fix(core): ensure stable fallback for restricted preview models (#26999)

packages/core/src/fallback/handler.test.ts

@@ -77,6 +77,7 @@ const createMockConfig = (overrides: Partial<Config> = {}): Config =>
     getModel: vi.fn(() => MOCK_PRO_MODEL),
     getUserTier: vi.fn(() => undefined),
     isInteractive: vi.fn(() => false),
+    getHasAccessToPreviewModel: vi.fn(() => false),
     ...overrides,
   }) as unknown as Config;
 
@@ -234,6 +235,7 @@ describe('handleFallback', () => {
       vi.mocked(policyConfig.getModel).mockReturnValue(
         PREVIEW_GEMINI_MODEL_AUTO,
       );
+      vi.mocked(policyConfig.getHasAccessToPreviewModel).mockReturnValue(true);
 
       const result = await handleFallback(
         policyConfig,

packages/core/src/fallback/handler.ts

@@ -20,6 +20,8 @@ import {
   applyAvailabilityTransition,
 } from '../availability/policyHelpers.js';
 
+import { isPreviewModel } from '../config/models.js';
+
 export const UPGRADE_URL_PAGE = 'https://goo.gle/set-up-gemini-code-assist';
 
 export async function handleFallback(
@@ -28,13 +30,19 @@ export async function handleFallback(
   authType?: string,
   error?: unknown,
 ): Promise<string | boolean | null> {
+  const failureKind = classifyFailureKind(error);
+
+  // If a preview model is not found, record that the user lacks preview access.
+  if (failureKind === 'not_found' && isPreviewModel(failedModel, config)) {
+    config.setHasAccessToPreviewModel?.(false);
+  }
+
   const chain = resolvePolicyChain(config);
   const { failedPolicy, candidates } = buildFallbackPolicyContext(
     chain,
     failedModel,
   );
 
-  const failureKind = classifyFailureKind(error);
   const availability = config.getModelAvailabilityService();
   const getAvailabilityContext = () => {
     if (!failedPolicy) return undefined;