Don't prematurely end convo w/ Gemini.

- There seems to be a root model bug where the model will preemptively bail on conversations without trying harder. Typically the stops are VERY obvious and bug-looking where you need to prmopt the model to "continue".
- This PR attempts to fix the above by running a 2.0-flash request (don't need somethign more powerful) at the end of every full interaction to see who should speak (user or model).
- Add tests for nextSpeakerChecker

Fixes https://b.corp.google.com/issues/416826051

packages/server/src/utils/nextSpeakerChecker.test.ts

@@ -0,0 +1,186 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect, vi, beforeEach, Mock, afterEach } from 'vitest';
+import { Chat, Content } from '@google/genai';
+import { GeminiClient } from '../core/client.js';
+import { Config } from '../config/config.js'; // Added Config import
+import { checkNextSpeaker, NextSpeakerResponse } from './nextSpeakerChecker.js';
+
+// Mock GeminiClient and Config constructor
+vi.mock('../core/client.js');
+vi.mock('../config/config.js');
+
+// Mock @google/genai
+const mockGetHistory = vi.fn();
+const mockCreateChat = vi.fn(() => ({
+  getHistory: mockGetHistory,
+}));
+
+vi.mock('@google/genai', async () => {
+  const actualGenAI =
+    await vi.importActual<typeof import('@google/genai')>('@google/genai');
+  return {
+    ...actualGenAI,
+    GoogleGenAI: vi.fn().mockImplementation(() => ({
+      chats: {
+        create: mockCreateChat,
+      },
+    })),
+    // Keep Chat constructor mock for type safety if direct instantiation is attempted,
+    // but primary path is via client.chats.create
+    Chat: vi.fn().mockImplementation(() => ({
+      getHistory: mockGetHistory,
+    })),
+  };
+});
+
+describe('checkNextSpeaker', () => {
+  let mockChat: Chat;
+  let mockGeminiClient: GeminiClient;
+  let MockConfig: Mock;
+
+  beforeEach(() => {
+    // Dynamically import and assign the mock
+    // Must be done within beforeEach or test to ensure mocks are reset
+    MockConfig = vi.mocked(Config);
+    // Create a mock instance of Config
+    const mockConfigInstance = new MockConfig(
+      'test-api-key',
+      'gemini-pro',
+      false,
+      '.',
+      false,
+      undefined,
+      false,
+      undefined,
+      undefined,
+      undefined,
+    );
+    // Mock any methods on mockConfigInstance if needed, e.g., mockConfigInstance.getToolRegistry = vi.fn()...
+
+    mockGeminiClient = new GeminiClient(mockConfigInstance);
+    // Simulate chat creation as done in GeminiClient
+    mockChat = { getHistory: mockGetHistory } as unknown as Chat;
+  });
+
+  afterEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it('should return null if history is empty', async () => {
+    (mockChat.getHistory as Mock).mockResolvedValue([]);
+    const result = await checkNextSpeaker(mockChat, mockGeminiClient);
+    expect(result).toBeNull();
+    expect(mockGeminiClient.generateJson).not.toHaveBeenCalled();
+  });
+
+  it('should return null if the last speaker was the user', async () => {
+    (mockChat.getHistory as Mock).mockResolvedValue([
+      { role: 'user', parts: [{ text: 'Hello' }] },
+    ] as Content[]);
+    const result = await checkNextSpeaker(mockChat, mockGeminiClient);
+    expect(result).toBeNull();
+    expect(mockGeminiClient.generateJson).not.toHaveBeenCalled();
+  });
+
+  it("should return { next_speaker: 'model' } when model intends to continue", async () => {
+    (mockChat.getHistory as Mock).mockResolvedValue([
+      { role: 'model', parts: [{ text: 'I will now do something.' }] },
+    ] as Content[]);
+    const mockApiResponse: NextSpeakerResponse = {
+      reasoning: 'Model stated it will do something.',
+      next_speaker: 'model',
+    };
+    (mockGeminiClient.generateJson as Mock).mockResolvedValue(mockApiResponse);
+
+    const result = await checkNextSpeaker(mockChat, mockGeminiClient);
+    expect(result).toEqual(mockApiResponse);
+    expect(mockGeminiClient.generateJson).toHaveBeenCalledTimes(1);
+  });
+
+  it("should return { next_speaker: 'user' } when model asks a question", async () => {
+    (mockChat.getHistory as Mock).mockResolvedValue([
+      { role: 'model', parts: [{ text: 'What would you like to do?' }] },
+    ] as Content[]);
+    const mockApiResponse: NextSpeakerResponse = {
+      reasoning: 'Model asked a question.',
+      next_speaker: 'user',
+    };
+    (mockGeminiClient.generateJson as Mock).mockResolvedValue(mockApiResponse);
+
+    const result = await checkNextSpeaker(mockChat, mockGeminiClient);
+    expect(result).toEqual(mockApiResponse);
+  });
+
+  it("should return { next_speaker: 'user' } when model makes a statement", async () => {
+    (mockChat.getHistory as Mock).mockResolvedValue([
+      { role: 'model', parts: [{ text: 'This is a statement.' }] },
+    ] as Content[]);
+    const mockApiResponse: NextSpeakerResponse = {
+      reasoning: 'Model made a statement, awaiting user input.',
+      next_speaker: 'user',
+    };
+    (mockGeminiClient.generateJson as Mock).mockResolvedValue(mockApiResponse);
+
+    const result = await checkNextSpeaker(mockChat, mockGeminiClient);
+    expect(result).toEqual(mockApiResponse);
+  });
+
+  it('should return null if geminiClient.generateJson throws an error', async () => {
+    const consoleWarnSpy = vi
+      .spyOn(console, 'warn')
+      .mockImplementation(() => {});
+    (mockChat.getHistory as Mock).mockResolvedValue([
+      { role: 'model', parts: [{ text: 'Some model output.' }] },
+    ] as Content[]);
+    (mockGeminiClient.generateJson as Mock).mockRejectedValue(
+      new Error('API Error'),
+    );
+
+    const result = await checkNextSpeaker(mockChat, mockGeminiClient);
+    expect(result).toBeNull();
+    consoleWarnSpy.mockRestore();
+  });
+
+  it('should return null if geminiClient.generateJson returns invalid JSON (missing next_speaker)', async () => {
+    (mockChat.getHistory as Mock).mockResolvedValue([
+      { role: 'model', parts: [{ text: 'Some model output.' }] },
+    ] as Content[]);
+    (mockGeminiClient.generateJson as Mock).mockResolvedValue({
+      reasoning: 'This is incomplete.',
+    } as unknown as NextSpeakerResponse); // Type assertion to simulate invalid response
+
+    const result = await checkNextSpeaker(mockChat, mockGeminiClient);
+    expect(result).toBeNull();
+  });
+
+  it('should return null if geminiClient.generateJson returns a non-string next_speaker', async () => {
+    (mockChat.getHistory as Mock).mockResolvedValue([
+      { role: 'model', parts: [{ text: 'Some model output.' }] },
+    ] as Content[]);
+    (mockGeminiClient.generateJson as Mock).mockResolvedValue({
+      reasoning: 'Model made a statement, awaiting user input.',
+      next_speaker: 123, // Invalid type
+    } as unknown as NextSpeakerResponse);
+
+    const result = await checkNextSpeaker(mockChat, mockGeminiClient);
+    expect(result).toBeNull();
+  });
+
+  it('should return null if geminiClient.generateJson returns an invalid next_speaker string value', async () => {
+    (mockChat.getHistory as Mock).mockResolvedValue([
+      { role: 'model', parts: [{ text: 'Some model output.' }] },
+    ] as Content[]);
+    (mockGeminiClient.generateJson as Mock).mockResolvedValue({
+      reasoning: 'Model made a statement, awaiting user input.',
+      next_speaker: 'neither', // Invalid enum value
+    } as unknown as NextSpeakerResponse);
+
+    const result = await checkNextSpeaker(mockChat, mockGeminiClient);
+    expect(result).toBeNull();
+  });
+});