feat(agentchannel): add DependsOn to SubtaskSpec so a decomposing task can order its own subtasks

3 files changed, 78 insertions, 0 deletions

diff --git a/internal/agentchannel/agentchannel.go b/internal/agentchannel/agentchannel.go
index 202283e..98104df 100644
--- a/internal/agentchannel/agentchannel.go
+++ b/internal/agentchannel/agentchannel.go
@@ -44,6 +44,14 @@ type SubtaskSpec struct {
 	// any other role-typed task. Empty (the default) preserves the prior
 	// hardcoded "claude" behavior for backward compatibility.
 	Role string
+	// DependsOn, when non-empty, names sibling task IDs (returned by a prior
+	// SpawnSubtask call in the same decomposition) this new subtask must wait
+	// for — set directly on the created child's task.DependsOn, so the
+	// existing dispatch-gating and cascade-fail-on-dependency-failure
+	// machinery (already built for top-level tasks) applies unchanged. Empty
+	// (the default) preserves today's behavior: every spawned subtask is
+	// structurally independent/parallel.
+	DependsOn []string
 }
 
 // EpicProposal describes an epic a discovery/planner-role agent wants to
diff --git a/internal/executor/channel.go b/internal/executor/channel.go
index 5afe22e..9392c77 100644
--- a/internal/executor/channel.go
+++ b/internal/executor/channel.go
@@ -156,6 +156,7 @@ func (c *storeChannel) SpawnSubtask(_ context.Context, spec SubtaskSpec) (string
 		ParentTaskID: c.taskID,
 		Agent:        agent,
 		Priority:     task.PriorityNormal,
+		DependsOn:    spec.DependsOn,
 		State:        task.StatePending,
 		CreatedAt:    now,
 		UpdatedAt:    now,
diff --git a/internal/executor/channel_test.go b/internal/executor/channel_test.go
index 522f23a..84cad89 100644
--- a/internal/executor/channel_test.go
+++ b/internal/executor/channel_test.go
@@ -269,6 +269,75 @@ func TestStoreChannel_SpawnSubtask_PropagatesError(t *testing.T) {
 	}
 }
 
+// TestStoreChannel_SpawnSubtask_SetsDependsOn proves a caller can chain a
+// new subtask after a sibling it already spawned, by passing the sibling's
+// returned ID back in DependsOn -- the mechanism a decomposing task uses to
+// express "this step must wait for that one", not just fan-out-and-wait.
+func TestStoreChannel_SpawnSubtask_SetsDependsOn(t *testing.T) {
+	store := &fakeChannelStore{}
+	ch := newStoreChannel(store, "parent-task-1")
+
+	firstID, err := ch.SpawnSubtask(context.Background(), agentchannel.SubtaskSpec{
+		Name:         "step 1",
+		Instructions: "do the first thing",
+	})
+	if err != nil {
+		t.Fatalf("spawn first subtask: %v", err)
+	}
+
+	secondID, err := ch.SpawnSubtask(context.Background(), agentchannel.SubtaskSpec{
+		Name:         "step 2",
+		Instructions: "do the second thing",
+		DependsOn:    []string{firstID},
+	})
+	if err != nil {
+		t.Fatalf("spawn second subtask: %v", err)
+	}
+
+	var second *task.Task
+	for _, ct := range store.createdTasks {
+		if ct.ID == secondID {
+			second = ct
+		}
+	}
+	if second == nil {
+		t.Fatal("second subtask not found in store.createdTasks")
+	}
+	if len(second.DependsOn) != 1 || second.DependsOn[0] != firstID {
+		t.Errorf("expected DependsOn=[%q], got %v", firstID, second.DependsOn)
+	}
+}
+
+// TestStoreChannel_SpawnSubtask_DependsOnDefaultsEmpty proves the
+// backward-compatible default: a caller that never sets DependsOn (every
+// pre-existing caller) gets a child task with no dependencies, exactly as
+// before this change.
+func TestStoreChannel_SpawnSubtask_DependsOnDefaultsEmpty(t *testing.T) {
+	store := &fakeChannelStore{}
+	ch := newStoreChannel(store, "parent-task-1")
+
+	id, err := ch.SpawnSubtask(context.Background(), agentchannel.SubtaskSpec{
+		Name:         "solo step",
+		Instructions: "do the thing",
+	})
+	if err != nil {
+		t.Fatalf("spawn subtask: %v", err)
+	}
+
+	var got *task.Task
+	for _, ct := range store.createdTasks {
+		if ct.ID == id {
+			got = ct
+		}
+	}
+	if got == nil {
+		t.Fatal("subtask not found in store.createdTasks")
+	}
+	if len(got.DependsOn) != 0 {
+		t.Errorf("expected empty DependsOn by default, got %v", got.DependsOn)
+	}
+}
+
 func TestStoreChannel_RecordProgress_EmitsAgentMessage(t *testing.T) {
 	store := &fakeChannelStore{}
 	ch := newStoreChannel(store, "task-1")