feat(core): Implement parallel FC for read only tools. (#18791)

integration-tests/parallel-tools.responses

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+{"method":"generateContentStream","response":[{"candidates":[{"content":{"parts":[{"functionCall":{"name":"read_file","args":{"file_path":"file1.txt"}}},{"functionCall":{"name":"read_file","args":{"file_path":"file2.txt"}}},{"functionCall":{"name":"write_file","args":{"file_path":"output.txt","content":"wave2"}}},{"functionCall":{"name":"read_file","args":{"file_path":"file3.txt"}}},{"functionCall":{"name":"read_file","args":{"file_path":"file4.txt"}}}, {"text":"All waves completed successfully."}]},"finishReason":"STOP","index":0}]}]}

integration-tests/parallel-tools.test.ts

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+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect, beforeEach, afterEach } from 'vitest';
+import { TestRig } from './test-helper.js';
+import { join } from 'node:path';
+import fs from 'node:fs';
+
+describe('Parallel Tool Execution Integration', () => {
+  let rig: TestRig;
+
+  beforeEach(() => {
+    rig = new TestRig();
+  });
+
+  afterEach(async () => {
+    await rig.cleanup();
+  });
+
+  it('should execute [read, read, write, read, read] in correct waves with user approval', async () => {
+    rig.setup('parallel-wave-execution', {
+      fakeResponsesPath: join(import.meta.dirname, 'parallel-tools.responses'),
+      settings: {
+        tools: {
+          core: ['read_file', 'write_file'],
+          approval: 'ASK', // Disable YOLO mode to show permission prompts
+          confirmationRequired: ['write_file'],
+        },
+      },
+    });
+
+    rig.createFile('file1.txt', 'c1');
+    rig.createFile('file2.txt', 'c2');
+    rig.createFile('file3.txt', 'c3');
+    rig.createFile('file4.txt', 'c4');
+    rig.sync();
+
+    const run = await rig.runInteractive({ approvalMode: 'default' });
+
+    // 1. Trigger the wave
+    await run.type('ok');
+    await run.type('\r');
+
+    // 3. Wait for the write_file prompt.
+    await run.expectText('Allow', 5000);
+
+    // 4. Press Enter to approve the write_file.
+    await run.type('y');
+    await run.type('\r');
+
+    // 5. Wait for the final model response
+    await run.expectText('All waves completed successfully.', 5000);
+
+    // Verify all tool calls were made and succeeded in the logs
+    await rig.expectToolCallSuccess(['write_file']);
+    const toolLogs = rig.readToolLogs();
+
+    const readFiles = toolLogs.filter(
+      (l) => l.toolRequest.name === 'read_file',
+    );
+    const writeFiles = toolLogs.filter(
+      (l) => l.toolRequest.name === 'write_file',
+    );
+
+    expect(readFiles.length).toBe(4);
+    expect(writeFiles.length).toBe(1);
+    expect(toolLogs.every((l) => l.toolRequest.success)).toBe(true);
+
+    // Check that output.txt was actually written
+    expect(fs.readFileSync(join(rig.testDir!, 'output.txt'), 'utf8')).toBe(
+      'wave2',
+    );
+  });
+});