Phase 5 done

.opencode/plans/dap-debugging.md

@@ -0,0 +1,536 @@
+# DAP-Based Debugging for GitHub Actions Runner
+
+**Status:** Draft  
+**Author:** GitHub Actions Team  
+**Date:** January 2026
+
+## Progress Checklist
+
+- [x] **Phase 1:** DAP Protocol Infrastructure (DapMessages.cs, DapServer.cs, basic DapDebugSession.cs)
+- [x] **Phase 2:** Debug Session Logic (DapVariableProvider.cs, variable inspection, step history tracking)
+- [x] **Phase 3:** StepsRunner Integration (pause hooks before/after step execution)
+- [x] **Phase 4:** Expression Evaluation & Shell (REPL)
+- [x] **Phase 5:** Startup Integration (JobRunner.cs modifications)
+
+## Overview
+
+This document describes the implementation of Debug Adapter Protocol (DAP) support in the GitHub Actions runner, enabling rich debugging of workflow jobs from any DAP-compatible editor (nvim-dap, VS Code, etc.).
+
+## Goals
+
+- **Primary:** Create a working demo to demonstrate the feasibility of DAP-based workflow debugging
+- **Non-goal:** Production-ready, polished implementation (this is proof-of-concept)
+
+## User Experience
+
+1. User re-runs a failed job with "Enable debug logging" checked in GitHub UI
+2. Runner (running locally) detects debug mode and starts DAP server on port 4711
+3. Runner prints "Waiting for debugger on port 4711..." and pauses
+4. User opens editor (nvim with nvim-dap), connects to debugger
+5. Job execution begins, pausing before the first step
+6. User can:
+   - **Inspect variables:** View `github`, `env`, `inputs`, `steps`, `secrets` (redacted), `runner`, `job` contexts
+   - **Evaluate expressions:** `${{ github.event.pull_request.title }}`
+   - **Execute shell commands:** Run arbitrary commands in the job's environment (REPL)
+   - **Step through job:** `next` moves to next step, `continue` runs to end
+   - **Pause after steps:** Inspect step outputs before continuing
+
+## Activation
+
+DAP debugging activates automatically when the job is in debug mode:
+
+- User enables "Enable debug logging" when re-running a job in GitHub UI
+- Server sends `ACTIONS_STEP_DEBUG=true` in job variables
+- Runner sets `Global.WriteDebug = true` and `runner.debug = "1"`
+- DAP server starts on port 4711
+
+**No additional configuration required.**
+
+### Optional Configuration
+
+| Environment Variable | Default | Description |
+|---------------------|---------|-------------|
+| `ACTIONS_DAP_PORT` | `4711` | TCP port for DAP server (optional override) |
+
+## Architecture
+
+```
+┌─────────────────────┐                    ┌─────────────────────────────────────────┐
+│     nvim-dap        │                    │           Runner.Worker                 │
+│   (DAP Client)      │◄───TCP:4711───────►│  ┌─────────────────────────────────┐   │
+│                     │                    │  │     DapServer                    │   │
+└─────────────────────┘                    │  │  - TCP listener                  │   │
+                                           │  │  - DAP JSON protocol             │   │
+                                           │  └──────────────┬──────────────────┘   │
+                                           │                 │                       │
+                                           │  ┌──────────────▼──────────────────┐   │
+                                           │  │     DapDebugSession              │   │
+                                           │  │  - Debug state management        │   │
+                                           │  │  - Step coordination             │   │
+                                           │  │  - Variable exposure             │   │
+                                           │  │  - Expression evaluation         │   │
+                                           │  │  - Shell execution (REPL)        │   │
+                                           │  └──────────────┬──────────────────┘   │
+                                           │                 │                       │
+                                           │  ┌──────────────▼──────────────────┐   │
+                                           │  │     StepsRunner (modified)       │   │
+                                           │  │  - Pause before/after steps      │   │
+                                           │  │  - Notify debug session          │   │
+                                           │  └─────────────────────────────────┘   │
+                                           └─────────────────────────────────────────┘
+```
+
+## DAP Concept Mapping
+
+| DAP Concept | Actions Runner Equivalent |
+|-------------|---------------------------|
+| Thread | Single job execution |
+| Stack Frame | Current step + completed steps (step history) |
+| Scope | Context category: `github`, `env`, `inputs`, `steps`, `secrets`, `runner`, `job` |
+| Variable | Individual context values |
+| Breakpoint | Pause before specific step (future enhancement) |
+| Step Over (Next) | Execute current step, pause before next |
+| Continue | Run until job end |
+| Evaluate | Evaluate `${{ }}` expressions OR execute shell commands (REPL) |
+
+## File Structure
+
+```
+src/Runner.Worker/
+├── Dap/
+│   ├── DapServer.cs              # TCP listener, JSON protocol handling
+│   ├── DapDebugSession.cs        # Debug state, step coordination  
+│   ├── DapMessages.cs            # DAP protocol message types
+│   └── DapVariableProvider.cs    # Converts ExecutionContext to DAP variables
+```
+
+## Implementation Phases
+
+### Phase 1: DAP Protocol Infrastructure
+
+#### 1.1 Protocol Messages (`Dap/DapMessages.cs`)
+
+Base message types following DAP spec:
+
+```csharp
+public abstract class ProtocolMessage
+{
+    public int seq { get; set; }
+    public string type { get; set; }  // "request", "response", "event"
+}
+
+public class Request : ProtocolMessage
+{
+    public string command { get; set; }
+    public object arguments { get; set; }
+}
+
+public class Response : ProtocolMessage
+{
+    public int request_seq { get; set; }
+    public bool success { get; set; }
+    public string command { get; set; }
+    public string message { get; set; }
+    public object body { get; set; }
+}
+
+public class Event : ProtocolMessage
+{
+    public string @event { get; set; }
+    public object body { get; set; }
+}
+```
+
+Message framing: `Content-Length: N\r\n\r\n{json}`
+
+#### 1.2 DAP Server (`Dap/DapServer.cs`)
+
+```csharp
+[ServiceLocator(Default = typeof(DapServer))]
+public interface IDapServer : IRunnerService
+{
+    Task StartAsync(int port);
+    Task WaitForConnectionAsync();
+    Task StopAsync();
+    void SendEvent(Event evt);
+}
+
+public sealed class DapServer : RunnerService, IDapServer
+{
+    private TcpListener _listener;
+    private TcpClient _client;
+    private IDapDebugSession _session;
+    
+    // TCP listener on configurable port
+    // Single-client connection
+    // Async read/write loop
+    // Dispatch requests to DapDebugSession
+}
+```
+
+### Phase 2: Debug Session Logic
+
+#### 2.1 Debug Session (`Dap/DapDebugSession.cs`)
+
+```csharp
+public enum DapCommand { Continue, Next, Pause, Disconnect }
+public enum PauseReason { Entry, Step, Breakpoint, Pause }
+
+[ServiceLocator(Default = typeof(DapDebugSession))]
+public interface IDapDebugSession : IRunnerService
+{
+    bool IsActive { get; }
+    
+    // Called by DapServer
+    void Initialize(InitializeRequestArguments args);
+    void Attach(AttachRequestArguments args);
+    void ConfigurationDone();
+    Task<DapCommand> WaitForCommandAsync();
+    
+    // Called by StepsRunner
+    Task OnStepStartingAsync(IStep step, IExecutionContext jobContext);
+    void OnStepCompleted(IStep step);
+    
+    // DAP requests
+    ThreadsResponse GetThreads();
+    StackTraceResponse GetStackTrace(int threadId);
+    ScopesResponse GetScopes(int frameId);
+    VariablesResponse GetVariables(int variablesReference);
+    EvaluateResponse Evaluate(string expression, string context);
+}
+
+public sealed class DapDebugSession : RunnerService, IDapDebugSession
+{
+    private IExecutionContext _jobContext;
+    private IStep _currentStep;
+    private readonly List<IStep> _completedSteps = new();
+    private TaskCompletionSource<DapCommand> _commandTcs;
+    private bool _pauseAfterStep = false;
+    
+    // Object reference management for nested variables
+    private int _nextVariableReference = 1;
+    private readonly Dictionary<int, object> _variableReferences = new();
+}
+```
+
+Core state machine:
+1. **Waiting for client:** Server started, no client connected
+2. **Initializing:** Client connected, exchanging capabilities
+3. **Ready:** `configurationDone` received, waiting to start
+4. **Paused (before step):** Stopped before step execution, waiting for command
+5. **Running:** Executing a step
+6. **Paused (after step):** Stopped after step execution, waiting for command
+
+#### 2.2 Variable Provider (`Dap/DapVariableProvider.cs`)
+
+Maps `ExecutionContext.ExpressionValues` to DAP scopes and variables:
+
+| Scope | Source | Notes |
+|-------|--------|-------|
+| `github` | `ExpressionValues["github"]` | Full github context |
+| `env` | `ExpressionValues["env"]` | Environment variables |
+| `inputs` | `ExpressionValues["inputs"]` | Step inputs (when available) |
+| `steps` | `Global.StepsContext.GetScope()` | Completed step outputs |
+| `secrets` | `ExpressionValues["secrets"]` | Keys shown, values = `[REDACTED]` |
+| `runner` | `ExpressionValues["runner"]` | Runner context |
+| `job` | `ExpressionValues["job"]` | Job status |
+
+Nested objects (e.g., `github.event.pull_request`) become expandable variables with child references.
+
+### Phase 3: StepsRunner Integration
+
+#### 3.1 Modify `StepsRunner.cs`
+
+Add debug hooks at step boundaries:
+
+```csharp
+public async Task RunAsync(IExecutionContext jobContext)
+{
+    // Get debug session if available
+    var debugSession = HostContext.TryGetService<IDapDebugSession>();
+    bool isFirstStep = true;
+    
+    while (jobContext.JobSteps.Count > 0 || !checkPostJobActions)
+    {
+        // ... existing dequeue logic ...
+        
+        var step = jobContext.JobSteps.Dequeue();
+        
+        // Pause BEFORE step execution
+        if (debugSession?.IsActive == true)
+        {
+            var reason = isFirstStep ? PauseReason.Entry : PauseReason.Step;
+            await debugSession.OnStepStartingAsync(step, jobContext, reason);
+            isFirstStep = false;
+        }
+        
+        // ... existing step execution (condition eval, RunStepAsync, etc.) ...
+        
+        // Pause AFTER step execution (if requested)
+        if (debugSession?.IsActive == true)
+        {
+            debugSession.OnStepCompleted(step);
+            // Session may pause here to let user inspect outputs
+        }
+    }
+}
+```
+
+### Phase 4: Expression Evaluation & Shell (REPL)
+
+#### 4.1 Expression Evaluation
+
+Reuse existing `PipelineTemplateEvaluator`:
+
+```csharp
+private EvaluateResponseBody EvaluateExpression(string expression, IExecutionContext context)
+{
+    // Strip ${{ }} wrapper if present
+    var expr = expression.Trim();
+    if (expr.StartsWith("${{") && expr.EndsWith("}}"))
+    {
+        expr = expr.Substring(3, expr.Length - 5).Trim();
+    }
+    
+    var expressionToken = new BasicExpressionToken(fileId: null, line: null, column: null, expression: expr);
+    var templateEvaluator = context.ToPipelineTemplateEvaluator();
+    
+    var result = templateEvaluator.EvaluateStepDisplayName(
+        expressionToken, 
+        context.ExpressionValues, 
+        context.ExpressionFunctions
+    );
+    
+    // Mask secrets and determine type
+    result = HostContext.SecretMasker.MaskSecrets(result ?? "null");
+    
+    return new EvaluateResponseBody
+    {
+        Result = result,
+        Type = DetermineResultType(result),
+        VariablesReference = 0
+    };
+}
+```
+
+**Supported expression formats:**
+- Plain expression: `github.ref`, `steps.build.outputs.result`
+- Wrapped expression: `${{ github.event.pull_request.title }}`
+
+#### 4.2 Shell Execution (REPL)
+
+Shell execution is triggered when:
+1. The evaluate request has `context: "repl"`, OR
+2. The expression starts with `!` (e.g., `!ls -la`), OR  
+3. The expression starts with `$` followed by a shell command (e.g., `$env`)
+
+**Usage examples in debug console:**
+```
+!ls -la                          # List files in workspace
+!env | grep GITHUB               # Show GitHub environment variables
+!cat $GITHUB_EVENT_PATH          # View the event payload
+!echo ${{ github.ref }}          # Mix shell and expression (evaluated first)
+```
+
+**Implementation:**
+
+```csharp
+private async Task<EvaluateResponseBody> ExecuteShellCommandAsync(string command, IExecutionContext context)
+{
+    var processInvoker = HostContext.CreateService<IProcessInvoker>();
+    var output = new StringBuilder();
+    
+    processInvoker.OutputDataReceived += (sender, args) =>
+    {
+        output.AppendLine(args.Data);
+        // Stream to client in real-time via DAP output event
+        _server?.SendEvent(new Event
+        {
+            EventType = "output",
+            Body = new OutputEventBody { Category = "stdout", Output = args.Data + "\n" }
+        });
+    };
+    
+    processInvoker.ErrorDataReceived += (sender, args) =>
+    {
+        _server?.SendEvent(new Event
+        {
+            EventType = "output",
+            Body = new OutputEventBody { Category = "stderr", Output = args.Data + "\n" }
+        });
+    };
+    
+    // Build environment from job context (includes GITHUB_*, env context, prepend path)
+    var env = BuildShellEnvironment(context);
+    var workDir = GetWorkingDirectory(context);  // Uses github.workspace
+    var (shell, shellArgs) = GetDefaultShell();  // Platform-specific detection
+    
+    int exitCode = await processInvoker.ExecuteAsync(
+        workingDirectory: workDir,
+        fileName: shell,
+        arguments: string.Format(shellArgs, command),
+        environment: env,
+        requireExitCodeZero: false,
+        cancellationToken: CancellationToken.None
+    );
+    
+    return new EvaluateResponseBody
+    {
+        Result = HostContext.SecretMasker.MaskSecrets(output.ToString()),
+        Type = exitCode == 0 ? "string" : "error",
+        VariablesReference = 0
+    };
+}
+```
+
+**Shell detection by platform:**
+
+| Platform | Priority | Shell | Arguments |
+|----------|----------|-------|-----------|
+| Windows | 1 | `pwsh` | `-NoProfile -NonInteractive -Command "{0}"` |
+| Windows | 2 | `powershell` | `-NoProfile -NonInteractive -Command "{0}"` |
+| Windows | 3 | `cmd.exe` | `/C "{0}"` |
+| Unix | 1 | `bash` | `-c "{0}"` |
+| Unix | 2 | `sh` | `-c "{0}"` |
+
+**Environment built for shell commands:**
+- Current system environment variables
+- GitHub Actions context variables (from `IEnvironmentContextData.GetRuntimeEnvironmentVariables()`)
+- Prepend path from job context added to `PATH`
+
+### Phase 5: Startup Integration
+
+#### 5.1 Modify `JobRunner.cs`
+
+Add DAP server startup after debug mode is detected (around line 159):
+
+```csharp
+if (jobContext.Global.WriteDebug)
+{
+    jobContext.SetRunnerContext("debug", "1");
+    
+    // Start DAP server for interactive debugging
+    var dapServer = HostContext.GetService<IDapServer>();
+    var port = int.Parse(
+        Environment.GetEnvironmentVariable("ACTIONS_DAP_PORT") ?? "4711");
+    
+    await dapServer.StartAsync(port);
+    Trace.Info($"DAP server listening on port {port}");
+    jobContext.Output($"DAP debugger waiting for connection on port {port}...");
+    
+    // Block until debugger connects
+    await dapServer.WaitForConnectionAsync();
+    Trace.Info("DAP client connected, continuing job execution");
+}
+```
+
+## DAP Capabilities
+
+Capabilities to advertise in `InitializeResponse`:
+
+```json
+{
+    "supportsConfigurationDoneRequest": true,
+    "supportsEvaluateForHovers": true,
+    "supportsTerminateDebuggee": true,
+    "supportsStepBack": false,
+    "supportsSetVariable": false,
+    "supportsRestartFrame": false,
+    "supportsGotoTargetsRequest": false,
+    "supportsStepInTargetsRequest": false,
+    "supportsCompletionsRequest": false,
+    "supportsModulesRequest": false,
+    "supportsExceptionOptions": false,
+    "supportsValueFormattingOptions": false,
+    "supportsExceptionInfoRequest": false,
+    "supportsDelayedStackTraceLoading": false,
+    "supportsLoadedSourcesRequest": false,
+    "supportsProgressReporting": false,
+    "supportsRunInTerminalRequest": false
+}
+```
+
+## Client Configuration (nvim-dap)
+
+Example configuration for nvim-dap:
+
+```lua
+local dap = require('dap')
+
+dap.adapters.actions = {
+    type = 'server',
+    host = '127.0.0.1',
+    port = 4711,
+}
+
+dap.configurations.yaml = {
+    {
+        type = 'actions',
+        request = 'attach',
+        name = 'Attach to Actions Runner',
+    }
+}
+```
+
+## Demo Flow
+
+1. Trigger job re-run with "Enable debug logging" checked in GitHub UI
+2. Runner starts, detects debug mode (`Global.WriteDebug == true`)
+3. DAP server starts, console shows: `DAP debugger waiting for connection on port 4711...`
+4. In nvim: `:lua require('dap').continue()`
+5. Connection established, capabilities exchanged
+6. Job begins, pauses before first step
+7. nvim shows "stopped" state, variables panel shows contexts
+8. User explores variables, evaluates expressions, runs shell commands
+9. User presses `n` (next) to advance to next step
+10. After step completes, user can inspect outputs before continuing
+11. Repeat until job completes
+
+## Testing Strategy
+
+1. **Unit tests:** DAP protocol serialization, variable provider mapping
+2. **Integration tests:** Mock DAP client verifying request/response sequences
+3. **Manual testing:** Real job with nvim-dap attached
+
+## Future Enhancements (Out of Scope for Demo)
+
+- Composite action step-in (expand into sub-steps)
+- Breakpoints on specific step names
+- Watch expressions
+- Conditional breakpoints
+- Remote debugging (runner not on localhost)
+- VS Code extension
+
+## Estimated Effort
+
+| Phase | Effort |
+|-------|--------|
+| Phase 1: Protocol Infrastructure | 4-6 hours |
+| Phase 2: Debug Session Logic | 4-6 hours |
+| Phase 3: StepsRunner Integration | 2-3 hours |
+| Phase 4: Expression & Shell | 3-4 hours |
+| Phase 5: Startup & Polish | 2-3 hours |
+| **Total** | **~2-3 days** |
+
+## Key Files to Modify
+
+| File | Changes |
+|------|---------|
+| `src/Runner.Worker/JobRunner.cs` | Start DAP server when debug mode enabled |
+| `src/Runner.Worker/StepsRunner.cs` | Add pause hooks before/after step execution |
+| `src/Runner.Worker/Runner.Worker.csproj` | Add new Dap/ folder files |
+
+## Key Files to Create
+
+| File | Purpose |
+|------|---------|
+| `src/Runner.Worker/Dap/DapServer.cs` | TCP server, protocol framing |
+| `src/Runner.Worker/Dap/DapDebugSession.cs` | Debug state machine, command handling |
+| `src/Runner.Worker/Dap/DapMessages.cs` | Protocol message types |
+| `src/Runner.Worker/Dap/DapVariableProvider.cs` | Context → DAP variable conversion |
+
+## Reference Links
+
+- [DAP Overview](https://microsoft.github.io/debug-adapter-protocol/overview)
+- [DAP Specification](https://microsoft.github.io/debug-adapter-protocol/specification)
+- [Enable Debug Logging (GitHub Docs)](https://docs.github.com/en/actions/how-tos/monitor-workflows/enable-debug-logging)

releaseVersion

@@ -1 +1 @@
-2.331.0
+<Update to ./src/runnerversion when creating release>

src/Runner.Worker/Dap/DapServer.cs

@@ -0,0 +1,480 @@
+using System;
+using System.IO;
+using System.Net;
+using System.Net.Sockets;
+using System.Text;
+using System.Threading;
+using System.Threading.Tasks;
+using GitHub.Runner.Common;
+using Newtonsoft.Json;
+
+namespace GitHub.Runner.Worker.Dap
+{
+    /// <summary>
+    /// DAP Server interface for handling Debug Adapter Protocol connections.
+    /// </summary>
+    [ServiceLocator(Default = typeof(DapServer))]
+    public interface IDapServer : IRunnerService, IDisposable
+    {
+        /// <summary>
+        /// Starts the DAP TCP server on the specified port.
+        /// </summary>
+        /// <param name="port">The port to listen on (default: 4711)</param>
+        /// <param name="cancellationToken">Cancellation token</param>
+        Task StartAsync(int port, CancellationToken cancellationToken);
+
+        /// <summary>
+        /// Blocks until a debug client connects.
+        /// </summary>
+        /// <param name="cancellationToken">Cancellation token</param>
+        Task WaitForConnectionAsync(CancellationToken cancellationToken);
+
+        /// <summary>
+        /// Stops the DAP server and closes all connections.
+        /// </summary>
+        Task StopAsync();
+
+        /// <summary>
+        /// Sets the debug session that will handle DAP requests.
+        /// </summary>
+        /// <param name="session">The debug session</param>
+        void SetSession(IDapDebugSession session);
+
+        /// <summary>
+        /// Sends an event to the connected debug client.
+        /// </summary>
+        /// <param name="evt">The event to send</param>
+        void SendEvent(Event evt);
+
+        /// <summary>
+        /// Gets whether a debug client is currently connected.
+        /// </summary>
+        bool IsConnected { get; }
+    }
+
+    /// <summary>
+    /// TCP server implementation of the Debug Adapter Protocol.
+    /// Handles message framing (Content-Length headers) and JSON serialization.
+    /// </summary>
+    public sealed class DapServer : RunnerService, IDapServer
+    {
+        private const string ContentLengthHeader = "Content-Length: ";
+        private const string HeaderTerminator = "\r\n\r\n";
+
+        private TcpListener _listener;
+        private TcpClient _client;
+        private NetworkStream _stream;
+        private IDapDebugSession _session;
+        private CancellationTokenSource _cts;
+        private Task _messageLoopTask;
+        private TaskCompletionSource<bool> _connectionTcs;
+        private int _nextSeq = 1;
+        private readonly object _sendLock = new object();
+        private bool _disposed = false;
+
+        public bool IsConnected => _client?.Connected == true;
+
+        public override void Initialize(IHostContext hostContext)
+        {
+            base.Initialize(hostContext);
+            Trace.Info("DapServer initialized");
+        }
+
+        public void SetSession(IDapDebugSession session)
+        {
+            _session = session;
+            Trace.Info("Debug session set");
+        }
+
+        public async Task StartAsync(int port, CancellationToken cancellationToken)
+        {
+            Trace.Info($"Starting DAP server on port {port}");
+
+            _cts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
+            _connectionTcs = new TaskCompletionSource<bool>(TaskCreationOptions.RunContinuationsAsynchronously);
+
+            try
+            {
+                _listener = new TcpListener(IPAddress.Loopback, port);
+                _listener.Start();
+                Trace.Info($"DAP server listening on 127.0.0.1:{port}");
+
+                // Start accepting connections in the background
+                _ = AcceptConnectionAsync(_cts.Token);
+            }
+            catch (Exception ex)
+            {
+                Trace.Error($"Failed to start DAP server: {ex.Message}");
+                throw;
+            }
+
+            await Task.CompletedTask;
+        }
+
+        private async Task AcceptConnectionAsync(CancellationToken cancellationToken)
+        {
+            try
+            {
+                Trace.Info("Waiting for debug client connection...");
+
+                // Use cancellation-aware accept
+                using (cancellationToken.Register(() => _listener?.Stop()))
+                {
+                    _client = await _listener.AcceptTcpClientAsync();
+                }
+
+                if (cancellationToken.IsCancellationRequested)
+                {
+                    return;
+                }
+
+                _stream = _client.GetStream();
+                var remoteEndPoint = _client.Client.RemoteEndPoint;
+                Trace.Info($"Debug client connected from {remoteEndPoint}");
+
+                // Signal that connection is established
+                _connectionTcs.TrySetResult(true);
+
+                // Start processing messages
+                _messageLoopTask = ProcessMessagesAsync(_cts.Token);
+            }
+            catch (ObjectDisposedException) when (cancellationToken.IsCancellationRequested)
+            {
+                // Expected when cancellation stops the listener
+                Trace.Info("Connection accept cancelled");
+                _connectionTcs.TrySetCanceled();
+            }
+            catch (SocketException ex) when (cancellationToken.IsCancellationRequested)
+            {
+                // Expected when cancellation stops the listener
+                Trace.Info($"Connection accept cancelled: {ex.Message}");
+                _connectionTcs.TrySetCanceled();
+            }
+            catch (Exception ex)
+            {
+                Trace.Error($"Error accepting connection: {ex.Message}");
+                _connectionTcs.TrySetException(ex);
+            }
+        }
+
+        public async Task WaitForConnectionAsync(CancellationToken cancellationToken)
+        {
+            Trace.Info("Waiting for debug client to connect...");
+
+            using (cancellationToken.Register(() => _connectionTcs.TrySetCanceled()))
+            {
+                await _connectionTcs.Task;
+            }
+
+            Trace.Info("Debug client connected");
+        }
+
+        public async Task StopAsync()
+        {
+            Trace.Info("Stopping DAP server");
+
+            _cts?.Cancel();
+
+            // Wait for message loop to complete
+            if (_messageLoopTask != null)
+            {
+                try
+                {
+                    await _messageLoopTask;
+                }
+                catch (OperationCanceledException)
+                {
+                    // Expected
+                }
+                catch (Exception ex)
+                {
+                    Trace.Warning($"Message loop ended with error: {ex.Message}");
+                }
+            }
+
+            // Clean up resources
+            _stream?.Close();
+            _client?.Close();
+            _listener?.Stop();
+
+            Trace.Info("DAP server stopped");
+        }
+
+        public void SendEvent(Event evt)
+        {
+            if (!IsConnected)
+            {
+                Trace.Warning($"Cannot send event '{evt.EventType}': no client connected");
+                return;
+            }
+
+            try
+            {
+                lock (_sendLock)
+                {
+                    evt.Seq = _nextSeq++;
+                    SendMessageInternal(evt);
+                }
+                Trace.Info($"Sent event: {evt.EventType}");
+            }
+            catch (Exception ex)
+            {
+                Trace.Error($"Failed to send event '{evt.EventType}': {ex.Message}");
+            }
+        }
+
+        private async Task ProcessMessagesAsync(CancellationToken cancellationToken)
+        {
+            Trace.Info("Starting DAP message processing loop");
+
+            try
+            {
+                while (!cancellationToken.IsCancellationRequested && IsConnected)
+                {
+                    var json = await ReadMessageAsync(cancellationToken);
+                    if (json == null)
+                    {
+                        Trace.Info("Client disconnected (end of stream)");
+                        break;
+                    }
+
+                    await ProcessMessageAsync(json, cancellationToken);
+                }
+            }
+            catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
+            {
+                Trace.Info("Message processing cancelled");
+            }
+            catch (IOException ex)
+            {
+                Trace.Info($"Connection closed: {ex.Message}");
+            }
+            catch (Exception ex)
+            {
+                Trace.Error($"Error in message loop: {ex}");
+            }
+
+            Trace.Info("DAP message processing loop ended");
+        }
+
+        private async Task ProcessMessageAsync(string json, CancellationToken cancellationToken)
+        {
+            Request request = null;
+            try
+            {
+                // Parse the incoming message
+                request = JsonConvert.DeserializeObject<Request>(json);
+                if (request == null || request.Type != "request")
+                {
+                    Trace.Warning($"Received non-request message: {json}");
+                    return;
+                }
+
+                Trace.Info($"Received request: seq={request.Seq}, command={request.Command}");
+
+                // Dispatch to session for handling
+                if (_session == null)
+                {
+                    Trace.Error("No debug session configured");
+                    SendErrorResponse(request, "No debug session configured");
+                    return;
+                }
+
+                var response = await _session.HandleRequestAsync(request);
+                response.RequestSeq = request.Seq;
+                response.Command = request.Command;
+                response.Type = "response";
+
+                lock (_sendLock)
+                {
+                    response.Seq = _nextSeq++;
+                    SendMessageInternal(response);
+                }
+
+                Trace.Info($"Sent response: seq={response.Seq}, command={response.Command}, success={response.Success}");
+            }
+            catch (JsonException ex)
+            {
+                Trace.Error($"Failed to parse request: {ex.Message}");
+                Trace.Error($"JSON: {json}");
+            }
+            catch (Exception ex)
+            {
+                Trace.Error($"Error processing request: {ex}");
+                if (request != null)
+                {
+                    SendErrorResponse(request, ex.Message);
+                }
+            }
+        }
+
+        private void SendErrorResponse(Request request, string message)
+        {
+            var response = new Response
+            {
+                Type = "response",
+                RequestSeq = request.Seq,
+                Command = request.Command,
+                Success = false,
+                Message = message,
+                Body = new ErrorResponseBody
+                {
+                    Error = new Message
+                    {
+                        Id = 1,
+                        Format = message,
+                        ShowUser = true
+                    }
+                }
+            };
+
+            lock (_sendLock)
+            {
+                response.Seq = _nextSeq++;
+                SendMessageInternal(response);
+            }
+        }
+
+        /// <summary>
+        /// Reads a DAP message from the stream.
+        /// DAP uses HTTP-like message framing: Content-Length: N\r\n\r\n{json}
+        /// </summary>
+        private async Task<string> ReadMessageAsync(CancellationToken cancellationToken)
+        {
+            // Read headers until we find Content-Length
+            var headerBuilder = new StringBuilder();
+            int contentLength = -1;
+
+            while (true)
+            {
+                var line = await ReadLineAsync(cancellationToken);
+                if (line == null)
+                {
+                    // End of stream
+                    return null;
+                }
+
+                if (line.Length == 0)
+                {
+                    // Empty line marks end of headers
+                    break;
+                }
+
+                headerBuilder.AppendLine(line);
+
+                if (line.StartsWith(ContentLengthHeader, StringComparison.OrdinalIgnoreCase))
+                {
+                    var lengthStr = line.Substring(ContentLengthHeader.Length).Trim();
+                    if (!int.TryParse(lengthStr, out contentLength))
+                    {
+                        throw new InvalidDataException($"Invalid Content-Length: {lengthStr}");
+                    }
+                }
+            }
+
+            if (contentLength < 0)
+            {
+                throw new InvalidDataException("Missing Content-Length header");
+            }
+
+            // Read the JSON body
+            var buffer = new byte[contentLength];
+            var totalRead = 0;
+            while (totalRead < contentLength)
+            {
+                var bytesRead = await _stream.ReadAsync(buffer, totalRead, contentLength - totalRead, cancellationToken);
+                if (bytesRead == 0)
+                {
+                    throw new EndOfStreamException("Connection closed while reading message body");
+                }
+                totalRead += bytesRead;
+            }
+
+            var json = Encoding.UTF8.GetString(buffer);
+            Trace.Verbose($"Received: {json}");
+            return json;
+        }
+
+        /// <summary>
+        /// Reads a line from the stream (terminated by \r\n).
+        /// </summary>
+        private async Task<string> ReadLineAsync(CancellationToken cancellationToken)
+        {
+            var lineBuilder = new StringBuilder();
+            var buffer = new byte[1];
+            var previousWasCr = false;
+
+            while (true)
+            {
+                var bytesRead = await _stream.ReadAsync(buffer, 0, 1, cancellationToken);
+                if (bytesRead == 0)
+                {
+                    // End of stream
+                    return lineBuilder.Length > 0 ? lineBuilder.ToString() : null;
+                }
+
+                var c = (char)buffer[0];
+
+                if (c == '\n' && previousWasCr)
+                {
+                    // Found \r\n, return the line (without the \r)
+                    if (lineBuilder.Length > 0 && lineBuilder[lineBuilder.Length - 1] == '\r')
+                    {
+                        lineBuilder.Length--;
+                    }
+                    return lineBuilder.ToString();
+                }
+
+                previousWasCr = (c == '\r');
+                lineBuilder.Append(c);
+            }
+        }
+
+        /// <summary>
+        /// Sends a DAP message to the stream with Content-Length framing.
+        /// Must be called within the _sendLock.
+        /// </summary>
+        private void SendMessageInternal(ProtocolMessage message)
+        {
+            var json = JsonConvert.SerializeObject(message, new JsonSerializerSettings
+            {
+                NullValueHandling = NullValueHandling.Ignore
+            });
+
+            var bodyBytes = Encoding.UTF8.GetBytes(json);
+            var header = $"Content-Length: {bodyBytes.Length}\r\n\r\n";
+            var headerBytes = Encoding.UTF8.GetBytes(header);
+
+            _stream.Write(headerBytes, 0, headerBytes.Length);
+            _stream.Write(bodyBytes, 0, bodyBytes.Length);
+            _stream.Flush();
+
+            Trace.Verbose($"Sent: {json}");
+        }
+
+        public void Dispose()
+        {
+            Dispose(true);
+            GC.SuppressFinalize(this);
+        }
+
+        private void Dispose(bool disposing)
+        {
+            if (_disposed)
+            {
+                return;
+            }
+
+            if (disposing)
+            {
+                _cts?.Cancel();
+                _stream?.Dispose();
+                _client?.Dispose();
+                _listener?.Stop();
+                _cts?.Dispose();
+            }
+
+            _disposed = true;
+        }
+    }
+}

src/Runner.Worker/Dap/DapVariableProvider.cs

@@ -0,0 +1,293 @@
+using System;
+using System.Collections.Generic;
+using GitHub.DistributedTask.Pipelines.ContextData;
+using GitHub.Runner.Common;
+
+namespace GitHub.Runner.Worker.Dap
+{
+    /// <summary>
+    /// Provides DAP variable information from the execution context.
+    /// Maps workflow contexts (github, env, runner, job, steps, secrets) to DAP scopes and variables.
+    /// </summary>
+    public sealed class DapVariableProvider
+    {
+        // Well-known scope names that map to top-level contexts
+        private static readonly string[] ScopeNames = { "github", "env", "runner", "job", "steps", "secrets", "inputs", "vars", "matrix", "needs" };
+
+        // Reserved variable reference ranges for scopes (1-100)
+        private const int ScopeReferenceBase = 1;
+        private const int ScopeReferenceMax = 100;
+
+        // Dynamic variable references start after scope range
+        private const int DynamicReferenceBase = 101;
+
+        private readonly IHostContext _hostContext;
+        private readonly Dictionary<int, (PipelineContextData Data, string Path)> _variableReferences = new();
+        private int _nextVariableReference = DynamicReferenceBase;
+
+        public DapVariableProvider(IHostContext hostContext)
+        {
+            _hostContext = hostContext;
+        }
+
+        /// <summary>
+        /// Resets the variable reference state. Call this when the execution context changes.
+        /// </summary>
+        public void Reset()
+        {
+            _variableReferences.Clear();
+            _nextVariableReference = DynamicReferenceBase;
+        }
+
+        /// <summary>
+        /// Gets the list of scopes for a given execution context.
+        /// Each scope represents a top-level context like 'github', 'env', etc.
+        /// </summary>
+        public List<Scope> GetScopes(IExecutionContext context, int frameId)
+        {
+            var scopes = new List<Scope>();
+
+            if (context?.ExpressionValues == null)
+            {
+                return scopes;
+            }
+
+            for (int i = 0; i < ScopeNames.Length; i++)
+            {
+                var scopeName = ScopeNames[i];
+                if (context.ExpressionValues.TryGetValue(scopeName, out var value) && value != null)
+                {
+                    var variablesRef = ScopeReferenceBase + i;
+                    var scope = new Scope
+                    {
+                        Name = scopeName,
+                        VariablesReference = variablesRef,
+                        Expensive = false,
+                        // Secrets get a special presentation hint
+                        PresentationHint = scopeName == "secrets" ? "registers" : null
+                    };
+
+                    // Count named variables if it's a dictionary
+                    if (value is DictionaryContextData dict)
+                    {
+                        scope.NamedVariables = dict.Count;
+                    }
+                    else if (value is CaseSensitiveDictionaryContextData csDict)
+                    {
+                        scope.NamedVariables = csDict.Count;
+                    }
+
+                    scopes.Add(scope);
+                }
+            }
+
+            return scopes;
+        }
+
+        /// <summary>
+        /// Gets variables for a given variable reference.
+        /// </summary>
+        public List<Variable> GetVariables(IExecutionContext context, int variablesReference)
+        {
+            var variables = new List<Variable>();
+
+            if (context?.ExpressionValues == null)
+            {
+                return variables;
+            }
+
+            PipelineContextData data = null;
+            string basePath = null;
+            bool isSecretsScope = false;
+
+            // Check if this is a scope reference (1-100)
+            if (variablesReference >= ScopeReferenceBase && variablesReference <= ScopeReferenceMax)
+            {
+                var scopeIndex = variablesReference - ScopeReferenceBase;
+                if (scopeIndex < ScopeNames.Length)
+                {
+                    var scopeName = ScopeNames[scopeIndex];
+                    isSecretsScope = scopeName == "secrets";
+                    if (context.ExpressionValues.TryGetValue(scopeName, out data))
+                    {
+                        basePath = scopeName;
+                    }
+                }
+            }
+            // Check dynamic references
+            else if (_variableReferences.TryGetValue(variablesReference, out var refData))
+            {
+                data = refData.Data;
+                basePath = refData.Path;
+                // Check if we're inside the secrets scope
+                isSecretsScope = basePath?.StartsWith("secrets", StringComparison.OrdinalIgnoreCase) == true;
+            }
+
+            if (data == null)
+            {
+                return variables;
+            }
+
+            // Convert the data to variables
+            ConvertToVariables(data, basePath, isSecretsScope, variables);
+
+            return variables;
+        }
+
+        /// <summary>
+        /// Converts PipelineContextData to DAP Variable objects.
+        /// </summary>
+        private void ConvertToVariables(PipelineContextData data, string basePath, bool isSecretsScope, List<Variable> variables)
+        {
+            switch (data)
+            {
+                case DictionaryContextData dict:
+                    ConvertDictionaryToVariables(dict, basePath, isSecretsScope, variables);
+                    break;
+
+                case CaseSensitiveDictionaryContextData csDict:
+                    ConvertCaseSensitiveDictionaryToVariables(csDict, basePath, isSecretsScope, variables);
+                    break;
+
+                case ArrayContextData array:
+                    ConvertArrayToVariables(array, basePath, isSecretsScope, variables);
+                    break;
+
+                default:
+                    // Scalar value - shouldn't typically get here for a container
+                    break;
+            }
+        }
+
+        private void ConvertDictionaryToVariables(DictionaryContextData dict, string basePath, bool isSecretsScope, List<Variable> variables)
+        {
+            foreach (var pair in dict)
+            {
+                var variable = CreateVariable(pair.Key, pair.Value, basePath, isSecretsScope);
+                variables.Add(variable);
+            }
+        }
+
+        private void ConvertCaseSensitiveDictionaryToVariables(CaseSensitiveDictionaryContextData dict, string basePath, bool isSecretsScope, List<Variable> variables)
+        {
+            foreach (var pair in dict)
+            {
+                var variable = CreateVariable(pair.Key, pair.Value, basePath, isSecretsScope);
+                variables.Add(variable);
+            }
+        }
+
+        private void ConvertArrayToVariables(ArrayContextData array, string basePath, bool isSecretsScope, List<Variable> variables)
+        {
+            for (int i = 0; i < array.Count; i++)
+            {
+                var item = array[i];
+                var variable = CreateVariable($"[{i}]", item, basePath, isSecretsScope);
+                variable.Name = $"[{i}]";
+                variables.Add(variable);
+            }
+        }
+
+        private Variable CreateVariable(string name, PipelineContextData value, string basePath, bool isSecretsScope)
+        {
+            var childPath = string.IsNullOrEmpty(basePath) ? name : $"{basePath}.{name}";
+            var variable = new Variable
+            {
+                Name = name,
+                EvaluateName = $"${{{{ {childPath} }}}}"
+            };
+
+            if (value == null)
+            {
+                variable.Value = "null";
+                variable.Type = "null";
+                variable.VariablesReference = 0;
+                return variable;
+            }
+
+            switch (value)
+            {
+                case StringContextData str:
+                    if (isSecretsScope)
+                    {
+                        // Always mask secrets regardless of value
+                        variable.Value = "[REDACTED]";
+                    }
+                    else
+                    {
+                        // Mask any secret values that might be in non-secret contexts
+                        variable.Value = MaskSecrets(str.Value);
+                    }
+                    variable.Type = "string";
+                    variable.VariablesReference = 0;
+                    break;
+
+                case NumberContextData num:
+                    variable.Value = num.ToString();
+                    variable.Type = "number";
+                    variable.VariablesReference = 0;
+                    break;
+
+                case BooleanContextData boolVal:
+                    variable.Value = boolVal.Value ? "true" : "false";
+                    variable.Type = "boolean";
+                    variable.VariablesReference = 0;
+                    break;
+
+                case DictionaryContextData dict:
+                    variable.Value = $"Object ({dict.Count} properties)";
+                    variable.Type = "object";
+                    variable.VariablesReference = RegisterVariableReference(dict, childPath);
+                    variable.NamedVariables = dict.Count;
+                    break;
+
+                case CaseSensitiveDictionaryContextData csDict:
+                    variable.Value = $"Object ({csDict.Count} properties)";
+                    variable.Type = "object";
+                    variable.VariablesReference = RegisterVariableReference(csDict, childPath);
+                    variable.NamedVariables = csDict.Count;
+                    break;
+
+                case ArrayContextData array:
+                    variable.Value = $"Array ({array.Count} items)";
+                    variable.Type = "array";
+                    variable.VariablesReference = RegisterVariableReference(array, childPath);
+                    variable.IndexedVariables = array.Count;
+                    break;
+
+                default:
+                    // Unknown type - convert to string representation
+                    var rawValue = value.ToJToken()?.ToString() ?? "unknown";
+                    variable.Value = MaskSecrets(rawValue);
+                    variable.Type = value.GetType().Name;
+                    variable.VariablesReference = 0;
+                    break;
+            }
+
+            return variable;
+        }
+
+        /// <summary>
+        /// Registers a nested variable reference and returns its ID.
+        /// </summary>
+        private int RegisterVariableReference(PipelineContextData data, string path)
+        {
+            var reference = _nextVariableReference++;
+            _variableReferences[reference] = (data, path);
+            return reference;
+        }
+
+        /// <summary>
+        /// Masks any secret values in the string using the host context's secret masker.
+        /// </summary>
+        private string MaskSecrets(string value)
+        {
+            if (string.IsNullOrEmpty(value))
+            {
+                return value ?? string.Empty;
+            }
+
+            return _hostContext.SecretMasker.MaskSecrets(value);
+        }
+    }
+}

src/Runner.Worker/JobRunner.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
@@ -13,6 +13,7 @@ using GitHub.DistributedTask.WebApi;
 using GitHub.Runner.Common;
 using GitHub.Runner.Common.Util;
 using GitHub.Runner.Sdk;
+using GitHub.Runner.Worker.Dap;
 using GitHub.Services.Common;
 using GitHub.Services.WebApi;
 using Sdk.RSWebApi.Contracts;
@@ -112,6 +113,7 @@ namespace GitHub.Runner.Worker
 
             IExecutionContext jobContext = null;
             CancellationTokenRegistration? runnerShutdownRegistration = null;
+            IDapServer dapServer = null;
             try
             {
                 // Create the job execution context.
@@ -159,6 +161,47 @@ namespace GitHub.Runner.Worker
                 if (jobContext.Global.WriteDebug)
                 {
                     jobContext.SetRunnerContext("debug", "1");
+
+                    // Start DAP server for interactive debugging
+                    // This allows debugging workflow jobs with DAP-compatible editors (nvim-dap, VS Code, etc.)
+                    try
+                    {
+                        var port = 4711;
+                        var portEnv = Environment.GetEnvironmentVariable("ACTIONS_DAP_PORT");
+                        if (!string.IsNullOrEmpty(portEnv) && int.TryParse(portEnv, out var customPort))
+                        {
+                            port = customPort;
+                        }
+
+                        dapServer = HostContext.GetService<IDapServer>();
+                        var debugSession = HostContext.GetService<IDapDebugSession>();
+
+                        // Wire up the server and session
+                        dapServer.SetSession(debugSession);
+                        debugSession.SetDapServer(dapServer);
+
+                        await dapServer.StartAsync(port, jobRequestCancellationToken);
+                        Trace.Info($"DAP server listening on port {port}");
+                        jobContext.Output($"DAP debugger waiting for connection on port {port}...");
+                        jobContext.Output($"Connect your DAP client (nvim-dap, VS Code, etc.) to attach to this job.");
+
+                        // Block until debugger connects
+                        await dapServer.WaitForConnectionAsync(jobRequestCancellationToken);
+                        Trace.Info("DAP client connected, continuing job execution");
+                        jobContext.Output("Debugger connected. Job execution will pause before each step.");
+                    }
+                    catch (OperationCanceledException)
+                    {
+                        // Job was cancelled before debugger connected
+                        Trace.Info("Job cancelled while waiting for DAP client connection");
+                    }
+                    catch (Exception ex)
+                    {
+                        // Log but don't fail the job if DAP server fails to start
+                        Trace.Warning($"Failed to start DAP server: {ex.Message}");
+                        jobContext.Warning($"DAP debugging unavailable: {ex.Message}");
+                        dapServer = null;
+                    }
                 }
 
                 jobContext.SetRunnerContext("os", VarUtil.OS);
@@ -259,6 +302,20 @@ namespace GitHub.Runner.Worker
                     runnerShutdownRegistration = null;
                 }
 
+                // Stop DAP server if it was started
+                if (dapServer != null)
+                {
+                    try
+                    {
+                        Trace.Info("Stopping DAP server");
+                        await dapServer.StopAsync();
+                    }
+                    catch (Exception ex)
+                    {
+                        Trace.Warning($"Error stopping DAP server: {ex.Message}");
+                    }
+                }
+
                 await ShutdownQueue(throwOnFailure: false);
             }
         }

src/Runner.Worker/StepsRunner.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.Threading;
 using System.Threading.Tasks;
@@ -10,6 +10,7 @@ using GitHub.DistributedTask.WebApi;
 using GitHub.Runner.Common;
 using GitHub.Runner.Common.Util;
 using GitHub.Runner.Sdk;
+using GitHub.Runner.Worker.Dap;
 using GitHub.Runner.Worker.Expressions;
 
 namespace GitHub.Runner.Worker
@@ -50,6 +51,12 @@ namespace GitHub.Runner.Worker
             jobContext.JobContext.Status = (jobContext.Result ?? TaskResult.Succeeded).ToActionResult();
             var scopeInputs = new Dictionary<string, PipelineContextData>(StringComparer.OrdinalIgnoreCase);
             bool checkPostJobActions = false;
+
+            // Get debug session for DAP debugging support
+            // The session's IsActive property determines if debugging is actually enabled
+            var debugSession = HostContext.GetService<IDapDebugSession>();
+            bool isFirstStep = true;
+
             while (jobContext.JobSteps.Count > 0 || !checkPostJobActions)
             {
                 if (jobContext.JobSteps.Count == 0 && !checkPostJobActions)
@@ -181,6 +188,14 @@ namespace GitHub.Runner.Worker
                             }
                         }
 
+                        // Pause for DAP debugger BEFORE step execution
+                        // This happens after expression values are set up so the debugger can inspect variables
+                        if (debugSession?.IsActive == true)
+                        {
+                            await debugSession.OnStepStartingAsync(step, jobContext, isFirstStep);
+                            isFirstStep = false;
+                        }
+
                         // Evaluate condition
                         step.ExecutionContext.Debug($"Evaluating condition for step: '{step.DisplayName}'");
                         var conditionTraceWriter = new ConditionTraceWriter(Trace, step.ExecutionContext);
@@ -253,8 +268,17 @@ namespace GitHub.Runner.Worker
                     Trace.Info($"No need for updating job result with current step result '{step.ExecutionContext.Result}'.");
                 }
 
+                // Notify DAP debugger AFTER step execution
+                if (debugSession?.IsActive == true)
+                {
+                    debugSession.OnStepCompleted(step);
+                }
+
                 Trace.Info($"Current state: job state = '{jobContext.Result}'");
             }
+
+            // Notify DAP debugger that the job has completed
+            debugSession?.OnJobCompleted();
         }
 
         private async Task RunStepAsync(IStep step, CancellationToken jobCancellationToken)