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

packages/core/src/availability/autoRoutingFallback.integration.test.ts

@@ -29,6 +29,9 @@ describe('Auto Routing Fallback Integration', () => {
 
   beforeEach(() => {
     vi.useFakeTimers();
+    vi.spyOn(Config.prototype, 'getHasAccessToPreviewModel').mockReturnValue(
+      true,
+    );
 
     // Mock fs to avoid real file system access
     vi.mocked(fs.existsSync).mockReturnValue(true);

packages/core/src/availability/fallbackIntegration.test.ts

@@ -28,6 +28,7 @@ describe('Fallback Integration', () => {
       getActiveModel: () => PREVIEW_GEMINI_MODEL_AUTO,
       setActiveModel: vi.fn(),
       getUserTier: () => undefined,
+      getHasAccessToPreviewModel: () => true,
       getModelAvailabilityService: () => availabilityService,
       modelConfigService: undefined as unknown as ModelConfigService,
     } as unknown as Config;

packages/core/src/availability/policyHelpers.ts

@@ -55,7 +55,10 @@ export function resolvePolicyChain(
   const useGemini31FlashLite =
     config.getGemini31FlashLiteLaunchedSync?.() ?? false;
   const useCustomToolModel = config.getUseCustomToolModelSync?.() ?? false;
-  const hasAccessToPreview = config.getHasAccessToPreviewModel?.() ?? true;
+  const hasAccessToPreview = config.getHasAccessToPreviewModel?.() ?? false;
+
+  // Capture the original family intent before any normalization or early downgrade.
+  const isOriginallyGemini3 = isGemini3Model(modelFromConfig, config);
 
   const resolvedModel = normalizeModelId(
     resolveModel(
@@ -75,10 +78,7 @@ export function resolvePolicyChain(
   // We always wrap around for Gemini 3 chains to ensure maximum availability
   // between models in the same family (e.g. fallback to Pro if Flash is exhausted).
   const effectiveWrapsAround =
-    wrapsAround ||
-    isAutoPreferred ||
-    isAutoConfigured ||
-    isGemini3Model(resolvedModel, config);
+    wrapsAround || isAutoPreferred || isAutoConfigured || isOriginallyGemini3;
 
   // --- DYNAMIC PATH ---
   if (config.getExperimentalDynamicModelConfiguration?.() === true) {
@@ -91,11 +91,7 @@ export function resolvePolicyChain(
 
     if (resolvedModel === DEFAULT_GEMINI_FLASH_LITE_MODEL) {
       chain = config.modelConfigService.resolveChain('lite', context);
-    } else if (
-      isGemini3Model(normalizeModelId(resolvedModel), config) ||
-      isAutoPreferred ||
-      isAutoConfigured
-    ) {
+    } else if (isOriginallyGemini3 || isAutoPreferred || isAutoConfigured) {
       // 1. Try to find a chain specifically for the current configured alias
       if (
         isAutoConfigured &&
@@ -132,15 +128,11 @@ export function resolvePolicyChain(
 
     if (resolvedModel === DEFAULT_GEMINI_FLASH_LITE_MODEL) {
       chain = getFlashLitePolicyChain();
-    } else if (
-      isGemini3Model(resolvedModel, config) ||
-      isAutoPreferred ||
-      isAutoConfigured
-    ) {
+    } else if (isOriginallyGemini3 || isAutoPreferred || isAutoConfigured) {
       const isAutoSelection = isAutoPreferred || isAutoConfigured;
       if (hasAccessToPreview) {
         const previewEnabled =
-          isGemini3Model(resolvedModel, config) ||
+          isOriginallyGemini3 ||
           normalizedPreferredModel === PREVIEW_GEMINI_MODEL_AUTO ||
           configuredModel === PREVIEW_GEMINI_MODEL_AUTO;
         chain = getModelPolicyChain({