feat(core): fallback to 2.5 models with no access for toolcalls (#21283)

Author: sehoon38

packages/core/src/availability/policyHelpers.ts

@@ -49,15 +49,16 @@ export function resolvePolicyChain(
   const useCustomToolModel =
     useGemini31 &&
     config.getContentGeneratorConfig?.()?.authType === AuthType.USE_GEMINI;
+  const hasAccessToPreview = config.getHasAccessToPreviewModel?.() ?? true;
 
   const resolvedModel = resolveModel(
     modelFromConfig,
     useGemini31,
     useCustomToolModel,
+    hasAccessToPreview,
   );
   const isAutoPreferred = preferredModel ? isAutoModel(preferredModel) : false;
   const isAutoConfigured = isAutoModel(configuredModel);
-  const hasAccessToPreview = config.getHasAccessToPreviewModel?.() ?? true;
 
   if (resolvedModel === DEFAULT_GEMINI_FLASH_LITE_MODEL) {
     chain = getFlashLitePolicyChain();
@@ -80,7 +81,7 @@ export function resolvePolicyChain(
     } else {
       // User requested Gemini 3 but has no access. Proactively downgrade
       // to the stable Gemini 2.5 chain.
-      return getModelPolicyChain({
+      chain = getModelPolicyChain({
         previewEnabled: false,
         userTier: config.getUserTier(),
         useGemini31,

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

@@ -217,6 +217,38 @@ describe('resolveModel', () => {
       expect(model).toBe(customModel);
     });
   });
+
+  describe('hasAccessToPreview logic', () => {
+    it('should return default model when access to preview is false and preview model is requested', () => {
+      expect(resolveModel(PREVIEW_GEMINI_MODEL, false, false, false)).toBe(
+        DEFAULT_GEMINI_MODEL,
+      );
+    });
+
+    it('should return default flash model when access to preview is false and preview flash model is requested', () => {
+      expect(
+        resolveModel(PREVIEW_GEMINI_FLASH_MODEL, false, false, false),
+      ).toBe(DEFAULT_GEMINI_FLASH_MODEL);
+    });
+
+    it('should return default model when access to preview is false and auto-gemini-3 is requested', () => {
+      expect(resolveModel(PREVIEW_GEMINI_MODEL_AUTO, false, false, false)).toBe(
+        DEFAULT_GEMINI_MODEL,
+      );
+    });
+
+    it('should return default model when access to preview is false and Gemini 3.1 is requested', () => {
+      expect(resolveModel(PREVIEW_GEMINI_MODEL_AUTO, true, false, false)).toBe(
+        DEFAULT_GEMINI_MODEL,
+      );
+    });
+
+    it('should still return default model when access to preview is false and auto-gemini-2.5 is requested', () => {
+      expect(resolveModel(DEFAULT_GEMINI_MODEL_AUTO, false, false, false)).toBe(
+        DEFAULT_GEMINI_MODEL,
+      );
+    });
+  });
 });
 
 describe('isGemini2Model', () => {

packages/core/src/config/models.ts

@@ -43,38 +43,70 @@ export const DEFAULT_THINKING_MODE = 8192;
  *
  * @param requestedModel The model alias or concrete model name requested by the user.
  * @param useGemini3_1 Whether to use Gemini 3.1 Pro Preview for auto/pro aliases.
+ * @param hasAccessToPreview Whether the user has access to preview models.
  * @returns The resolved concrete model name.
  */
 export function resolveModel(
   requestedModel: string,
   useGemini3_1: boolean = false,
   useCustomToolModel: boolean = false,
+  hasAccessToPreview: boolean = true,
 ): string {
+  let resolved: string;
   switch (requestedModel) {
     case PREVIEW_GEMINI_MODEL:
     case PREVIEW_GEMINI_MODEL_AUTO:
     case GEMINI_MODEL_ALIAS_AUTO:
     case GEMINI_MODEL_ALIAS_PRO: {
       if (useGemini3_1) {
-        return useCustomToolModel
+        resolved = useCustomToolModel
           ? PREVIEW_GEMINI_3_1_CUSTOM_TOOLS_MODEL
           : PREVIEW_GEMINI_3_1_MODEL;
+      } else {
+        resolved = PREVIEW_GEMINI_MODEL;
       }
-      return PREVIEW_GEMINI_MODEL;
+      break;
     }
     case DEFAULT_GEMINI_MODEL_AUTO: {
-      return DEFAULT_GEMINI_MODEL;
+      resolved = DEFAULT_GEMINI_MODEL;
+      break;
     }
     case GEMINI_MODEL_ALIAS_FLASH: {
-      return PREVIEW_GEMINI_FLASH_MODEL;
+      resolved = PREVIEW_GEMINI_FLASH_MODEL;
+      break;
     }
     case GEMINI_MODEL_ALIAS_FLASH_LITE: {
-      return DEFAULT_GEMINI_FLASH_LITE_MODEL;
+      resolved = DEFAULT_GEMINI_FLASH_LITE_MODEL;
+      break;
     }
     default: {
-      return requestedModel;
+      resolved = requestedModel;
+      break;
+    }
+  }
+
+  if (!hasAccessToPreview && isPreviewModel(resolved)) {
+    // Downgrade to stable models if user lacks preview access.
+    switch (resolved) {
+      case PREVIEW_GEMINI_FLASH_MODEL:
+        return DEFAULT_GEMINI_FLASH_MODEL;
+      case PREVIEW_GEMINI_MODEL:
+      case PREVIEW_GEMINI_3_1_MODEL:
+      case PREVIEW_GEMINI_3_1_CUSTOM_TOOLS_MODEL:
+        return DEFAULT_GEMINI_MODEL;
+      default:
+        // Fallback for unknown preview models, preserving original logic.
+        if (resolved.includes('flash-lite')) {
+          return DEFAULT_GEMINI_FLASH_LITE_MODEL;
+        }
+        if (resolved.includes('flash')) {
+          return DEFAULT_GEMINI_FLASH_MODEL;
+        }
+        return DEFAULT_GEMINI_MODEL;
     }
   }
+
+  return resolved;
 }
 
 /**