fix(a2a-server): Resolve race condition in tool completion waiting (#26568)

packages/a2a-server/src/agent/race-condition.test.ts

@@ -0,0 +1,173 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect, vi, beforeEach, type Mock } from 'vitest';
+import { Task } from './task.js';
+import {
+  MessageBusType,
+  CoreToolCallStatus,
+  type Config,
+  type MessageBus,
+} from '@google/gemini-cli-core';
+import { createMockConfig } from '../utils/testing_utils.js';
+import type { RequestContext } from '@a2a-js/sdk/server';
+
+describe('Task Race Condition', () => {
+  let mockConfig: Config;
+  let messageBus: MessageBus;
+
+  beforeEach(() => {
+    messageBus = {
+      subscribe: vi.fn(),
+      unsubscribe: vi.fn(),
+      publish: vi.fn(),
+    } as unknown as MessageBus;
+    mockConfig = createMockConfig({
+      messageBus,
+    }) as Config;
+  });
+
+  it('should not hang when multiple tool confirmations are processed while waiting', async () => {
+    // @ts-expect-error - private constructor
+    const task = new Task('task-id', 'context-id', mockConfig);
+
+    // 1. Register two tools as scheduled
+    task['_registerToolCall']('tool-1', 'scheduled');
+    task['_registerToolCall']('tool-2', 'scheduled');
+
+    // 2. Both transition to awaiting_approval
+    const updateHandler = (messageBus.subscribe as Mock).mock.calls.find(
+      (c: unknown[]) => c[0] === MessageBusType.TOOL_CALLS_UPDATE,
+    )?.[1];
+
+    updateHandler({
+      type: MessageBusType.TOOL_CALLS_UPDATE,
+      schedulerId: 'task-id',
+      toolCalls: [
+        {
+          request: { callId: 'tool-1', name: 't1' },
+          status: CoreToolCallStatus.AwaitingApproval,
+          correlationId: 'corr-1',
+          confirmationDetails: { type: 'info' },
+        },
+        {
+          request: { callId: 'tool-2', name: 't2' },
+          status: CoreToolCallStatus.AwaitingApproval,
+          correlationId: 'corr-2',
+          confirmationDetails: { type: 'info' },
+        },
+      ],
+    });
+
+    // 3. Confirm Tool 1. This makes isAwaitingApprovalOnly() return false.
+    for await (const _ of task.acceptUserMessage(
+      {
+        userMessage: {
+          parts: [
+            {
+              kind: 'data',
+              data: { callId: 'tool-1', outcome: 'proceed_once' },
+            },
+          ],
+        },
+      } as unknown as RequestContext,
+      new AbortController().signal,
+    )) {
+      // consume generator
+    }
+
+    // 4. Start waiting. This should now block because Tool 1 is confirmed (so we are waiting for its execution).
+    const waitPromise = task.waitForPendingTools();
+
+    // 5. Confirm Tool 2 while waiting.
+    for await (const _ of task.acceptUserMessage(
+      {
+        userMessage: {
+          parts: [
+            {
+              kind: 'data',
+              data: { callId: 'tool-2', outcome: 'proceed_once' },
+            },
+          ],
+        },
+      } as unknown as RequestContext,
+      new AbortController().signal,
+    )) {
+      // consume generator
+    }
+
+    // 6. Both tools complete successfully
+    updateHandler({
+      type: MessageBusType.TOOL_CALLS_UPDATE,
+      schedulerId: 'task-id',
+      toolCalls: [
+        {
+          request: { callId: 'tool-1', name: 't1' },
+          status: CoreToolCallStatus.Success,
+          response: { responseParts: [] },
+        },
+        {
+          request: { callId: 'tool-2', name: 't2' },
+          status: CoreToolCallStatus.Success,
+          response: { responseParts: [] },
+        },
+      ],
+    });
+
+    // 7. Verify that the original waitPromise resolves.
+    await expect(waitPromise).resolves.toBeUndefined();
+  });
+
+  it('should reject waitForPendingTools when tools are cancelled', async () => {
+    // @ts-expect-error - private constructor
+    const task = new Task('task-id', 'context-id', mockConfig);
+
+    // 1. Register a tool
+    task['_registerToolCall']('tool-1', 'scheduled');
+
+    // 2. Start waiting
+    const waitPromise = task.waitForPendingTools();
+
+    // 3. Cancel pending tools
+    task.cancelPendingTools('User requested cancellation');
+
+    // 4. Verify waitPromise rejects with the reason
+    await expect(waitPromise).rejects.toThrow('User requested cancellation');
+  });
+
+  it('should handle concurrent tool scheduling correctly', async () => {
+    // @ts-expect-error - private constructor
+    const task = new Task('task-id', 'context-id', mockConfig);
+
+    // 1. Register a tool and start waiting
+    task['_registerToolCall']('tool-1', 'scheduled');
+    const waitPromise = task.waitForPendingTools();
+
+    // 2. Schedule another tool concurrently (e.g. from a secondary user message)
+    // This should NOT resolve the current waitPromise until both are done
+    await task.scheduleToolCalls(
+      [{ callId: 'tool-2', name: 't2', args: {} }],
+      new AbortController().signal,
+    );
+
+    expect(task['pendingToolCalls'].size).toBe(2);
+
+    // 3. Resolve tool 1
+    task['_resolveToolCall']('tool-1');
+
+    // 4. Verify waitPromise is still pending
+    let resolved = false;
+    waitPromise.then(() => (resolved = true));
+    await new Promise((resolve) => setTimeout(resolve, 10));
+    expect(resolved).toBe(false);
+
+    // 5. Resolve tool 2
+    task['_resolveToolCall']('tool-2');
+
+    // 6. Now it should resolve
+    await expect(waitPromise).resolves.toBeUndefined();
+  });
+});

packages/a2a-server/src/agent/task-event-driven.test.ts

@@ -12,6 +12,7 @@ import {
   ApprovalMode,
   Scheduler,
   type MessageBus,
+  type ToolLiveOutput,
 } from '@google/gemini-cli-core';
 import { createMockConfig } from '../utils/testing_utils.js';
 import type { ExecutionEventBus } from '@a2a-js/sdk/server';
@@ -608,6 +609,74 @@ describe('Task Event-Driven Scheduler', () => {
     );
   });
 
+  it('should handle multi-turn tool resolution correctly', async () => {
+    // @ts-expect-error - Calling private constructor
+    const task = new Task('task-id', 'context-id', mockConfig);
+
+    task['_registerToolCall']('1', 'scheduled');
+    task['_registerToolCall']('2', 'scheduled');
+
+    const handler = (messageBus.subscribe as Mock).mock.calls.find(
+      (call: unknown[]) => call[0] === MessageBusType.TOOL_CALLS_UPDATE,
+    )?.[1];
+
+    // Turn 1: Resolve tool 1
+    handler({
+      type: MessageBusType.TOOL_CALLS_UPDATE,
+      toolCalls: [
+        {
+          request: { callId: '1', name: 't1' },
+          status: 'success',
+          response: { responseParts: [] },
+        },
+      ],
+      schedulerId: 'task-id',
+    });
+
+    expect(task['pendingToolCalls'].size).toBe(1);
+    expect(task['pendingToolCalls'].has('2')).toBe(true);
+
+    // Turn 2: Resolve tool 2
+    handler({
+      type: MessageBusType.TOOL_CALLS_UPDATE,
+      toolCalls: [
+        {
+          request: { callId: '2', name: 't2' },
+          status: 'success',
+          response: { responseParts: [] },
+        },
+      ],
+      schedulerId: 'task-id',
+    });
+
+    expect(task['pendingToolCalls'].size).toBe(0);
+  });
+
+  it('should handle subagent progress events from the scheduler', async () => {
+    // @ts-expect-error - Calling private constructor
+    const task = new Task('task-id', 'context-id', mockConfig, mockEventBus);
+
+    // Trigger _schedulerOutputUpdate with subagent progress
+    task['_schedulerOutputUpdate']('tool-1', {
+      isSubagentProgress: true,
+      agentName: 'researcher',
+      recentActivity: [],
+    } as ToolLiveOutput);
+
+    expect(mockEventBus.publish).toHaveBeenCalledWith(
+      expect.objectContaining({
+        kind: 'artifact-update',
+        artifact: expect.objectContaining({
+          parts: [
+            expect.objectContaining({
+              text: expect.stringContaining('researcher'),
+            }),
+          ],
+        }),
+      }),
+    );
+  });
+
   it('should wait for executing tools before transitioning to input-required state', async () => {
     // @ts-expect-error - Calling private constructor
     const task = new Task('task-id', 'context-id', mockConfig, mockEventBus);

packages/a2a-server/src/agent/task.ts

@@ -52,6 +52,7 @@ import type {
   Artifact,
 } from '@a2a-js/sdk';
 import { v4 as uuidv4 } from 'uuid';
+import { EventEmitter } from 'node:events';
 import { logger } from '../utils/logger.js';
 import * as fs from 'node:fs/promises';
 import * as path from 'node:path';
@@ -99,11 +100,8 @@ export class Task {
   private pendingOutcomes: Map<string, ToolConfirmationOutcome | undefined> =
     new Map(); // toolCallId --> outcome
   private toolsAlreadyConfirmed: Set<string> = new Set();
-  private toolCompletionPromise?: Promise<void>;
+  private toolUpdateEmitter = new EventEmitter();
-  private toolCompletionNotifier?: {
+  private cancellationError?: Error;
-    resolve: () => void;
-    reject: (reason?: Error) => void;
-  };
 
   private constructor(
     id: string,
@@ -124,7 +122,6 @@ export class Task {
     this.taskState = 'submitted';
     this.eventBus = eventBus;
     this.completedToolCalls = [];
-    this._resetToolCompletionPromise();
     this.autoExecute = autoExecute;
     this.config.setFallbackModelHandler(
       // For a2a-server, we want to automatically switch to the fallback model
@@ -179,22 +176,9 @@ export class Task {
     return metadata;
   }
 
-  private _resetToolCompletionPromise(): void {
-    this.toolCompletionPromise = new Promise((resolve, reject) => {
-      this.toolCompletionNotifier = { resolve, reject };
-    });
-    // If there are no pending calls when reset, resolve immediately.
-    if (this.pendingToolCalls.size === 0 && this.toolCompletionNotifier) {
-      this.toolCompletionNotifier.resolve();
-    }
-  }
-
   private _registerToolCall(toolCallId: string, status: string): void {
-    const wasEmpty = this.pendingToolCalls.size === 0;
     this.pendingToolCalls.set(toolCallId, status);
-    if (wasEmpty) {
+    this.toolUpdateEmitter.emit('update');
-      this._resetToolCompletionPromise();
-    }
     logger.info(
       `[Task] Registered tool call: ${toolCallId}. Pending: ${this.pendingToolCalls.size}`,
     );
@@ -203,23 +187,47 @@ export class Task {
   private _resolveToolCall(toolCallId: string): void {
     if (this.pendingToolCalls.has(toolCallId)) {
       this.pendingToolCalls.delete(toolCallId);
+      this.toolUpdateEmitter.emit('update');
       logger.info(
         `[Task] Resolved tool call: ${toolCallId}. Pending: ${this.pendingToolCalls.size}`,
       );
-      if (this.pendingToolCalls.size === 0 && this.toolCompletionNotifier) {
-        this.toolCompletionNotifier.resolve();
-      }
     }
   }
 
-  async waitForPendingTools(): Promise<void> {
+  private isAwaitingApprovalOnly(): boolean {
     if (this.pendingToolCalls.size === 0) {
-      return Promise.resolve();
+      return false;
+    }
+    for (const [callId, status] of this.pendingToolCalls.entries()) {
+      if (
+        status !== CoreToolCallStatus.AwaitingApproval ||
+        this.toolsAlreadyConfirmed.has(callId)
+      ) {
+        return false;
+      }
+    }
+    return true;
+  }
+
+  async waitForPendingTools(): Promise<void> {
+    while (this.pendingToolCalls.size > 0 && !this.isAwaitingApprovalOnly()) {
+      if (this.cancellationError) {
+        const error = this.cancellationError;
+        this.cancellationError = undefined;
+        throw error;
+      }
+      logger.info(
+        `[Task] Waiting for ${this.pendingToolCalls.size} pending tool(s)...`,
+      );
+      await new Promise((resolve) =>
+        this.toolUpdateEmitter.once('update', resolve),
+      );
+    }
+    if (this.cancellationError) {
+      const error = this.cancellationError;
+      this.cancellationError = undefined;
+      throw error;
     }
-    logger.info(
-      `[Task] Waiting for ${this.pendingToolCalls.size} pending tool(s)...`,
-    );
-    await this.toolCompletionPromise;
   }
 
   cancelPendingTools(reason: string): void {
@@ -228,15 +236,13 @@ export class Task {
         `[Task] Cancelling all ${this.pendingToolCalls.size} pending tool calls. Reason: ${reason}`,
       );
     }
-    if (this.toolCompletionNotifier) {
+    this.cancellationError = new Error(reason);
-      this.toolCompletionNotifier.reject(new Error(reason));
-    }
     this.pendingToolCalls.clear();
     this.pendingCorrelationIds.clear();
+    this.toolsAlreadyConfirmed.clear();
 
     this.scheduler.cancelAll();
-    // Reset the promise for any future operations, ensuring it's in a clean state.
+    this.toolUpdateEmitter.emit('update');
-    this._resetToolCompletionPromise();
   }
 
   private _createTextMessage(
@@ -552,8 +558,8 @@ export class Task {
 
       // Unblock waitForPendingTools to correctly end the executor loop and release the HTTP response stream.
       // The IDE client will open a new stream with the confirmation reply.
-      if (!wasAlreadyInputRequired && this.toolCompletionNotifier) {
+      if (!wasAlreadyInputRequired) {
-        this.toolCompletionNotifier.resolve();
+        this.toolUpdateEmitter.emit('update');
       }
     }
   }
@@ -917,6 +923,7 @@ export class Task {
     const outcomeString = part.data['outcome'];
 
     this.toolsAlreadyConfirmed.add(callId);
+    this.toolUpdateEmitter.emit('update');
 
     let confirmationOutcome: ToolConfirmationOutcome | undefined;
 
@@ -1130,10 +1137,6 @@ export class Task {
       if (confirmationHandled) {
         anyConfirmationHandled = true;
         // If a confirmation was handled, the scheduler will now run the tool (or cancel it).
-        // We resolve the toolCompletionPromise manually in checkInputRequiredState
-        // to break the original execution loop, so we must reset it here so the
-        // new loop correctly awaits the tool's final execution.
-        this._resetToolCompletionPromise();
         // We don't send anything to the LLM for this part.
         // The subsequent tool execution will eventually lead to resolveToolCall.
         continue;