExecutionViewEntry>());
+
+            Assert.Empty(result.EntryStartLines);
+            Assert.Contains("  - step: Setup job\n", result.Yaml);
+            Assert.Contains("  - step: Complete job\n", result.Yaml);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_NoPerEntryPhaseField()
+        {
+            // The phase: <value> per-entry field is gone — the section
+            // header is the phase indicator. Guard against accidental
+            // regressions.
+            var result = JobExecutionViewRenderer.Render("build", WorkedExampleEntries());
+            Assert.DoesNotContain("phase:", result.Yaml);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_ThrowsOnNullJobId()
+        {
+            Assert.Throws<ArgumentException>(
+                () => JobExecutionViewRenderer.Render(null, new List<JobExecutionViewEntry>()));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_ThrowsOnWhitespaceJobId()
+        {
+            Assert.Throws<ArgumentException>(
+                () => JobExecutionViewRenderer.Render("   ", new List<JobExecutionViewEntry>()));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_ThrowsOnNullEntries()
+        {
+            Assert.Throws<ArgumentNullException>(
+                () => JobExecutionViewRenderer.Render("j", null));
+        }
+
+        [Theory]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        [InlineData(null, 1)]
+        [InlineData("", 1)]
+        [InlineData("   ", 1)]
+        public void Entry_Constructor_RejectsBadDisplayName(string displayName, int sourceLine)
+        {
+            Assert.Throws<ArgumentException>(
+                () => new JobExecutionViewEntry(JobExecutionPhase.Main, displayName, sourceLine: sourceLine));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Entry_Constructor_RejectsZeroLineWhenSourcePathSet()
+        {
+            Assert.Throws<ArgumentException>(
+                () => new JobExecutionViewEntry(
+                    JobExecutionPhase.Main,
+                    "ok",
+                    sourcePath: "ci.yml",
+                    sourceLine: 0));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Render_AlwaysUsesLfLineBreaks()
+        {
+            // Regression: YamlDotNet's Emitter calls WriteLine, which on
+            // Windows produces CRLF (the host's Environment.NewLine).
+            // The renderer's hand-emitted skeleton always uses '\n'; this
+            // test asserts the scalar formatter doesn't sneak CRLF in.
+            var entry = new JobExecutionViewEntry(JobExecutionPhase.Main, "with: colon", id: "step-1", uses: "actions/checkout@v4");
+            var result = JobExecutionViewRenderer.Render("job-1", new[] { entry });
+            Assert.DoesNotContain("\r", result.Yaml);
+        }
+    }
+}

src/Test/L0/Worker/YamlScalarFormatterL0.cs

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

src/dev.sh

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

src/global.json

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