fix(worker): claim Running before creating run resources

TaskRunner.RunAsync created the worktree (PrepareRunDirectoryAsync)
before claiming Running via StartRunningAsync. RunNow dispatches by
task id with no atomic claim of their own, so a Queued task racing
the queue picker's atomic SQL claim could hit WorktreeManager's
branch-collision self-heal, which force-removes and recreates the
winner's live worktree mid-run.

Move the claim before any resource creation and bail out immediately
when it's rejected. OverrideSlotService.RunNow also fast-rejects a
task already Running in the DB (defense in depth). RunCancellationRegistry
now refuses (and logs) a double registration instead of silently
overwriting the first CTS, so a losing dispatch's cleanup can no
longer unregister the winner's cancellation token.

src/ClaudeDo.Worker/Queue/OverrideSlotService.cs

@@ -33,9 +33,15 @@ public sealed class OverrideSlotService
     {
         using (var context = _dbFactory.CreateDbContext())
         {
-            var exists = await new TaskRepository(context).GetByIdAsync(taskId);
-            if (exists is null)
+            var task = await new TaskRepository(context).GetByIdAsync(taskId);
+            if (task is null)
                 throw new KeyNotFoundException($"Task '{taskId}' not found.");
+
+            // Fast-fail precheck only — not the race guard. A task sitting Queued still
+            // passes here and can race the queue picker's atomic claim; TaskRunner.RunAsync
+            // resolves that by claiming Running before creating any resources.
+            if (task.Status == Data.Models.TaskStatus.Running)
+                throw new InvalidOperationException("task is already running");
         }
 
         StartInSlot(taskId, ct => RunInSlotAsync(taskId, ct), "RunInSlotAsync failed for task {TaskId}");

src/ClaudeDo.Worker/Queue/RunCancellationRegistry.cs

@@ -10,8 +10,23 @@ namespace ClaudeDo.Worker.Queue;
 public sealed class RunCancellationRegistry
 {
     private readonly ConcurrentDictionary<string, CancellationTokenSource> _running = new(StringComparer.Ordinal);
+    private readonly ILogger<RunCancellationRegistry> _logger;
 
-    public void Register(string taskId, CancellationTokenSource cts) => _running[taskId] = cts;
+    public RunCancellationRegistry(ILogger<RunCancellationRegistry> logger) => _logger = logger;
+
+    /// Registers the CTS driving <paramref name="taskId"/>'s run. Refuses (and logs) instead of
+    /// silently overwriting when a CTS is already registered for this task id — an overwrite
+    /// would mean a double-dispatch is in flight, and the caller that lost the race would then
+    /// unregister the winner's CTS out from under it, leaving CancelAsync unable to reach the
+    /// still-running process.
+    public bool Register(string taskId, CancellationTokenSource cts)
+    {
+        if (_running.TryAdd(taskId, cts)) return true;
+
+        _logger.LogWarning(
+            "Task {TaskId} already has a registered run; refusing to overwrite (double-dispatch?)", taskId);
+        return false;
+    }
 
     /// Removes the registration only if <paramref name="cts"/> is still the registered
     /// one — a re-run may already have registered a newer CTS under the same task id.

src/ClaudeDo.Worker/Runner/TaskRunner.cs

@@ -87,6 +87,32 @@ public sealed class TaskRunner
                     attachmentPaths = attachments.Select(a => Path.Combine(_attachments.TaskDir(task.Id), a.FileName)).ToList();
             }
 
+            var now = DateTime.UtcNow;
+            // Claim Running before creating any resources (worktree, MCP token file, ...):
+            // the queue picker claims Queued→Running atomically (incl. StartedAt) before
+            // dispatching, so only unclaimed dispatches (override slot) need to claim here.
+            // Claiming first means a losing double-dispatch (RunNow racing the picker for the
+            // same row) bails out immediately instead of creating a worktree the winner then
+            // has to self-heal past.
+            if (!alreadyClaimed)
+            {
+                var startResult = await _state.StartRunningAsync(task.Id, now, ct);
+                if (!startResult.Ok)
+                {
+                    _logger.LogWarning("Task {TaskId} skipped: StartRunningAsync rejected ({Reason})", task.Id, startResult.Reason);
+                    return;
+                }
+            }
+            else
+            {
+                // Queue-claimed dispatches skip StartRunningAsync (the atomic SQL claim in
+                // QueuePicker already flipped the row to Running), so it never broadcasts
+                // TaskUpdated for this transition. Send it here so the task-list badge flips
+                // live instead of staying on "Queued" until the run finishes.
+                await _broadcaster.TaskUpdated(task.Id);
+            }
+            await _broadcaster.TaskStarted(slot, task.Id, now);
+
             // Determine working directory: worktree or sandbox.
             var prep = await PrepareRunDirectoryAsync(task, list, ct);
             if (prep.FailureReason is not null)
@@ -117,28 +143,6 @@ public sealed class TaskRunner
                     : "mcp__claudedo_run__AskUser",
             };
 
-            var now = DateTime.UtcNow;
-            // The queue picker claims Queued→Running atomically (incl. StartedAt) before
-            // dispatching; only unclaimed dispatches (override slot) claim here.
-            if (!alreadyClaimed)
-            {
-                var startResult = await _state.StartRunningAsync(task.Id, now, ct);
-                if (!startResult.Ok)
-                {
-                    _logger.LogWarning("Task {TaskId} skipped: StartRunningAsync rejected ({Reason})", task.Id, startResult.Reason);
-                    return;
-                }
-            }
-            else
-            {
-                // Queue-claimed dispatches skip StartRunningAsync (the atomic SQL claim in
-                // QueuePicker already flipped the row to Running), so it never broadcasts
-                // TaskUpdated for this transition. Send it here so the task-list badge flips
-                // live instead of staying on "Queued" until the run finishes.
-                await _broadcaster.TaskUpdated(task.Id);
-            }
-            await _broadcaster.TaskStarted(slot, task.Id, now);
-
             await _skillSeeder.SeedAsync(runDir, resolvedConfig.SkillNames, wtCtx is not null, ct);
 
             // Build prompt: title + description + only the OPEN sub-tasks (resolved ones are dropped).

tests/ClaudeDo.Worker.Tests/External/AddSubtaskToolTests.cs

@@ -74,7 +74,7 @@ public sealed class AddSubtaskToolTests : IDisposable
             NullLogger<TaskRunner>.Instance, state, new TaskRunTokenRegistry(), new AttachmentStore(), new FakeSessionSkillSeeder(), new FakeTranscriptUsageReader());
         var waker = new ClaudeDo.Worker.Queue.QueueWaker();
         var picker = new ClaudeDo.Worker.Queue.QueuePicker(dbFactory);
-        var runCancels = new RunCancellationRegistry();
+        var runCancels = new RunCancellationRegistry(NullLogger<RunCancellationRegistry>.Instance);
         var overrideSlot = new OverrideSlotService(dbFactory, runner, NullLogger<OverrideSlotService>.Instance, runCancels);
         var queue = new QueueService(dbFactory, runner, cfg, NullLogger<QueueService>.Instance, waker, picker, overrideSlot, state, runCancels,
             new FakeUsageGate(), new UsageState(), broadcaster);

tests/ClaudeDo.Worker.Tests/External/BatchMcpToolsTests.cs

@@ -94,7 +94,7 @@ public sealed class BatchMcpToolsTests : IDisposable
         var state = TaskStateServiceBuilder.Build(dbFactory).State;
         var runner = new TaskRunner(new FakeClaudeProcess(), dbFactory, broadcaster, wtManager, new ClaudeArgsBuilder(), cfg,
             NullLogger<TaskRunner>.Instance, state, new TaskRunTokenRegistry(), new AttachmentStore(), new FakeSessionSkillSeeder(), new FakeTranscriptUsageReader());
-        var runCancels = new RunCancellationRegistry();
+        var runCancels = new RunCancellationRegistry(NullLogger<RunCancellationRegistry>.Instance);
         var overrideSlot = new OverrideSlotService(dbFactory, runner, NullLogger<OverrideSlotService>.Instance, runCancels);
         return new QueueService(dbFactory, runner, cfg, NullLogger<QueueService>.Instance,
             new QueueWaker(), new QueuePicker(dbFactory), overrideSlot, state, runCancels,

tests/ClaudeDo.Worker.Tests/External/ExternalMcpServiceTests.cs

@@ -160,7 +160,7 @@ public sealed class ExternalMcpServiceTests : IDisposable
             NullLogger<TaskRunner>.Instance, state, new TaskRunTokenRegistry(), new AttachmentStore(), new FakeSessionSkillSeeder(), new FakeTranscriptUsageReader());
         var waker = new ClaudeDo.Worker.Queue.QueueWaker();
         var picker = new ClaudeDo.Worker.Queue.QueuePicker(dbFactory);
-        var runCancels = new RunCancellationRegistry();
+        var runCancels = new RunCancellationRegistry(NullLogger<RunCancellationRegistry>.Instance);
         var overrideSlot = new OverrideSlotService(dbFactory, runner, NullLogger<OverrideSlotService>.Instance, runCancels);
         return new QueueService(dbFactory, runner, cfg, NullLogger<QueueService>.Instance, waker, picker, overrideSlot, state, runCancels,
             new FakeUsageGate(), new UsageState(), broadcaster);

tests/ClaudeDo.Worker.Tests/Infrastructure/TaskStateServiceBuilder.cs

@@ -25,7 +25,7 @@ public static class TaskStateServiceBuilder
         var hub = new CapturingHubContext();
         var broadcaster = new HubBroadcaster(hub);
         var waker = new CountingQueueWaker();
-        var runCancels = new RunCancellationRegistry();
+        var runCancels = new RunCancellationRegistry(NullLogger<RunCancellationRegistry>.Instance);
 
         TaskStateService? state = null;
         var chain = new PlanningChainCoordinator(dbFactory, () => state!);

tests/ClaudeDo.Worker.Tests/Queue/OverrideSlotServiceTests.cs

@@ -0,0 +1,89 @@
+using ClaudeDo.Data;
+using ClaudeDo.Data.Models;
+using ClaudeDo.Data.Repositories;
+using ClaudeDo.Worker.Config;
+using ClaudeDo.Worker.Hub;
+using ClaudeDo.Worker.Queue;
+using ClaudeDo.Worker.Runner;
+using ClaudeDo.Worker.Tests.Infrastructure;
+using Microsoft.Extensions.Logging.Abstractions;
+using TaskStatus = ClaudeDo.Data.Models.TaskStatus;
+
+namespace ClaudeDo.Worker.Tests.Queue;
+
+/// OverrideSlotService.RunNow's own precheck — distinct from QueueService.EnsureNotInQueueSlot,
+/// which only guards a task already tracked in its in-memory queue-slot dict. This covers a
+/// task the DB already shows Running (e.g. the queue picker's atomic claim landed) regardless
+/// of local slot bookkeeping. The real race-safety net is TaskRunner.RunAsync claiming Running
+/// before creating any resources; this precheck just fails fast for the common case.
+public sealed class OverrideSlotServiceTests : IDisposable
+{
+    private readonly DbFixture _db = new();
+    private readonly string _tempDir;
+    private readonly WorkerConfig _cfg;
+
+    public OverrideSlotServiceTests()
+    {
+        _tempDir = Path.Combine(Path.GetTempPath(), $"cd_override_{Guid.NewGuid():N}");
+        Directory.CreateDirectory(_tempDir);
+        _cfg = new WorkerConfig { SandboxRoot = _tempDir, LogRoot = _tempDir };
+    }
+
+    public void Dispose() { _db.Dispose(); try { Directory.Delete(_tempDir, true); } catch { } }
+
+    private OverrideSlotService BuildService()
+    {
+        var dbFactory = _db.CreateFactory();
+        var state = TaskStateServiceBuilder.Build(dbFactory).State;
+        var wt = new WorktreeManager(new ClaudeDo.Data.Git.GitService(), dbFactory, _cfg, NullLogger<WorktreeManager>.Instance);
+        var runner = new TaskRunner(new FakeClaudeProcess(), dbFactory, new HubBroadcaster(new CapturingHubContext()), wt,
+            new ClaudeArgsBuilder(), _cfg, NullLogger<TaskRunner>.Instance, state, new TaskRunTokenRegistry(),
+            new AttachmentStore(), new FakeSessionSkillSeeder(), new FakeTranscriptUsageReader());
+        var runCancels = new RunCancellationRegistry(NullLogger<RunCancellationRegistry>.Instance);
+        return new OverrideSlotService(dbFactory, runner, NullLogger<OverrideSlotService>.Instance, runCancels);
+    }
+
+    [Fact]
+    public async Task RunNow_TaskAlreadyRunningInDb_ThrowsWithoutDispatching()
+    {
+        string listId = Guid.NewGuid().ToString(), taskId = Guid.NewGuid().ToString();
+        using (var ctx = _db.CreateContext())
+        {
+            ctx.Lists.Add(new ListEntity { Id = listId, Name = "L", CreatedAt = DateTime.UtcNow });
+            ctx.Tasks.Add(new TaskEntity
+            {
+                Id = taskId, ListId = listId, Title = "T", Status = TaskStatus.Running,
+                StartedAt = DateTime.UtcNow, CreatedAt = DateTime.UtcNow,
+            });
+            await ctx.SaveChangesAsync();
+        }
+
+        var service = BuildService();
+
+        var ex = await Assert.ThrowsAsync<InvalidOperationException>(() => service.RunNow(taskId));
+        Assert.Contains("already running", ex.Message);
+        Assert.Null(service.CurrentSlot);
+    }
+
+    [Fact]
+    public async Task RunNow_TaskQueued_DoesNotThrow()
+    {
+        string listId = Guid.NewGuid().ToString(), taskId = Guid.NewGuid().ToString();
+        using (var ctx = _db.CreateContext())
+        {
+            ctx.Lists.Add(new ListEntity { Id = listId, Name = "L", CreatedAt = DateTime.UtcNow });
+            ctx.Tasks.Add(new TaskEntity
+            {
+                Id = taskId, ListId = listId, Title = "T", Status = TaskStatus.Queued,
+                CreatedAt = DateTime.UtcNow,
+            });
+            await ctx.SaveChangesAsync();
+        }
+
+        var service = BuildService();
+
+        // The dispatch itself races with FakeClaudeProcess's immediate completion in the
+        // background, so this only asserts the precheck doesn't reject a claimable task.
+        await service.RunNow(taskId);
+    }
+}

tests/ClaudeDo.Worker.Tests/Queue/RunCancellationRegistryTests.cs

@@ -1,4 +1,5 @@
 using ClaudeDo.Worker.Queue;
+using Microsoft.Extensions.Logging.Abstractions;
 
 namespace ClaudeDo.Worker.Tests.Queue;
 
@@ -7,9 +8,9 @@ public sealed class RunCancellationRegistryTests
     [Fact]
     public void TryCancel_RegisteredTask_CancelsAndReturnsTrue()
     {
-        var sut = new RunCancellationRegistry();
+        var sut = new RunCancellationRegistry(NullLogger<RunCancellationRegistry>.Instance);
         using var cts = new CancellationTokenSource();
-        sut.Register("t1", cts);
+        Assert.True(sut.Register("t1", cts));
 
         Assert.True(sut.TryCancel("t1"));
         Assert.True(cts.IsCancellationRequested);
@@ -18,30 +19,50 @@ public sealed class RunCancellationRegistryTests
     [Fact]
     public void TryCancel_UnknownTask_ReturnsFalse()
     {
-        var sut = new RunCancellationRegistry();
+        var sut = new RunCancellationRegistry(NullLogger<RunCancellationRegistry>.Instance);
         Assert.False(sut.TryCancel("nope"));
     }
 
+    [Fact]
+    public void Register_SecondCallForSameTask_RefusesAndKeepsFirstRegistration()
+    {
+        // A double-dispatch (e.g. RunNow racing the queue picker for the same task) must not
+        // let the second registration silently clobber the first — that would leave CancelAsync
+        // pointing at the wrong (or already-finished) CTS.
+        var sut = new RunCancellationRegistry(NullLogger<RunCancellationRegistry>.Instance);
+        using var first = new CancellationTokenSource();
+        using var second = new CancellationTokenSource();
+
+        Assert.True(sut.Register("t1", first));
+        Assert.False(sut.Register("t1", second));
+
+        Assert.True(sut.TryCancel("t1"));
+        Assert.True(first.IsCancellationRequested);
+        Assert.False(second.IsCancellationRequested);
+    }
+
     [Fact]
     public void Unregister_RemovesOnlyTheGivenRegistration()
     {
-        var sut = new RunCancellationRegistry();
-        using var stale = new CancellationTokenSource();
-        using var current = new CancellationTokenSource();
+        var sut = new RunCancellationRegistry(NullLogger<RunCancellationRegistry>.Instance);
+        using var first = new CancellationTokenSource();
 
-        sut.Register("t1", stale);
-        sut.Register("t1", current); // re-run replaced the registration
-        sut.Unregister("t1", stale); // late cleanup of the old slot must not evict the new one
+        sut.Register("t1", first);
+        sut.Unregister("t1", first);
+
+        // Slot is free again once the run that held it cleans up properly, so a genuine
+        // sequential re-run can register a fresh CTS under the same task id.
+        using var second = new CancellationTokenSource();
+        Assert.True(sut.Register("t1", second));
 
         Assert.True(sut.TryCancel("t1"));
-        Assert.True(current.IsCancellationRequested);
-        Assert.False(stale.IsCancellationRequested);
+        Assert.True(second.IsCancellationRequested);
     }
 
     [Fact]
     public void TryCancel_DisposedCts_ReturnsFalse()
     {
-        var sut = new RunCancellationRegistry();
+        var sut = new RunCancellationRegistry(NullLogger<RunCancellationRegistry>.Instance);
         var cts = new CancellationTokenSource();
         sut.Register("t1", cts);
         cts.Dispose();

tests/ClaudeDo.Worker.Tests/Runner/ConcurrentDispatchRaceTests.cs

@@ -0,0 +1,122 @@
+using ClaudeDo.Data;
+using ClaudeDo.Data.Git;
+using ClaudeDo.Data.Models;
+using ClaudeDo.Data.Repositories;
+using ClaudeDo.Worker.Config;
+using ClaudeDo.Worker.Hub;
+using ClaudeDo.Worker.Queue;
+using ClaudeDo.Worker.Runner;
+using ClaudeDo.Worker.Tests.Infrastructure;
+using Microsoft.Extensions.Logging.Abstractions;
+using TaskStatus = ClaudeDo.Data.Models.TaskStatus;
+
+namespace ClaudeDo.Worker.Tests.Runner;
+
+/// Regression test for the RunNow/queue-picker double-dispatch race: a Queued task
+/// dispatched both via the override slot (RunNow, unclaimed) and the queue picker (atomic
+/// SQL Queued->Running claim) must not create its worktree twice. Before the fix,
+/// TaskRunner.RunAsync created the worktree (PrepareRunDirectoryAsync) BEFORE claiming
+/// Running, so the losing dispatch reached WorktreeManager's "branch already exists"
+/// self-heal and force-removed the winner's live worktree out from under its still-running
+/// process. The fix claims Running first and bails out before touching git when the claim
+/// is rejected.
+public sealed class ConcurrentDispatchRaceTests : IDisposable
+{
+    private readonly DbFixture _db = new();
+    private readonly GitRepoFixture? _repo;
+    private readonly string _tempDir;
+    private readonly WorkerConfig _cfg;
+
+    private static bool GitAvailable => GitRepoFixture.IsGitAvailable();
+
+    public ConcurrentDispatchRaceTests()
+    {
+        _tempDir = Path.Combine(Path.GetTempPath(), $"cd_race_{Guid.NewGuid():N}");
+        Directory.CreateDirectory(_tempDir);
+        _cfg = new WorkerConfig { SandboxRoot = _tempDir, LogRoot = _tempDir };
+        if (GitAvailable) _repo = new GitRepoFixture();
+    }
+
+    public void Dispose()
+    {
+        _db.Dispose();
+        _repo?.Dispose();
+        try { Directory.Delete(_tempDir, true); } catch { /* best effort */ }
+    }
+
+    [Fact]
+    public async Task RunNow_RacingQueuePicker_LoserBailsBeforeTouchingWorktree()
+    {
+        if (!GitAvailable) { Assert.True(true, "git not available -- skipping"); return; }
+
+        var dbFactory = _db.CreateFactory();
+        string listId = Guid.NewGuid().ToString(), taskId = Guid.NewGuid().ToString();
+        using (var ctx = _db.CreateContext())
+        {
+            ctx.Lists.Add(new ListEntity { Id = listId, Name = "L", WorkingDir = _repo!.RepoDir, CreatedAt = DateTime.UtcNow });
+            ctx.Tasks.Add(new TaskEntity
+            {
+                Id = taskId, ListId = listId, Title = "Racing task", Status = TaskStatus.Queued,
+                CreatedAt = DateTime.UtcNow,
+            });
+            await ctx.SaveChangesAsync();
+        }
+
+        var processStarted = new TaskCompletionSource();
+        var releaseProcess = new TaskCompletionSource();
+        var fake = new FakeClaudeProcess(async (_, _, _, _, _) =>
+        {
+            processStarted.TrySetResult();
+            await releaseProcess.Task;
+            return new RunResult { ExitCode = 0, ResultMarkdown = "ok" };
+        });
+
+        var state = TaskStateServiceBuilder.Build(dbFactory).State;
+        var wt = new WorktreeManager(new GitService(), dbFactory, _cfg, NullLogger<WorktreeManager>.Instance);
+        var runner = new TaskRunner(fake, dbFactory, new HubBroadcaster(new CapturingHubContext()), wt,
+            new ClaudeArgsBuilder(), _cfg, NullLogger<TaskRunner>.Instance, state, new TaskRunTokenRegistry(),
+            new AttachmentStore(), new FakeSessionSkillSeeder(), new FakeTranscriptUsageReader());
+
+        // Winner: mirrors QueuePicker's atomic Queued->Running claim, then a queue-slot
+        // dispatch (alreadyClaimed: true, matching QueueService.RunInSlotAsync).
+        var picker = new QueuePicker(dbFactory);
+        var claimed = await picker.ClaimNextAsync(DateTime.UtcNow, CancellationToken.None);
+        Assert.NotNull(claimed);
+
+        var winnerTask = runner.RunAsync(claimed!, "queue", CancellationToken.None, alreadyClaimed: true);
+        await processStarted.Task; // worktree exists, "Claude" is now running in it
+
+        WorktreeEntity? wtRowBefore;
+        using (var ctx = _db.CreateContext())
+            wtRowBefore = await new WorktreeRepository(ctx).GetByTaskIdAsync(taskId);
+        Assert.NotNull(wtRowBefore);
+
+        var markerPath = Path.Combine(wtRowBefore!.Path, "in-progress.marker");
+        File.WriteAllText(markerPath, "winner's work");
+
+        // Loser: a RunNow-style dispatch (unclaimed) of the SAME task arriving after the
+        // picker already claimed it. Before the fix this would recreate the worktree via
+        // WorktreeManager's self-heal, wiping the winner's live directory (and this marker
+        // file) out from under the still-running process.
+        TaskEntity taskForOverride;
+        using (var ctx = _db.CreateContext())
+            taskForOverride = (await new TaskRepository(ctx).GetByIdAsync(taskId))!;
+
+        await runner.RunAsync(taskForOverride, "override", CancellationToken.None);
+
+        Assert.True(File.Exists(markerPath), "loser dispatch touched the winner's live worktree");
+        Assert.Equal(1, fake.CallCount);
+
+        releaseProcess.TrySetResult();
+        await winnerTask;
+
+        using var verify = _db.CreateContext();
+        Assert.Single(await new TaskRunRepository(verify).GetByTaskIdAsync(taskId));
+
+        var wtRowAfter = await new WorktreeRepository(verify).GetByTaskIdAsync(taskId);
+        Assert.NotNull(wtRowAfter);
+        Assert.Equal(WorktreeState.Active, wtRowAfter!.State);
+        Assert.Equal(wtRowBefore.Path, wtRowAfter.Path);
+        Assert.True(Directory.Exists(wtRowAfter.Path));
+    }
+}