refactor: Centralize and improve model fallback handling (#7634)

packages/core/src/fallback/handler.ts

@@ -0,0 +1,69 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import type { Config } from '../config/config.js';
+import { AuthType } from '../core/contentGenerator.js';
+import { DEFAULT_GEMINI_FLASH_MODEL } from '../config/models.js';
+import { logFlashFallback, FlashFallbackEvent } from '../telemetry/index.js';
+
+export async function handleFallback(
+  config: Config,
+  failedModel: string,
+  authType?: string,
+  error?: unknown,
+): Promise<string | boolean | null> {
+  // Applicability Checks
+  if (authType !== AuthType.LOGIN_WITH_GOOGLE) return null;
+
+  const fallbackModel = DEFAULT_GEMINI_FLASH_MODEL;
+
+  if (failedModel === fallbackModel) return null;
+
+  // Consult UI Handler for Intent
+  const fallbackModelHandler = config.fallbackModelHandler;
+  if (typeof fallbackModelHandler !== 'function') return null;
+
+  try {
+    // Pass the specific failed model to the UI handler.
+    const intent = await fallbackModelHandler(
+      failedModel,
+      fallbackModel,
+      error,
+    );
+
+    // Process Intent and Update State
+    switch (intent) {
+      case 'retry':
+        // Activate fallback mode. The NEXT retry attempt will pick this up.
+        activateFallbackMode(config, authType);
+        return true; // Signal retryWithBackoff to continue.
+
+      case 'stop':
+        activateFallbackMode(config, authType);
+        return false;
+
+      case 'auth':
+        return false;
+
+      default:
+        throw new Error(
+          `Unexpected fallback intent received from fallbackModelHandler: "${intent}"`,
+        );
+    }
+  } catch (handlerError) {
+    console.error('Fallback UI handler failed:', handlerError);
+    return null;
+  }
+}
+
+function activateFallbackMode(config: Config, authType: string | undefined) {
+  if (!config.isInFallbackMode()) {
+    config.setFallbackMode(true);
+    if (authType) {
+      logFlashFallback(config, new FlashFallbackEvent(authType));
+    }
+  }
+}