fix: prevent subtask restart on cancellation

- Added wasSubtaskCancelled flag to Task class to track cancelled subtasks
- Modified completeSubtask to accept wasCancelled parameter
- Updated cancelTask to detect subtasks and handle them differently
- Subtasks now notify parent of cancellation instead of triggering rehydration
- Added comprehensive tests for subtask cancellation behavior

Fixes #8504

Author: roomote

src/core/task/Task.ts

@@ -146,6 +146,7 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 	readonly rootTaskId?: string
 	readonly parentTaskId?: string
 	childTaskId?: string
+	wasSubtaskCancelled?: boolean
 
 	readonly instanceId: string
 	readonly metadata: TaskMetadata
@@ -1649,32 +1650,55 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 		})
 	}
 
-	public async completeSubtask(lastMessage: string) {
+	public async completeSubtask(lastMessage: string, wasCancelled: boolean = false) {
 		this.isPaused = false
 		this.childTaskId = undefined
+		this.wasSubtaskCancelled = wasCancelled
 
 		this.emit(RooCodeEventName.TaskUnpaused, this.taskId)
 
-		// Fake an answer from the subtask that it has completed running and
-		// this is the result of what it has done add the message to the chat
-		// history and to the webview ui.
-		try {
-			await this.say("subtask_result", lastMessage)
+		// Only add the subtask result if it wasn't cancelled
+		if (!wasCancelled) {
+			// Fake an answer from the subtask that it has completed running and
+			// this is the result of what it has done add the message to the chat
+			// history and to the webview ui.
+			try {
+				await this.say("subtask_result", lastMessage)
 
-			await this.addToApiConversationHistory({
-				role: "user",
-				content: [{ type: "text", text: `[new_task completed] Result: ${lastMessage}` }],
-			})
+				await this.addToApiConversationHistory({
+					role: "user",
+					content: [{ type: "text", text: `[new_task completed] Result: ${lastMessage}` }],
+				})
 
-			// Set skipPrevResponseIdOnce to ensure the next API call sends the full conversation
-			// including the subtask result, not just from before the subtask was created
-			this.skipPrevResponseIdOnce = true
-		} catch (error) {
-			this.providerRef
-				.deref()
-				?.log(`Error failed to add reply from subtask into conversation of parent task, error: ${error}`)
+				// Set skipPrevResponseIdOnce to ensure the next API call sends the full conversation
+				// including the subtask result, not just from before the subtask was created
+				this.skipPrevResponseIdOnce = true
+			} catch (error) {
+				this.providerRef
+					.deref()
+					?.log(`Error failed to add reply from subtask into conversation of parent task, error: ${error}`)
 
-			throw error
+				throw error
+			}
+		} else {
+			// If the subtask was cancelled, add a message indicating that
+			try {
+				await this.say("subtask_result", "Subtask was cancelled by user")
+
+				await this.addToApiConversationHistory({
+					role: "user",
+					content: [{ type: "text", text: `[new_task cancelled] The subtask was cancelled by the user.` }],
+				})
+
+				// Set skipPrevResponseIdOnce to ensure the next API call sends the full conversation
+				this.skipPrevResponseIdOnce = true
+			} catch (error) {
+				this.providerRef
+					.deref()
+					?.log(`Error failed to add cancellation message to parent task, error: ${error}`)
+
+				throw error
+			}
 		}
 	}
 

src/core/webview/ClineProvider.ts

@@ -481,13 +481,13 @@ export class ClineProvider
 	// exists).
 	// This is used when a subtask is finished and the parent task needs to be
 	// resumed.
-	async finishSubTask(lastMessage: string) {
+	async finishSubTask(lastMessage: string, wasCancelled: boolean = false) {
 		// Remove the last cline instance from the stack (this is the finished
 		// subtask).
 		await this.removeClineFromStack()
 		// Resume the last cline instance in the stack (if it exists - this is
 		// the 'parent' calling task).
-		await this.getCurrentTask()?.completeSubtask(lastMessage)
+		await this.getCurrentTask()?.completeSubtask(lastMessage, wasCancelled)
 	}
 	// Pending Edit Operations Management
 
@@ -2582,6 +2582,44 @@ export class ClineProvider
 
 		console.log(`[cancelTask] cancelling task ${task.taskId}.${task.instanceId}`)
 
+		// Check if this is a subtask
+		const isSubtask = task.parentTask !== undefined
+
+		if (isSubtask) {
+			// For subtasks, we handle cancellation differently to prevent automatic restart
+			console.log(`[cancelTask] Cancelling subtask ${task.taskId}.${task.instanceId}`)
+
+			// Mark this as a user-initiated cancellation
+			task.abortReason = "user_cancelled"
+
+			// Begin abort (non-blocking)
+			task.abortTask()
+
+			// Mark as abandoned to prevent residual activity
+			task.abandoned = true
+
+			// Wait for the task to finish aborting
+			await pWaitFor(
+				() =>
+					this.getCurrentTask()! === undefined ||
+					this.getCurrentTask()!.isStreaming === false ||
+					this.getCurrentTask()!.didFinishAbortingStream ||
+					this.getCurrentTask()!.isWaitingForFirstChunk,
+				{
+					timeout: 3_000,
+				},
+			).catch(() => {
+				console.error("Failed to abort subtask")
+			})
+
+			// Notify the parent task that the subtask was cancelled
+			await this.finishSubTask("Subtask was cancelled by user", true)
+
+			// Don't rehydrate - let the parent task handle what to do next
+			return
+		}
+
+		// For non-subtasks, use the original cancellation logic with rehydration
 		const { historyItem, uiMessagesFilePath } = await this.getTaskWithId(task.taskId)
 
 		// Preserve parent and root task information for history item.