feat(claude-do): depends_on als Menge statt Einzelfeld

Ein Task kann jetzt auf beliebig viele Vorgaenger warten. Die Einzelspalte
tasks.depends_on_task_id wird durch die Beziehungstabelle task_dependencies
ersetzt (Composite-PK, beide FKs ON DELETE CASCADE -- das reproduziert das
alte ON DELETE SET NULL: geloeschter Vorgaenger = Kante weg = Task wieder
pickbar). BlockedByTaskId bleibt unberuehrt.

- Migration AddTaskDependencies kopiert jede bestehende depends_on_task_id
  verlustfrei in die Tabelle, bevor die Spalte fallengelassen wird
- QueuePicker gatet per doppelt negiertem NOT EXISTS: kein Task mit 0 Kanten
  wird blockiert, alle Kanten muessen done sein, eine Kante auf einen
  verschwundenen Task blockiert weiter
- SetDependsOnAsync ersetzt die ganze Menge; Zykluspruefung ist jetzt eine
  BFS ueber den Graphen, faengt also auch A->B->C->A
- blockedReason nennt alle offenen Vorgaenger (External/TaskBlocking, von
  ExternalMcpService und TaskWaitMcpTools gemeinsam genutzt)
- add_task/update_task nehmen dependsOnTaskIds; die alte Skalarform bleibt
  als Ein-Element-Menge inkl. ""-Clear-Sentinel gueltig
- UI: Fan-in-Zeile bleibt flach und listet alle Vorgaenger im After-Label,
  die lineare Kette rendert unveraendert

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

@@ -408,8 +408,7 @@ public sealed class BatchMcpToolsTests : IDisposable
         var listId = await SeedListAsync();
         var predecessor = await SeedTaskAsync(listId, "predecessor", TaskStatus.Idle);
         var task = await SeedTaskAsync(listId, "blocked", TaskStatus.Queued);
-        task.DependsOnTaskId = predecessor.Id;
-        await _tasks.UpdateAsync(task);
+        await _ctx.SeedDependsOnAsync(task.Id, predecessor.Id);
         var sut = BuildSut();
 
         var results = await sut.BatchGetTasks(new[] { task.Id }, cancellationToken: CancellationToken.None);

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

@@ -118,6 +118,10 @@ public sealed class ExternalMcpServiceTests : IDisposable
         return id;
     }
 
+    // The dependency set has no defined order, and the ids are random GUIDs -- sort both sides.
+    private static List<string> Sorted(params string[] ids) => ids.OrderBy(x => x).ToList();
+    private static List<string> Sorted(IEnumerable<string> ids) => ids.OrderBy(x => x).ToList();
+
     private async Task<TaskEntity> SeedTaskAsync(
         string listId, string title = "t", TaskStatus status = TaskStatus.Idle, string? parentId = null)
     {
@@ -444,8 +448,7 @@ public sealed class ExternalMcpServiceTests : IDisposable
         var listId = await SeedListAsync();
         var predecessor = await SeedTaskAsync(listId, status: TaskStatus.Idle);
         var task = await SeedTaskAsync(listId, status: TaskStatus.Queued);
-        task.DependsOnTaskId = predecessor.Id;
-        await _tasks.UpdateAsync(task, CancellationToken.None);
+        await _ctx.SeedDependsOnAsync(task.Id, predecessor.Id);
         var sut = BuildSut(CreateQueue());
 
         var dto = await sut.GetTask(task.Id, CancellationToken.None);
@@ -460,8 +463,7 @@ public sealed class ExternalMcpServiceTests : IDisposable
         var listId = await SeedListAsync();
         var predecessor = await SeedTaskAsync(listId, status: TaskStatus.Done);
         var task = await SeedTaskAsync(listId, status: TaskStatus.Queued);
-        task.DependsOnTaskId = predecessor.Id;
-        await _tasks.UpdateAsync(task, CancellationToken.None);
+        await _ctx.SeedDependsOnAsync(task.Id, predecessor.Id);
         var sut = BuildSut(CreateQueue());
 
         var dto = await sut.GetTask(task.Id, CancellationToken.None);
@@ -478,8 +480,7 @@ public sealed class ExternalMcpServiceTests : IDisposable
         var listId = await SeedListAsync();
         var predecessor = await SeedTaskAsync(listId, status: TaskStatus.Idle);
         var task = await SeedTaskAsync(listId, status: TaskStatus.Idle);
-        task.DependsOnTaskId = predecessor.Id;
-        await _tasks.UpdateAsync(task, CancellationToken.None);
+        await _ctx.SeedDependsOnAsync(task.Id, predecessor.Id);
         var sut = BuildSut(CreateQueue());
 
         var dto = await sut.GetTask(task.Id, CancellationToken.None);
@@ -493,8 +494,7 @@ public sealed class ExternalMcpServiceTests : IDisposable
         var listId = await SeedListAsync();
         var predecessor = await SeedTaskAsync(listId, "predecessor", TaskStatus.Idle);
         var blocked = await SeedTaskAsync(listId, "blocked", TaskStatus.Queued);
-        blocked.DependsOnTaskId = predecessor.Id;
-        await _tasks.UpdateAsync(blocked, CancellationToken.None);
+        await _ctx.SeedDependsOnAsync(blocked.Id, predecessor.Id);
         var unblocked = await SeedTaskAsync(listId, "unblocked", TaskStatus.Queued);
         var sut = BuildSut(CreateQueue());
 
@@ -513,8 +513,7 @@ public sealed class ExternalMcpServiceTests : IDisposable
         var listId = await SeedListAsync();
         var predecessor = await SeedTaskAsync(listId, "predecessor", TaskStatus.Idle);
         var blocked = await SeedTaskAsync(listId, "blocked", TaskStatus.Queued);
-        blocked.DependsOnTaskId = predecessor.Id;
-        await _tasks.UpdateAsync(blocked, CancellationToken.None);
+        await _ctx.SeedDependsOnAsync(blocked.Id, predecessor.Id);
         var sut = BuildSut(CreateQueue());
 
         var result = await sut.ListTasks(listId, includeDescription: true, cancellationToken: CancellationToken.None);
@@ -555,10 +554,10 @@ public sealed class ExternalMcpServiceTests : IDisposable
 
         var dto = await sut.UpdateTask(task.Id, dependsOnTaskId: predecessor.Id, cancellationToken: CancellationToken.None);
 
-        Assert.Equal(predecessor.Id, dto.DependsOnTaskId);
+        Assert.Equal([predecessor.Id], dto.DependsOnTaskIds);
         Assert.True(dto.Blocked);
         var loaded = await _tasks.GetByIdAsync(task.Id);
-        Assert.Equal(predecessor.Id, loaded!.DependsOnTaskId);
+        Assert.Equal([predecessor.Id], loaded!.DependsOnTaskIds);
     }
 
     [Fact]
@@ -572,7 +571,7 @@ public sealed class ExternalMcpServiceTests : IDisposable
 
         var dto = await sut.UpdateTask(task.Id, dependsOnTaskId: "", cancellationToken: CancellationToken.None);
 
-        Assert.Null(dto.DependsOnTaskId);
+        Assert.Empty(dto.DependsOnTaskIds!);
         Assert.False(dto.Blocked);
     }
 
@@ -589,6 +588,75 @@ public sealed class ExternalMcpServiceTests : IDisposable
             sut.UpdateTask(a.Id, dependsOnTaskId: b.Id, cancellationToken: CancellationToken.None));
     }
 
+    [Fact]
+    public async Task UpdateTask_WithDependsOnTaskIds_SetsSeveralPredecessors()
+    {
+        var listId = await SeedListAsync();
+        var a = await SeedTaskAsync(listId, "a", TaskStatus.Idle);
+        var b = await SeedTaskAsync(listId, "b", TaskStatus.Idle);
+        var task = await SeedTaskAsync(listId, status: TaskStatus.Queued);
+        var sut = BuildSut(CreateQueue());
+
+        var dto = await sut.UpdateTask(
+            task.Id, dependsOnTaskIds: [a.Id, b.Id], cancellationToken: CancellationToken.None);
+
+        Assert.Equal(Sorted(a.Id, b.Id), Sorted(dto.DependsOnTaskIds!));
+        Assert.True(dto.Blocked);
+        // Naming only the first predecessor would send a caller off to unblock the wrong task.
+        Assert.Contains(a.Id, dto.BlockedReason);
+        Assert.Contains(b.Id, dto.BlockedReason);
+    }
+
+    [Fact]
+    public async Task UpdateTask_WithEmptyDependsOnTaskIds_ClearsTheWholeSet()
+    {
+        var listId = await SeedListAsync();
+        var a = await SeedTaskAsync(listId, "a", TaskStatus.Idle);
+        var b = await SeedTaskAsync(listId, "b", TaskStatus.Idle);
+        var task = await SeedTaskAsync(listId, status: TaskStatus.Queued);
+        var sut = BuildSut(CreateQueue());
+        await sut.UpdateTask(task.Id, dependsOnTaskIds: [a.Id, b.Id], cancellationToken: CancellationToken.None);
+
+        var dto = await sut.UpdateTask(task.Id, dependsOnTaskIds: [], cancellationToken: CancellationToken.None);
+
+        Assert.Empty(dto.DependsOnTaskIds!);
+        Assert.False(dto.Blocked);
+    }
+
+    [Fact]
+    public async Task UpdateTask_LegacyScalarDependsOnTaskId_StillReplacesTheWholeSet()
+    {
+        // Back-compat: existing callers and prompts only know the scalar form. It has to keep
+        // behaving as "this is the one predecessor", not append to whatever is already there.
+        var listId = await SeedListAsync();
+        var a = await SeedTaskAsync(listId, "a", TaskStatus.Idle);
+        var b = await SeedTaskAsync(listId, "b", TaskStatus.Idle);
+        var task = await SeedTaskAsync(listId, status: TaskStatus.Queued);
+        var sut = BuildSut(CreateQueue());
+        await sut.UpdateTask(task.Id, dependsOnTaskIds: [a.Id, b.Id], cancellationToken: CancellationToken.None);
+
+        var dto = await sut.UpdateTask(task.Id, dependsOnTaskId: a.Id, cancellationToken: CancellationToken.None);
+
+        Assert.Equal([a.Id], dto.DependsOnTaskIds);
+    }
+
+    [Fact]
+    public async Task UpdateTask_WithCyclicDependsOnTaskIds_Throws()
+    {
+        var listId = await SeedListAsync();
+        var a = await SeedTaskAsync(listId, "a", TaskStatus.Idle);
+        var b = await SeedTaskAsync(listId, "b", TaskStatus.Idle);
+        var c = await SeedTaskAsync(listId, "c", TaskStatus.Idle);
+        var innocent = await SeedTaskAsync(listId, "innocent", TaskStatus.Idle);
+        var sut = BuildSut(CreateQueue());
+        await sut.UpdateTask(b.Id, dependsOnTaskId: a.Id, cancellationToken: CancellationToken.None);
+        await sut.UpdateTask(c.Id, dependsOnTaskId: b.Id, cancellationToken: CancellationToken.None);
+
+        // a -> {innocent, c} closes a -> c -> b -> a over three edges.
+        await Assert.ThrowsAsync<InvalidOperationException>(() =>
+            sut.UpdateTask(a.Id, dependsOnTaskIds: [innocent.Id, c.Id], cancellationToken: CancellationToken.None));
+    }
+
     [Fact]
     public async Task ReviewTask_Approve_SetsDone()
     {
@@ -1488,11 +1556,29 @@ public sealed class ExternalMcpServiceTests : IDisposable
         var dto = await sut.AddTask(
             listId, "t", dependsOnTaskId: predecessor.Id, queueImmediately: true, cancellationToken: CancellationToken.None);
 
-        Assert.Equal(predecessor.Id, dto.Task.DependsOnTaskId);
+        Assert.Equal([predecessor.Id], dto.Task.DependsOnTaskIds);
         Assert.True(dto.Task.Blocked);
         Assert.Contains(predecessor.Id, dto.Task.BlockedReason);
         var loaded = await _tasks.GetByIdAsync(dto.Task.Id);
-        Assert.Equal(predecessor.Id, loaded!.DependsOnTaskId);
+        Assert.Equal([predecessor.Id], loaded!.DependsOnTaskIds);
+    }
+
+    [Fact]
+    public async Task AddTask_WithDependsOnTaskIds_PersistsEveryLink()
+    {
+        var listId = await SeedListAsync();
+        var a = await SeedTaskAsync(listId, "a", TaskStatus.Idle);
+        var b = await SeedTaskAsync(listId, "b", TaskStatus.Done);
+        var sut = NewService();
+
+        var dto = await sut.AddTask(
+            listId, "t", dependsOnTaskIds: [a.Id, b.Id], queueImmediately: true,
+            cancellationToken: CancellationToken.None);
+
+        Assert.Equal(Sorted(a.Id, b.Id), Sorted(dto.Task.DependsOnTaskIds!));
+        Assert.True(dto.Task.Blocked);
+        var loaded = await _tasks.GetByIdAsync(dto.Task.Id);
+        Assert.Equal(Sorted(a.Id, b.Id), Sorted(loaded!.DependsOnTaskIds));
     }
 
     [Fact]

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

@@ -220,8 +220,7 @@ public sealed class TaskWaitMcpToolsTests : IDisposable
     {
         var dependency = await SeedTaskAsync(TaskStatus.Idle);
         var task = await SeedTaskAsync(TaskStatus.Queued);
-        task.DependsOnTaskId = dependency.Id;
-        await _tasks.UpdateAsync(task);
+        await _ctx.SeedDependsOnAsync(task.Id, dependency.Id);
         var sut = BuildSut();
         var sw = Stopwatch.StartNew();
 
@@ -240,8 +239,7 @@ public sealed class TaskWaitMcpToolsTests : IDisposable
     {
         var dependency = await SeedTaskAsync(TaskStatus.Done);
         var task = await SeedTaskAsync(TaskStatus.Queued);
-        task.DependsOnTaskId = dependency.Id;
-        await _tasks.UpdateAsync(task);
+        await _ctx.SeedDependsOnAsync(task.Id, dependency.Id);
         var sut = BuildSut();
         var sw = Stopwatch.StartNew();
 

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

@@ -0,0 +1,20 @@
+using ClaudeDo.Data;
+using ClaudeDo.Data.Models;
+
+namespace ClaudeDo.Worker.Tests.Infrastructure;
+
+internal static class TestDependencies
+{
+    /// <summary>
+    /// Seeds task_dependencies rows straight through the context. The production writer is
+    /// <c>TaskStateService.SetDependsOnAsync</c>, which most read-path fixtures don't build —
+    /// this is the "arrange" shortcut, not a second writer.
+    /// </summary>
+    public static async Task SeedDependsOnAsync(
+        this ClaudeDoDbContext ctx, string taskId, params string[] dependsOnTaskIds)
+    {
+        ctx.TaskDependencies.AddRange(dependsOnTaskIds.Select(id =>
+            new TaskDependencyEntity { TaskId = taskId, DependsOnTaskId = id }));
+        await ctx.SaveChangesAsync();
+    }
+}

tests/ClaudeDo.Worker.Tests/Planning/PlanningMergeOrchestratorTests.cs

@@ -908,7 +908,7 @@ file sealed class ApproveObservingTaskStateService : ITaskStateService
     public Task<TransitionResult> FinalizePlanningAsync(string parentId, CancellationToken ct) => _inner.FinalizePlanningAsync(parentId, ct);
     public Task<TransitionResult> BlockOnAsync(string taskId, string predecessorTaskId, CancellationToken ct) => _inner.BlockOnAsync(taskId, predecessorTaskId, ct);
     public Task<TransitionResult> UnblockAsync(string taskId, CancellationToken ct) => _inner.UnblockAsync(taskId, ct);
-    public Task<TransitionResult> SetDependsOnAsync(string taskId, string? dependsOnTaskId, CancellationToken ct) => _inner.SetDependsOnAsync(taskId, dependsOnTaskId, ct);
+    public Task<TransitionResult> SetDependsOnAsync(string taskId, IReadOnlyList<string> dependsOnTaskIds, CancellationToken ct) => _inner.SetDependsOnAsync(taskId, dependsOnTaskIds, ct);
     public Task TryAdvanceParentAsync(string parentId) => _inner.TryAdvanceParentAsync(parentId);
     public Task<int> RecoverStaleRunningAsync(string reason, CancellationToken ct) => _inner.RecoverStaleRunningAsync(reason, ct);
 }

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

@@ -50,7 +50,7 @@ public sealed class QueuePickerTests : IDisposable
         bool taskAgentTag = false,
         int? sortOrder = null,
         bool isManual = false,
-        string? dependsOn = null)
+        params string[] dependsOn)
     {
         var task = new TaskEntity
         {
@@ -61,11 +61,12 @@ public sealed class QueuePickerTests : IDisposable
             CreatedAt = createdAt ?? DateTime.UtcNow,
             ScheduledFor = scheduledFor,
             BlockedByTaskId = blockedBy,
-            DependsOnTaskId = dependsOn,
             CommitType = "feat",
             IsManual = isManual,
         };
         await _tasks.AddAsync(task);
+        if (dependsOn.Length > 0)
+            await _ctx.SeedDependsOnAsync(task.Id, dependsOn);
         if (sortOrder is not null)
         {
             task.SortOrder = sortOrder.Value;
@@ -117,7 +118,7 @@ public sealed class QueuePickerTests : IDisposable
     {
         var listId = await CreateListAsync();
         var dependency = await SeedAsync(listId, status: TaskStatus.Idle);
-        await SeedAsync(listId, dependsOn: dependency.Id);
+        await SeedAsync(listId, dependsOn: [dependency.Id]);
 
         // The dependency is only Idle (not Done), so both stay unclaimed: the dependent is
         // gated on depends_on_task_id, and the dependency itself was never queued.
@@ -129,7 +130,7 @@ public sealed class QueuePickerTests : IDisposable
     {
         var listId = await CreateListAsync();
         var dependency = await SeedAsync(listId, status: TaskStatus.Done);
-        var dependent = await SeedAsync(listId, dependsOn: dependency.Id);
+        var dependent = await SeedAsync(listId, dependsOn: [dependency.Id]);
 
         var picked = await _picker.ClaimNextAsync(DateTime.UtcNow, CancellationToken.None);
 
@@ -142,7 +143,7 @@ public sealed class QueuePickerTests : IDisposable
     {
         var listId = await CreateListAsync();
         var dependency = await SeedAsync(listId, status: TaskStatus.Failed);
-        await SeedAsync(listId, dependsOn: dependency.Id);
+        await SeedAsync(listId, dependsOn: [dependency.Id]);
         var independent = await SeedAsync(listId, createdAt: DateTime.UtcNow.AddMinutes(1));
 
         // A Failed dependency does not cascade-cancel or auto-unblock the dependent (unlike the
@@ -155,6 +156,55 @@ public sealed class QueuePickerTests : IDisposable
         Assert.Null(second);
     }
 
+    [Fact]
+    public async Task ClaimNextAsync_Skips_TaskWhoseDependsOnCancelled()
+    {
+        var listId = await CreateListAsync();
+        var dependency = await SeedAsync(listId, status: TaskStatus.Cancelled);
+        await SeedAsync(listId, dependsOn: [dependency.Id]);
+
+        Assert.Null(await _picker.ClaimNextAsync(DateTime.UtcNow, CancellationToken.None));
+    }
+
+    [Fact]
+    public async Task ClaimNextAsync_Skips_TaskWithOneOfTwoDependenciesUnmet()
+    {
+        var listId = await CreateListAsync();
+        var done = await SeedAsync(listId, status: TaskStatus.Done);
+        var running = await SeedAsync(listId, status: TaskStatus.Running);
+        await SeedAsync(listId, dependsOn: [done.Id, running.Id]);
+
+        Assert.Null(await _picker.ClaimNextAsync(DateTime.UtcNow, CancellationToken.None));
+    }
+
+    [Fact]
+    public async Task ClaimNextAsync_Claims_TaskOnceAllDependenciesAreDone()
+    {
+        var listId = await CreateListAsync();
+        var first = await SeedAsync(listId, status: TaskStatus.Done);
+        var second = await SeedAsync(listId, status: TaskStatus.Done);
+        var dependent = await SeedAsync(listId, dependsOn: [first.Id, second.Id]);
+
+        var picked = await _picker.ClaimNextAsync(DateTime.UtcNow, CancellationToken.None);
+
+        Assert.NotNull(picked);
+        Assert.Equal(dependent.Id, picked!.Id);
+    }
+
+    [Fact]
+    public async Task ClaimNextAsync_Claims_TaskWithoutAnyDependency()
+    {
+        // The gating clause is "no edge whose predecessor isn't done" — a task with zero edges
+        // must stay eligible, which the doubly-negated form is easy to get wrong.
+        var listId = await CreateListAsync();
+        var task = await SeedAsync(listId);
+
+        var picked = await _picker.ClaimNextAsync(DateTime.UtcNow, CancellationToken.None);
+
+        Assert.NotNull(picked);
+        Assert.Equal(task.Id, picked!.Id);
+    }
+
     [Fact]
     public async Task ClaimNextAsync_Picks_TasksWithoutAgentTag()
     {

tests/ClaudeDo.Worker.Tests/State/TaskStateServiceTests.cs

@@ -102,6 +102,17 @@ public sealed class TaskStateServiceTests : IDisposable
         return await new TaskRepository(ctx).GetByIdAsync(id) ?? throw new InvalidOperationException($"task {id} not found");
     }
 
+    private async Task<List<string>> GetDependsOnAsync(string id)
+    {
+        await using var ctx = _factory.CreateDbContext();
+        return await ctx.TaskDependencies.Where(d => d.TaskId == id)
+            .Select(d => d.DependsOnTaskId).ToListAsync();
+    }
+
+    // The dependency set has no defined order, and the ids are random GUIDs -- sort both sides.
+    private static List<string> Sorted(params string[] ids) => ids.OrderBy(x => x).ToList();
+    private static List<string> Sorted(IEnumerable<string> ids) => ids.OrderBy(x => x).ToList();
+
     // ─── EnqueueAsync ─────────────────────────────────────────────────────
 
     [Fact]
@@ -659,31 +670,55 @@ public sealed class TaskStateServiceTests : IDisposable
     // ─── SetDependsOnAsync ────────────────────────────────────────────────
 
     [Fact]
-    public async Task SetDependsOnAsync_SetsDependsOnTaskId()
+    public async Task SetDependsOnAsync_SetsDependsOnTaskIds()
     {
         var predecessor = await SeedTaskAsync(TaskStatus.Idle);
         var task = await SeedTaskAsync(TaskStatus.Queued);
 
-        var result = await _sut.SetDependsOnAsync(task, predecessor, default);
+        var result = await _sut.SetDependsOnAsync(task, [predecessor], default);
 
         Assert.True(result.Ok);
-        var t = await GetTaskAsync(task);
-        Assert.Equal(predecessor, t.DependsOnTaskId);
+        Assert.Equal([predecessor], await GetDependsOnAsync(task));
     }
 
     [Fact]
-    public async Task SetDependsOnAsync_Null_ClearsDependsOnTaskId_AndWakesQueue()
+    public async Task SetDependsOnAsync_SetsSeveralPredecessors()
+    {
+        var a = await SeedTaskAsync(TaskStatus.Idle);
+        var b = await SeedTaskAsync(TaskStatus.Idle);
+        var task = await SeedTaskAsync(TaskStatus.Queued);
+
+        Assert.True((await _sut.SetDependsOnAsync(task, [a, b], default)).Ok);
+
+        Assert.Equal(Sorted(a, b), Sorted(await GetDependsOnAsync(task)));
+    }
+
+    [Fact]
+    public async Task SetDependsOnAsync_ReplacingSet_KeepsOverlapAndDropsTheRest()
+    {
+        var a = await SeedTaskAsync(TaskStatus.Idle);
+        var b = await SeedTaskAsync(TaskStatus.Idle);
+        var c = await SeedTaskAsync(TaskStatus.Idle);
+        var task = await SeedTaskAsync(TaskStatus.Queued);
+        Assert.True((await _sut.SetDependsOnAsync(task, [a, b], default)).Ok);
+
+        Assert.True((await _sut.SetDependsOnAsync(task, [b, c], default)).Ok);
+
+        Assert.Equal(Sorted(b, c), Sorted(await GetDependsOnAsync(task)));
+    }
+
+    [Fact]
+    public async Task SetDependsOnAsync_Empty_ClearsDependsOnTaskIds_AndWakesQueue()
     {
         var predecessor = await SeedTaskAsync(TaskStatus.Idle);
         var task = await SeedTaskAsync(TaskStatus.Queued);
-        await _sut.SetDependsOnAsync(task, predecessor, default);
+        await _sut.SetDependsOnAsync(task, [predecessor], default);
         var wakesBefore = _built.WakeCount();
 
-        var result = await _sut.SetDependsOnAsync(task, null, default);
+        var result = await _sut.SetDependsOnAsync(task, [], default);
 
         Assert.True(result.Ok);
-        var t = await GetTaskAsync(task);
-        Assert.Null(t.DependsOnTaskId);
+        Assert.Empty(await GetDependsOnAsync(task));
         Assert.True(_built.WakeCount() > wakesBefore);
     }
 
@@ -692,11 +727,10 @@ public sealed class TaskStateServiceTests : IDisposable
     {
         var task = await SeedTaskAsync(TaskStatus.Queued);
 
-        var result = await _sut.SetDependsOnAsync(task, task, default);
+        var result = await _sut.SetDependsOnAsync(task, [task], default);
 
         Assert.False(result.Ok);
-        var t = await GetTaskAsync(task);
-        Assert.Null(t.DependsOnTaskId);
+        Assert.Empty(await GetDependsOnAsync(task));
     }
 
     [Fact]
@@ -704,7 +738,7 @@ public sealed class TaskStateServiceTests : IDisposable
     {
         var task = await SeedTaskAsync(TaskStatus.Queued);
 
-        var result = await _sut.SetDependsOnAsync(task, "does-not-exist", default);
+        var result = await _sut.SetDependsOnAsync(task, ["does-not-exist"], default);
 
         Assert.False(result.Ok);
     }
@@ -714,7 +748,7 @@ public sealed class TaskStateServiceTests : IDisposable
     {
         var predecessor = await SeedTaskAsync(TaskStatus.Idle);
 
-        var result = await _sut.SetDependsOnAsync("does-not-exist", predecessor, default);
+        var result = await _sut.SetDependsOnAsync("does-not-exist", [predecessor], default);
 
         Assert.False(result.Ok);
     }
@@ -724,15 +758,14 @@ public sealed class TaskStateServiceTests : IDisposable
     {
         var a = await SeedTaskAsync(TaskStatus.Queued);
         var b = await SeedTaskAsync(TaskStatus.Queued);
-        var setup = await _sut.SetDependsOnAsync(b, a, default);
+        var setup = await _sut.SetDependsOnAsync(b, [a], default);
         Assert.True(setup.Ok);
 
         // a -> b would close the loop a -> b -> a.
-        var result = await _sut.SetDependsOnAsync(a, b, default);
+        var result = await _sut.SetDependsOnAsync(a, [b], default);
 
         Assert.False(result.Ok);
-        var t = await GetTaskAsync(a);
-        Assert.Null(t.DependsOnTaskId);
+        Assert.Empty(await GetDependsOnAsync(a));
     }
 
     [Fact]
@@ -741,15 +774,31 @@ public sealed class TaskStateServiceTests : IDisposable
         var a = await SeedTaskAsync(TaskStatus.Queued);
         var b = await SeedTaskAsync(TaskStatus.Queued);
         var c = await SeedTaskAsync(TaskStatus.Queued);
-        Assert.True((await _sut.SetDependsOnAsync(b, a, default)).Ok);
-        Assert.True((await _sut.SetDependsOnAsync(c, b, default)).Ok);
+        Assert.True((await _sut.SetDependsOnAsync(b, [a], default)).Ok);
+        Assert.True((await _sut.SetDependsOnAsync(c, [b], default)).Ok);
 
         // a -> c would close the loop a -> c -> b -> a.
-        var result = await _sut.SetDependsOnAsync(a, c, default);
+        var result = await _sut.SetDependsOnAsync(a, [c], default);
 
         Assert.False(result.Ok);
     }
 
+    [Fact]
+    public async Task SetDependsOnAsync_CycleViaOneOfSeveralProposedEdges_Rejected()
+    {
+        // Only the second proposed edge closes a loop -- the check must look at every one of
+        // them, not just the first, and must leave the whole set unwritten when one fails.
+        var a = await SeedTaskAsync(TaskStatus.Queued);
+        var b = await SeedTaskAsync(TaskStatus.Queued);
+        var innocent = await SeedTaskAsync(TaskStatus.Idle);
+        Assert.True((await _sut.SetDependsOnAsync(b, [a], default)).Ok);
+
+        var result = await _sut.SetDependsOnAsync(a, [innocent, b], default);
+
+        Assert.False(result.Ok);
+        Assert.Empty(await GetDependsOnAsync(a));
+    }
+
     // ─── RecoverStaleRunningAsync ─────────────────────────────────────────
 
     [Fact]