diff --git a/src/ClaudeDo.Worker/Planning/PlanningMergeOrchestrator.cs b/src/ClaudeDo.Worker/Planning/PlanningMergeOrchestrator.cs
index 503ee38..643b9a5 100644
--- a/src/ClaudeDo.Worker/Planning/PlanningMergeOrchestrator.cs
+++ b/src/ClaudeDo.Worker/Planning/PlanningMergeOrchestrator.cs
@@ -1,5 +1,6 @@
 using System.Collections.Concurrent;
 using ClaudeDo.Data;
+using ClaudeDo.Data.Git;
 using ClaudeDo.Data.Models;
 using ClaudeDo.Worker.Hub;
 using ClaudeDo.Worker.Services;
@@ -14,6 +15,7 @@ public sealed class PlanningMergeOrchestrator
     private readonly TaskMergeService _merge;
     private readonly PlanningAggregator _aggregator;
     private readonly HubBroadcaster _broadcaster;
+    private readonly GitService _git;
     private readonly ILogger<PlanningMergeOrchestrator> _logger;
 
     private sealed class State
@@ -30,30 +32,53 @@ public sealed class PlanningMergeOrchestrator
         TaskMergeService merge,
         PlanningAggregator aggregator,
         HubBroadcaster broadcaster,
+        GitService git,
         ILogger<PlanningMergeOrchestrator> logger)
     {
         _dbFactory = dbFactory;
         _merge = merge;
         _aggregator = aggregator;
         _broadcaster = broadcaster;
+        _git = git;
         _logger = logger;
     }
 
     public async Task StartAsync(string planningTaskId, string targetBranch, CancellationToken ct)
     {
+        string workingDir;
         List<TaskEntity> children;
+
         using (var ctx = _dbFactory.CreateDbContext())
         {
-            children = await ctx.Tasks
-                .Include(t => t.Worktree)
-                .Where(t => t.ParentTaskId == planningTaskId)
-                .OrderBy(t => t.SortOrder)
-                .ToListAsync(ct);
+            var planning = await ctx.Tasks
+                .Include(t => t.List)
+                .Include(t => t.Children).ThenInclude(c => c.Worktree)
+                .SingleOrDefaultAsync(t => t.Id == planningTaskId, ct)
+                ?? throw new KeyNotFoundException($"Planning task '{planningTaskId}' not found.");
+            workingDir = planning.List.WorkingDir
+                ?? throw new InvalidOperationException("List has no working directory.");
+            children = planning.Children.OrderBy(c => c.SortOrder).ToList();
         }
 
+        foreach (var c in children)
+        {
+            if (c.Status != TaskStatus.Done)
+                throw new InvalidOperationException($"subtask {c.Id} is not Done (status {c.Status})");
+            if (c.Worktree is null)
+                throw new InvalidOperationException($"subtask {c.Id} has no worktree");
+            if (c.Worktree.State != WorktreeState.Active && c.Worktree.State != WorktreeState.Merged)
+                throw new InvalidOperationException(
+                    $"subtask {c.Id} worktree state is {c.Worktree.State}");
+        }
+
+        if (await _git.IsMidMergeAsync(workingDir, ct))
+            throw new InvalidOperationException("repo is mid-merge");
+        if (await _git.HasChangesAsync(workingDir, ct))
+            throw new InvalidOperationException("working tree has uncommitted changes");
+
         var queue = new Queue<string>(
             children
-                .Where(c => c.Worktree is not null && c.Worktree.State != WorktreeState.Merged)
+                .Where(c => c.Worktree!.State == WorktreeState.Active)
                 .Select(c => c.Id));
 
         var state = new State { TargetBranch = targetBranch, RemainingSubtaskIds = queue };
diff --git a/tests/ClaudeDo.Worker.Tests/Planning/PlanningMergeOrchestratorTests.cs b/tests/ClaudeDo.Worker.Tests/Planning/PlanningMergeOrchestratorTests.cs
index 38a91b5..34b0cb3 100644
--- a/tests/ClaudeDo.Worker.Tests/Planning/PlanningMergeOrchestratorTests.cs
+++ b/tests/ClaudeDo.Worker.Tests/Planning/PlanningMergeOrchestratorTests.cs
@@ -259,8 +259,96 @@ public sealed class PlanningMergeOrchestratorTests : IDisposable
             factory, git,
             NullLogger<PlanningAggregator>.Instance);
         var orch = new PlanningMergeOrchestrator(
-            factory, merge, aggregator, broadcaster,
+            factory, merge, aggregator, broadcaster, git,
             NullLogger<PlanningMergeOrchestrator>.Instance);
         return (orch, spy.Calls);
     }
+
+    [Fact]
+    public async Task StartAsync_SubtaskStillRunning_ThrowsWithoutSideEffects()
+    {
+        var db = NewDb();
+        var repo = NewRepo();
+        GitRepoFixture.RunGit(repo.RepoDir, "branch", "-m", "main");
+
+        var (parentId, runningSub) = await SeedPlanningWithOneRunningChildAsync(db, repo);
+
+        var (orch, spy) = BuildOrchestrator(db);
+
+        var ex = await Assert.ThrowsAsync<InvalidOperationException>(
+            () => orch.StartAsync(parentId, "main", CancellationToken.None));
+        Assert.Contains(runningSub, ex.Message);
+
+        using var ctx = db.CreateContext();
+        Assert.Equal(TaskStatus.Planned, ctx.Tasks.Single(t => t.Id == parentId).Status);
+        Assert.Empty(spy);
+    }
+
+    [Fact]
+    public async Task StartAsync_DirtyRepo_ThrowsWithoutSideEffects()
+    {
+        var db = NewDb();
+        var repo = NewRepo();
+        GitRepoFixture.RunGit(repo.RepoDir, "branch", "-m", "main");
+        var (parentId, _, _) = await SeedPlanningWithTwoNonConflictingChildrenAsync(db, repo);
+
+        File.WriteAllText(Path.Combine(repo.RepoDir, "dirty.txt"), "unstaged\n");
+
+        var (orch, _) = BuildOrchestrator(db);
+
+        var ex = await Assert.ThrowsAsync<InvalidOperationException>(
+            () => orch.StartAsync(parentId, "main", CancellationToken.None));
+        Assert.Contains("uncommitted", ex.Message);
+    }
+
+    [Fact]
+    public async Task StartAsync_IdempotentRestart_SkipsAlreadyMergedWorktrees()
+    {
+        var db = NewDb();
+        var repo = NewRepo();
+        GitRepoFixture.RunGit(repo.RepoDir, "branch", "-m", "main");
+
+        var (parentId, subA, subB) = await SeedPlanningWithTwoNonConflictingChildrenAsync(db, repo);
+        using (var setup = db.CreateContext())
+        {
+            var wt = setup.Worktrees.Single(w => w.TaskId == subA);
+            wt.State = WorktreeState.Merged;
+            await setup.SaveChangesAsync();
+        }
+
+        var (orch, spy) = BuildOrchestrator(db);
+        await orch.StartAsync(parentId, "main", CancellationToken.None);
+
+        Assert.DoesNotContain(spy, c => c.Method == "PlanningSubtaskMerged" && (string)c.Args[1]! == subA);
+        Assert.Contains(spy, c => c.Method == "PlanningSubtaskMerged" && (string)c.Args[1]! == subB);
+        Assert.Contains(spy, c => c.Method == "PlanningCompleted");
+    }
+
+    private async Task<(string parentId, string runningChild)> SeedPlanningWithOneRunningChildAsync(
+        DbFixture db, GitRepoFixture repo)
+    {
+        using var ctx = db.CreateContext();
+        var listId = Guid.NewGuid().ToString();
+        ctx.Lists.Add(new ListEntity
+        {
+            Id = listId, Name = "test", CreatedAt = DateTime.UtcNow, WorkingDir = repo.RepoDir,
+        });
+        var parentId = Guid.NewGuid().ToString();
+        ctx.Tasks.Add(new TaskEntity
+        {
+            Id = parentId, ListId = listId, Title = "plan", CreatedAt = DateTime.UtcNow,
+            Status = TaskStatus.Planned, SortOrder = 0,
+        });
+        var running = Guid.NewGuid().ToString();
+        ctx.Tasks.Add(new TaskEntity
+        {
+            Id = running, ListId = listId, Title = "still running",
+            CreatedAt = DateTime.UtcNow, ParentTaskId = parentId,
+            Status = TaskStatus.Running, SortOrder = 1,
+        });
+        await ctx.SaveChangesAsync();
+        SeedWorktreeWithFile(ctx, repo, running, "fileR.txt", "R\n");
+        await ctx.SaveChangesAsync();
+        return (parentId, running);
+    }
 }