log fallback mode (#15817)

packages/cli/src/ui/hooks/useQuotaAndFallback.ts

@@ -133,8 +133,7 @@ export function useQuotaAndFallback({
         // Set the model to the fallback model for the current session.
         // This ensures the Footer updates and future turns use this model.
         // The change is not persisted, so the original model is restored on restart.
-        config.setModel(proQuotaRequest.fallbackModel, true);
-
+        config.activateFallbackMode(proQuotaRequest.fallbackModel);
         historyManager.addItem(
           {
             type: MessageType.INFO,

packages/core/src/config/config.ts

@@ -60,8 +60,11 @@ import { ideContextStore } from '../ide/ideContext.js';
 import { WriteTodosTool } from '../tools/write-todos.js';
 import type { FileSystemService } from '../services/fileSystemService.js';
 import { StandardFileSystemService } from '../services/fileSystemService.js';
-import { logRipgrepFallback } from '../telemetry/loggers.js';
-import { RipgrepFallbackEvent } from '../telemetry/types.js';
+import { logRipgrepFallback, logFlashFallback } from '../telemetry/loggers.js';
+import {
+  RipgrepFallbackEvent,
+  FlashFallbackEvent,
+} from '../telemetry/types.js';
 import type { FallbackModelHandler } from '../fallback/types.js';
 import { ModelAvailabilityService } from '../availability/modelAvailabilityService.js';
 import { ModelRouterService } from '../routing/modelRouterService.js';
@@ -911,6 +914,14 @@ export class Config {
     this.modelAvailabilityService.reset();
   }
 
+  activateFallbackMode(model: string): void {
+    this.setModel(model, true);
+    const authType = this.getContentGeneratorConfig()?.authType;
+    if (authType) {
+      logFlashFallback(this, new FlashFallbackEvent(authType));
+    }
+  }
+
   getActiveModel(): string {
     return this._activeModel ?? this.model;
   }

packages/core/src/config/flashFallback.test.ts

@@ -7,10 +7,16 @@
 import { describe, it, expect, beforeEach, vi } from 'vitest';
 import { Config } from './config.js';
 import { DEFAULT_GEMINI_MODEL, DEFAULT_GEMINI_FLASH_MODEL } from './models.js';
+import { logFlashFallback } from '../telemetry/loggers.js';
+import { FlashFallbackEvent } from '../telemetry/types.js';
 
 import fs from 'node:fs';
 
 vi.mock('node:fs');
+vi.mock('../telemetry/loggers.js', () => ({
+  logFlashFallback: vi.fn(),
+  logRipgrepFallback: vi.fn(),
+}));
 
 describe('Flash Model Fallback Configuration', () => {
   let config: Config;
@@ -57,4 +63,15 @@ describe('Flash Model Fallback Configuration', () => {
       expect(newConfig.getModel()).toBe('custom-model');
     });
   });
+
+  describe('activateFallbackMode', () => {
+    it('should set model to fallback and log event', () => {
+      config.activateFallbackMode(DEFAULT_GEMINI_FLASH_MODEL);
+      expect(config.getModel()).toBe(DEFAULT_GEMINI_FLASH_MODEL);
+      expect(logFlashFallback).toHaveBeenCalledWith(
+        config,
+        expect.any(FlashFallbackEvent),
+      );
+    });
+  });
 });