feat: Add model selection logic (#1678)

packages/core/src/core/geminiChat.test.ts

@@ -14,6 +14,7 @@ import {
 } from '@google/genai';
 import { GeminiChat } from './geminiChat.js';
 import { Config } from '../config/config.js';
+import { AuthType } from '../core/contentGenerator.js';
 import { setSimulate429 } from '../utils/testUtils.js';
 
 // Mocks
@@ -38,11 +39,14 @@ describe('GeminiChat', () => {
       getUsageStatisticsEnabled: () => true,
       getDebugMode: () => false,
       getContentGeneratorConfig: () => ({
-        authType: 'oauth-personal',
+        authType: AuthType.USE_GEMINI,
         model: 'test-model',
       }),
       getModel: vi.fn().mockReturnValue('gemini-pro'),
       setModel: vi.fn(),
+      getGeminiClient: vi.fn().mockReturnValue({
+        generateJson: vi.fn().mockResolvedValue({ model: 'pro' }),
+      }),
       flashFallbackHandler: undefined,
     } as unknown as Config;
 
@@ -110,7 +114,7 @@ describe('GeminiChat', () => {
       await chat.sendMessageStream({ message: 'hello' });
 
       expect(mockModelsModule.generateContentStream).toHaveBeenCalledWith({
-        model: 'gemini-pro',
+        model: 'gemini-2.5-pro',
         contents: [{ role: 'user', parts: [{ text: 'hello' }] }],
         config: {},
       });

packages/core/src/core/geminiChat.ts

@@ -34,7 +34,10 @@ import {
   ApiRequestEvent,
   ApiResponseEvent,
 } from '../telemetry/types.js';
-import { DEFAULT_GEMINI_FLASH_MODEL } from '../config/models.js';
+import {
+  DEFAULT_GEMINI_FLASH_MODEL,
+  DEFAULT_GEMINI_MODEL,
+} from '../config/models.js';
 
 /**
  * Returns true if the response is valid, false otherwise.
@@ -346,14 +349,20 @@ export class GeminiChat {
     await this.sendPromise;
     const userContent = createUserContent(params.message);
     const requestContents = this.getHistory(true).concat(userContent);
-    this._logApiRequest(requestContents, this.config.getModel());
+
+    const model = await this._selectModel(
+      requestContents,
+      params.config?.abortSignal ?? new AbortController().signal,
+    );
+
+    this._logApiRequest(requestContents, model);
 
     const startTime = Date.now();
 
     try {
       const apiCall = () =>
         this.contentGenerator.generateContentStream({
-          model: this.config.getModel(),
+          model,
           contents: requestContents,
           config: { ...this.generationConfig, ...params.config },
         });
@@ -397,6 +406,82 @@ export class GeminiChat {
     }
   }
 
+  /**
+   * Selects the model to use for the request.
+   *
+   * This is a placeholder for now.
+   */
+  private async _selectModel(
+    history: Content[],
+    signal: AbortSignal,
+  ): Promise<string> {
+    const currentModel = this.config.getModel();
+    if (currentModel === DEFAULT_GEMINI_FLASH_MODEL) {
+      return DEFAULT_GEMINI_FLASH_MODEL;
+    }
+
+    if (
+      history.length < 5 &&
+      this.config.getContentGeneratorConfig().authType === AuthType.USE_GEMINI
+    ) {
+      // There's currently a bug where for Gemini API key usage if we try and use flash as one of the first
+      // requests in our sequence that it will return an empty token.
+      return DEFAULT_GEMINI_MODEL;
+    }
+
+    const flashIndicator = 'flash';
+    const proIndicator = 'pro';
+    const modelChoicePrompt = `You are a super-intelligent router that decides which model to use for a given request. You have two models to choose from: "${flashIndicator}" and "${proIndicator}". "${flashIndicator}" is a smaller and faster model that is good for simple or well defined requests. "${proIndicator}" is a larger and slower model that is good for complex or undefined requests.
+
+Based on the user request, which model should be used? Respond with a JSON object that contains a single field, \`model\`, whose value is the name of the model to be used.
+
+For example, if you think "${flashIndicator}" should be used, respond with: { "model": "${flashIndicator}" }`;
+    const modelChoiceContent: Content[] = [
+      {
+        role: 'user',
+        parts: [{ text: modelChoicePrompt }],
+      },
+    ];
+
+    const client = this.config.getGeminiClient();
+    try {
+      const choice = await client.generateJson(
+        [...history, ...modelChoiceContent],
+        {
+          type: 'object',
+          properties: {
+            model: {
+              type: 'string',
+              enum: [flashIndicator, proIndicator],
+            },
+          },
+          required: ['model'],
+        },
+        signal,
+        DEFAULT_GEMINI_FLASH_MODEL,
+        {
+          temperature: 0,
+          maxOutputTokens: 25,
+          thinkingConfig: {
+            thinkingBudget: 0,
+          },
+        },
+      );
+
+      switch (choice.model) {
+        case flashIndicator:
+          return DEFAULT_GEMINI_FLASH_MODEL;
+        case proIndicator:
+          return DEFAULT_GEMINI_MODEL;
+        default:
+          return currentModel;
+      }
+    } catch (_e) {
+      // If the model selection fails, just use the default flash model.
+      return DEFAULT_GEMINI_FLASH_MODEL;
+    }
+  }
+
   /**
    * Returns the chat history.
    *