fix: correct queue advancement off-by-one and child mode tracking in advanceSubtaskQueue

- Fix off-by-one: dispatch subtaskQueue[currentIndex] instead of
  subtaskQueue[nextIndex], preventing the first queued item from being
  skipped
- Fix completedMode: fetch child history to get the child actual mode
  instead of incorrectly using the parent historyItem.mode
- Update tests to reflect corrected queue semantics (subtaskQueueIndex
  represents the next item to dispatch, not the currently running item)

src/__tests__/sequential-fan-out.spec.ts

@@ -64,7 +64,7 @@ describe("advanceSubtaskQueue", () => {
 			getCurrentTask: vi.fn().mockReturnValue({ taskId: "child-1" }),
 			removeClineFromStack: vi.fn().mockResolvedValue(undefined),
 			getTaskWithId: vi.fn().mockResolvedValue({
-				historyItem: makeHistoryItem({ id: "child-1", status: "active" }),
+				historyItem: makeHistoryItem({ id: "child-1", mode: "code", status: "active" }),
 			}),
 			updateTaskHistory: vi.fn().mockResolvedValue(undefined),
 			handleModeSwitch: vi.fn().mockResolvedValue(undefined),
@@ -73,6 +73,8 @@ describe("advanceSubtaskQueue", () => {
 			log: vi.fn(),
 		}
 
+		// Queue items represent ADDITIONAL subtasks after the initial child.
+		// subtaskQueueIndex=0 means queue[0] is the next to dispatch.
 		const subtaskQueue: SubtaskQueueItem[] = [
 			{ mode: "code", message: "Step 1" },
 			{ mode: "debug", message: "Step 2" },
@@ -88,7 +90,7 @@ describe("advanceSubtaskQueue", () => {
 		const result = await (ClineProvider.prototype as any).advanceSubtaskQueue.call(provider, {
 			parentTaskId: "parent-1",
 			childTaskId: "child-1",
-			completionResultSummary: "Step 1 done",
+			completionResultSummary: "Initial task done",
 			historyItem,
 		})
 
@@ -102,11 +104,11 @@ describe("advanceSubtaskQueue", () => {
 			expect.objectContaining({ id: "child-1", status: "completed" }),
 		)
 
-		// Should have switched mode to next subtask's mode
-		expect(provider.handleModeSwitch).toHaveBeenCalledWith("debug")
+		// Should have switched mode to queue[0]'s mode (the next item to dispatch)
+		expect(provider.handleModeSwitch).toHaveBeenCalledWith("code")
 
-		// Should have created the next child with the queued message
-		expect(provider.createTask).toHaveBeenCalledWith("Step 2", undefined, undefined, {
+		// Should have created the next child with queue[0]'s message
+		expect(provider.createTask).toHaveBeenCalledWith("Step 1", undefined, undefined, {
 			initialTodos: [],
 			initialStatus: "active",
 			startTask: false,
@@ -115,12 +117,13 @@ describe("advanceSubtaskQueue", () => {
 		// Should have started the next child
 		expect(mockChild.start).toHaveBeenCalled()
 
-		// Should have updated parent with advanced queue index
+		// Should have updated parent with advanced queue index (0 -> 1)
+		// completedMode comes from child's history (mode: "code")
 		expect(provider.updateTaskHistory).toHaveBeenCalledWith(
 			expect.objectContaining({
 				id: "parent-1",
 				subtaskQueueIndex: 1,
-				subtaskResults: [{ taskId: "child-1", mode: "unknown", summary: "Step 1 done" }],
+				subtaskResults: [{ taskId: "child-1", mode: "code", summary: "Initial task done" }],
 				awaitingChildId: "child-2",
 				delegatedToId: "child-2",
 			}),
@@ -131,7 +134,7 @@ describe("advanceSubtaskQueue", () => {
 			RooCodeEventName.TaskDelegationCompleted,
 			"parent-1",
 			"child-1",
-			"Step 1 done",
+			"Initial task done",
 		)
 		expect(emitSpy).toHaveBeenCalledWith(RooCodeEventName.TaskDelegated, "parent-1", "child-2")
 	})
@@ -141,7 +144,7 @@ describe("advanceSubtaskQueue", () => {
 			getCurrentTask: vi.fn().mockReturnValue({ taskId: "child-2" }),
 			removeClineFromStack: vi.fn().mockResolvedValue(undefined),
 			getTaskWithId: vi.fn().mockResolvedValue({
-				historyItem: makeHistoryItem({ id: "child-2", status: "active" }),
+				historyItem: makeHistoryItem({ id: "child-2", mode: "code", status: "active" }),
 			}),
 			updateTaskHistory: vi.fn().mockResolvedValue(undefined),
 			handleModeSwitch: vi.fn(),
@@ -151,11 +154,13 @@ describe("advanceSubtaskQueue", () => {
 			formatAggregatedQueueResults: (ClineProvider.prototype as any).formatAggregatedQueueResults,
 		}
 
+		// Queue has 1 item, subtaskQueueIndex=1 means queue[0] was already dispatched.
+		// Now that child completes and the queue is exhausted.
 		const subtaskQueue: SubtaskQueueItem[] = [{ mode: "code", message: "Step 1" }]
 
 		const historyItem = makeHistoryItem({
 			subtaskQueue,
-			subtaskQueueIndex: 0,
+			subtaskQueueIndex: 1,
 			subtaskResults: [{ taskId: "child-1", mode: "code", summary: "Step 1 done" }],
 			childIds: ["child-1", "child-2"],
 		})

src/core/webview/ClineProvider.ts

@@ -3397,15 +3397,10 @@ export class ClineProvider
 			return { handled: false, aggregatedSummary: completionResultSummary }
 		}
 
+		// currentIndex is the next queue item to dispatch (0-based).
+		// When the initial child (from mode/message params) completes, currentIndex is 0,
+		// meaning queue[0] should be dispatched first.
 		const currentIndex = subtaskQueueIndex ?? 0
-		const nextIndex = currentIndex + 1
-
-		// Record this child's result
-		const completedMode = historyItem.mode ?? "unknown"
-		const updatedResults = [
-			...(subtaskResults ?? []),
-			{ taskId: childTaskId, mode: completedMode, summary: completionResultSummary },
-		]
 
 		// Close current child if still open
 		const current = this.getCurrentTask()
@@ -3413,9 +3408,11 @@ export class ClineProvider
 			await this.removeClineFromStack()
 		}
 
-		// Mark child as completed
+		// Fetch child history to get the child's actual mode and mark it completed
+		let completedMode = "unknown"
 		try {
 			const { historyItem: childHistory } = await this.getTaskWithId(childTaskId)
+			completedMode = childHistory.mode ?? "unknown"
 			await this.updateTaskHistory({ ...childHistory, status: "completed" })
 		} catch (err) {
 			this.log(
@@ -3425,6 +3422,12 @@ export class ClineProvider
 			)
 		}
 
+		// Record this child's result using the child's actual mode
+		const updatedResults = [
+			...(subtaskResults ?? []),
+			{ taskId: childTaskId, mode: completedMode, summary: completionResultSummary },
+		]
+
 		// Emit completion event for the finished child
 		try {
 			this.emit(RooCodeEventName.TaskDelegationCompleted, parentTaskId, childTaskId, completionResultSummary)
@@ -3432,11 +3435,11 @@ export class ClineProvider
 			// non-fatal
 		}
 
-		if (nextIndex <= subtaskQueue.length - 1) {
+		if (currentIndex < subtaskQueue.length) {
 			// More subtasks in queue — start the next one
-			const nextSubtask = subtaskQueue[nextIndex]
+			const nextSubtask = subtaskQueue[currentIndex]
 			this.log(
-				`[advanceSubtaskQueue] Auto-advancing queue: subtask ${nextIndex + 1}/${subtaskQueue.length} (mode: ${nextSubtask.mode})`,
+				`[advanceSubtaskQueue] Auto-advancing queue: subtask ${currentIndex + 1}/${subtaskQueue.length} (mode: ${nextSubtask.mode})`,
 			)
 
 			// Switch mode
@@ -3466,7 +3469,7 @@ export class ClineProvider
 				awaitingChildId: nextChild.taskId,
 				childIds,
 				subtaskQueue,
-				subtaskQueueIndex: nextIndex,
+				subtaskQueueIndex: currentIndex + 1,
 				subtaskResults: updatedResults,
 			})