diff --git a/packages/core/src/utils/nextSpeakerChecker.test.ts b/packages/core/src/utils/nextSpeakerChecker.test.ts
index 475b5662..9141105f 100644
--- a/packages/core/src/utils/nextSpeakerChecker.test.ts
+++ b/packages/core/src/utils/nextSpeakerChecker.test.ts
@@ -6,6 +6,7 @@
 
 import { describe, it, expect, vi, beforeEach, Mock, afterEach } from 'vitest';
 import { Content, GoogleGenAI, Models } from '@google/genai';
+import { DEFAULT_GEMINI_FLASH_MODEL } from '../config/models.js';
 import { GeminiClient } from '../core/client.js';
 import { Config } from '../config/config.js';
 import { checkNextSpeaker, NextSpeakerResponse } from './nextSpeakerChecker.js';
@@ -231,4 +232,22 @@ describe('checkNextSpeaker', () => {
     );
     expect(result).toBeNull();
   });
+
+  it('should call generateJson with DEFAULT_GEMINI_FLASH_MODEL', async () => {
+    (chatInstance.getHistory as Mock).mockReturnValue([
+      { role: 'model', parts: [{ text: 'Some model output.' }] },
+    ] as Content[]);
+    const mockApiResponse: NextSpeakerResponse = {
+      reasoning: 'Model made a statement, awaiting user input.',
+      next_speaker: 'user',
+    };
+    (mockGeminiClient.generateJson as Mock).mockResolvedValue(mockApiResponse);
+
+    await checkNextSpeaker(chatInstance, mockGeminiClient, abortSignal);
+
+    expect(mockGeminiClient.generateJson).toHaveBeenCalled();
+    const generateJsonCall = (mockGeminiClient.generateJson as Mock).mock
+      .calls[0];
+    expect(generateJsonCall[3]).toBe(DEFAULT_GEMINI_FLASH_MODEL);
+  });
 });
diff --git a/packages/core/src/utils/nextSpeakerChecker.ts b/packages/core/src/utils/nextSpeakerChecker.ts
index 165f277a..9d428887 100644
--- a/packages/core/src/utils/nextSpeakerChecker.ts
+++ b/packages/core/src/utils/nextSpeakerChecker.ts
@@ -5,6 +5,7 @@
  */
 
 import { Content, SchemaUnion, Type } from '@google/genai';
+import { DEFAULT_GEMINI_FLASH_MODEL } from '../config/models.js';
 import { GeminiClient } from '../core/client.js';
 import { GeminiChat } from '../core/geminiChat.js';
 import { isFunctionResponse } from './messageInspectors.js';
@@ -131,6 +132,7 @@ export async function checkNextSpeaker(
       contents,
       RESPONSE_SCHEMA,
       abortSignal,
+      DEFAULT_GEMINI_FLASH_MODEL,
     )) as unknown as NextSpeakerResponse;
 
     if (