fix: prevent time-travel bug in parallel tool calling

## Problem
When multiple tools are called in parallel (e.g., update_todo_list + new_task),
tool_results could appear in API history BEFORE their corresponding tool_use
blocks, causing Anthropic API 400 errors:
"unexpected `tool_use_id` found in `tool_result` blocks"

## Root Cause
Tools executed during streaming via presentAssistantMessage() BEFORE the
assistant message was saved to history. When new_task triggered delegation
and called flushPendingToolResultsToHistory(), it saved a user message
(with tool_results) before the assistant message (with tool_uses) existed.

## Solution
Added assistantMessageSavedToHistory coordination flag to ensure correct
message ordering:

1. Flag is reset at the start of each API request
2. Flag is set to true after assistant message is saved to history
3. flushPendingToolResultsToHistory() waits for the flag before flushing
4. This ensures: [assistant with tool_uses] → [user with tool_results]

Also removed the "CRITICAL: This tool MUST be called alone" restriction
from new_task tool description to enable parallel tool calling.

## Changes
- src/core/task/Task.ts: Added assistantMessageSavedToHistory flag and
  waiting logic in flushPendingToolResultsToHistory()
- src/core/prompts/tools/native-tools/new_task.ts: Removed restriction
  preventing parallel tool calling
- src/core/task/__tests__/flushPendingToolResultsToHistory.spec.ts: Added
  tests for waiting behavior

## Testing
All 8 tests pass, including new tests verifying:
- Skips waiting when flag is already true
- Waits via pWaitFor when flag is false
- Aborts without flushing when task is aborted during wait

Author: daniel-lxs

src/core/task/Task.ts

@@ -355,6 +355,20 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 	userMessageContent: (Anthropic.TextBlockParam | Anthropic.ImageBlockParam | Anthropic.ToolResultBlockParam)[] = []
 	userMessageContentReady = false
 
+	/**
+	 * Flag indicating whether the assistant message for the current streaming session
+	 * has been saved to API conversation history.
+	 *
+	 * This is critical for parallel tool calling: tools should NOT execute until
+	 * the assistant message is saved. Otherwise, if a tool like `new_task` triggers
+	 * `flushPendingToolResultsToHistory()`, the user message with tool_results would
+	 * appear BEFORE the assistant message with tool_uses, causing API errors.
+	 *
+	 * Reset to `false` at the start of each API request.
+	 * Set to `true` after the assistant message is saved in `recursivelyMakeClineRequests`.
+	 */
+	assistantMessageSavedToHistory = false
+
 	/**
 	 * Push a tool_result block to userMessageContent, preventing duplicates.
 	 * Duplicate tool_use_ids cause API errors.
@@ -1063,6 +1077,36 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 			return
 		}
 
+		// CRITICAL: Wait for the assistant message to be saved to API history first.
+		// Without this, tool_result blocks would appear BEFORE tool_use blocks in the
+		// conversation history, causing API errors like:
+		// "unexpected `tool_use_id` found in `tool_result` blocks"
+		//
+		// This can happen when parallel tools are called (e.g., update_todo_list + new_task).
+		// Tools execute during streaming via presentAssistantMessage, BEFORE the assistant
+		// message is saved. When new_task triggers delegation, it calls this method to
+		// flush pending results - but the assistant message hasn't been saved yet.
+		//
+		// The assistantMessageSavedToHistory flag is:
+		// - Reset to false at the start of each API request
+		// - Set to true after the assistant message is saved in recursivelyMakeClineRequests
+		if (!this.assistantMessageSavedToHistory) {
+			await pWaitFor(() => this.assistantMessageSavedToHistory || this.abort, {
+				interval: 50,
+				timeout: 30_000, // 30 second timeout as safety net
+			}).catch(() => {
+				// If timeout or abort, log and proceed anyway to avoid hanging
+				console.warn(
+					`[Task#${this.taskId}] flushPendingToolResultsToHistory: timed out waiting for assistant message to be saved`,
+				)
+			})
+		}
+
+		// If task was aborted while waiting, don't flush
+		if (this.abort) {
+			return
+		}
+
 		// Save the user message with tool_result blocks
 		const userMessage: Anthropic.MessageParam = {
 			role: "user",
@@ -2707,6 +2751,7 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 				this.userMessageContentReady = false
 				this.didRejectTool = false
 				this.didAlreadyUseTool = false
+				this.assistantMessageSavedToHistory = false
 				// Reset tool failure flag for each new assistant turn - this ensures that tool failures
 				// only prevent attempt_completion within the same assistant message, not across turns
 				// (e.g., if a tool fails, then user sends a message saying "just complete anyway")
@@ -3488,6 +3533,7 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 						{ role: "assistant", content: assistantContent },
 						reasoningMessage || undefined,
 					)
+					this.assistantMessageSavedToHistory = true
 
 					TelemetryService.instance.captureConversationMessage(this.taskId, "assistant")
 				}

src/core/task/__tests__/flushPendingToolResultsToHistory.spec.ts

@@ -38,8 +38,12 @@ vi.mock("fs/promises", async (importOriginal) => {
 	}
 })
 
+const { mockPWaitFor } = vi.hoisted(() => {
+	return { mockPWaitFor: vi.fn().mockImplementation(async () => Promise.resolve()) }
+})
+
 vi.mock("p-wait-for", () => ({
-	default: vi.fn().mockImplementation(async () => Promise.resolve()),
+	default: mockPWaitFor,
 }))
 
 vi.mock("vscode", () => {
@@ -344,4 +348,99 @@ describe("flushPendingToolResultsToHistory", () => {
 		expect((task.apiConversationHistory[0] as any).ts).toBeGreaterThanOrEqual(beforeTs)
 		expect((task.apiConversationHistory[0] as any).ts).toBeLessThanOrEqual(afterTs)
 	})
+
+	it("should skip waiting for assistantMessageSavedToHistory when flag is already true", async () => {
+		const task = new Task({
+			provider: mockProvider,
+			apiConfiguration: mockApiConfig,
+			task: "test task",
+			startTask: false,
+		})
+
+		// Set flag to true (assistant message already saved)
+		task.assistantMessageSavedToHistory = true
+
+		// Set up pending tool result
+		task.userMessageContent = [
+			{
+				type: "tool_result",
+				tool_use_id: "tool-skip-wait",
+				content: "Result when flag is true",
+			},
+		]
+
+		// Clear mock call history
+		mockPWaitFor.mockClear()
+
+		await task.flushPendingToolResultsToHistory()
+
+		// Should not have called pWaitFor since flag was already true
+		expect(mockPWaitFor).not.toHaveBeenCalled()
+
+		// Should still save the message
+		expect(task.apiConversationHistory.length).toBe(1)
+		expect((task.apiConversationHistory[0].content as any[])[0].tool_use_id).toBe("tool-skip-wait")
+	})
+
+	it("should wait for assistantMessageSavedToHistory when flag is false", async () => {
+		const task = new Task({
+			provider: mockProvider,
+			apiConfiguration: mockApiConfig,
+			task: "test task",
+			startTask: false,
+		})
+
+		// Flag is false by default - assistant message not yet saved
+		expect(task.assistantMessageSavedToHistory).toBe(false)
+
+		// Set up pending tool result
+		task.userMessageContent = [
+			{
+				type: "tool_result",
+				tool_use_id: "tool-wait",
+				content: "Result when flag is false",
+			},
+		]
+
+		// Clear mock call history
+		mockPWaitFor.mockClear()
+
+		await task.flushPendingToolResultsToHistory()
+
+		// Should have called pWaitFor since flag was false
+		expect(mockPWaitFor).toHaveBeenCalled()
+
+		// Should still save the message (mock resolves immediately)
+		expect(task.apiConversationHistory.length).toBe(1)
+	})
+
+	it("should not flush when task is aborted during wait", async () => {
+		const task = new Task({
+			provider: mockProvider,
+			apiConfiguration: mockApiConfig,
+			task: "test task",
+			startTask: false,
+		})
+
+		// Flag is false - will need to wait
+		task.assistantMessageSavedToHistory = false
+
+		// Set up pending tool result
+		task.userMessageContent = [
+			{
+				type: "tool_result",
+				tool_use_id: "tool-aborted",
+				content: "Should not be saved",
+			},
+		]
+
+		// Set abort flag - this will cause the condition in pWaitFor to return true
+		// AND will cause early return after the wait
+		task.abort = true
+
+		await task.flushPendingToolResultsToHistory()
+
+		// Should not have saved anything since task was aborted
+		expect(task.apiConversationHistory.length).toBe(0)
+	})
 })