[JUNE 25] Permanent failover to Flash model for OAuth users after persistent 429 errors (#1376)

Co-authored-by: Scott Densmore <scottdensmore@mac.com>

packages/core/src/core/client.ts

@@ -38,6 +38,7 @@ import {
 } from './contentGenerator.js';
 import { ProxyAgent, setGlobalDispatcher } from 'undici';
 import { DEFAULT_GEMINI_FLASH_MODEL } from '../config/models.js';
+import { AuthType } from './contentGenerator.js';
 
 function isThinkingSupported(model: string) {
   if (model.startsWith('gemini-2.5')) return true;
@@ -276,7 +277,11 @@ export class GeminiClient {
           contents,
         });
 
-      const result = await retryWithBackoff(apiCall);
+      const result = await retryWithBackoff(apiCall, {
+        onPersistent429: async (authType?: string) =>
+          await this.handleFlashFallback(authType),
+        authType: this.config.getContentGeneratorConfig()?.authType,
+      });
 
       const text = getResponseText(result);
       if (!text) {
@@ -360,7 +365,11 @@ export class GeminiClient {
           contents,
         });
 
-      const result = await retryWithBackoff(apiCall);
+      const result = await retryWithBackoff(apiCall, {
+        onPersistent429: async (authType?: string) =>
+          await this.handleFlashFallback(authType),
+        authType: this.config.getContentGeneratorConfig()?.authType,
+      });
       return result;
     } catch (error: unknown) {
       if (abortSignal.aborted) {
@@ -489,4 +498,43 @@ export class GeminiClient {
         }
       : null;
   }
+
+  /**
+   * Handles fallback to Flash model when persistent 429 errors occur for OAuth users.
+   * Uses a fallback handler if provided by the config, otherwise returns null.
+   */
+  private async handleFlashFallback(authType?: string): Promise<string | null> {
+    // Only handle fallback for OAuth users
+    if (
+      authType !== AuthType.LOGIN_WITH_GOOGLE_PERSONAL &&
+      authType !== AuthType.LOGIN_WITH_GOOGLE_ENTERPRISE
+    ) {
+      return null;
+    }
+
+    const currentModel = this.model;
+    const fallbackModel = DEFAULT_GEMINI_FLASH_MODEL;
+
+    // Don't fallback if already using Flash model
+    if (currentModel === fallbackModel) {
+      return null;
+    }
+
+    // Check if config has a fallback handler (set by CLI package)
+    const fallbackHandler = this.config.flashFallbackHandler;
+    if (typeof fallbackHandler === 'function') {
+      try {
+        const accepted = await fallbackHandler(currentModel, fallbackModel);
+        if (accepted) {
+          this.config.setModel(fallbackModel);
+          this.model = fallbackModel;
+          return fallbackModel;
+        }
+      } catch (error) {
+        console.warn('Flash fallback handler failed:', error);
+      }
+    }
+
+    return null;
+  }
 }