fix(core): ensure Vertex AI sets hasAccessToPreviewModels and remove aggressive 404 fallback revocation (#27067)

packages/core/src/config/config.ts

@@ -1627,7 +1627,10 @@ export class Config implements McpContext, AgentLoopContext {
     this.baseLlmClient = new BaseLlmClient(this.contentGenerator, this);
 
     const authType = this.contentGeneratorConfig.authType;
-    if (authType === AuthType.USE_GEMINI) {
+    if (
+      authType === AuthType.USE_GEMINI ||
+      authType === AuthType.USE_VERTEX_AI
+    ) {
       this.setHasAccessToPreviewModel(true);
     }
 

packages/core/src/fallback/handler.ts

@@ -20,8 +20,6 @@ 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(
@@ -32,11 +30,6 @@ export async function handleFallback(
 ): 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,