diff --git a/packages/core/src/code_assist/server.test.ts b/packages/core/src/code_assist/server.test.ts
index 67c2cab67d..9ea5ae6ce4 100644
--- a/packages/core/src/code_assist/server.test.ts
+++ b/packages/core/src/code_assist/server.test.ts
@@ -347,7 +347,7 @@ describe('CodeAssistServer', () => {
     });
 
     it('should construct the default URL correctly', () => {
-      const server = new CodeAssistServer({} as never);
+      const server = new CodeAssistServer({} as never, 'test-project');
       const url = server.getMethodUrl('testMethod');
       expect(url).toBe(
         'https://cloudcode-pa.googleapis.com/v1internal:testMethod',
@@ -356,26 +356,79 @@ describe('CodeAssistServer', () => {
 
     it('should use the CODE_ASSIST_ENDPOINT environment variable if set', () => {
       process.env['CODE_ASSIST_ENDPOINT'] = 'https://custom-endpoint.com';
-      const server = new CodeAssistServer({} as never);
+      const server = new CodeAssistServer({} as never, 'test-project');
       const url = server.getMethodUrl('testMethod');
       expect(url).toBe('https://custom-endpoint.com/v1internal:testMethod');
     });
 
     it('should use the CODE_ASSIST_API_VERSION environment variable if set', () => {
       process.env['CODE_ASSIST_API_VERSION'] = 'v2beta';
-      const server = new CodeAssistServer({} as never);
+      const server = new CodeAssistServer({} as never, 'test-project');
       const url = server.getMethodUrl('testMethod');
       expect(url).toBe('https://cloudcode-pa.googleapis.com/v2beta:testMethod');
     });
 
     it('should use default value if CODE_ASSIST_API_VERSION env var is empty', () => {
       process.env['CODE_ASSIST_API_VERSION'] = '';
-      const server = new CodeAssistServer({} as never);
+      const server = new CodeAssistServer({} as never, 'test-project');
       const url = server.getMethodUrl('testMethod');
       expect(url).toBe(
         'https://cloudcode-pa.googleapis.com/v1internal:testMethod',
       );
     });
+
+    it('should use standard API endpoint when no projectId is provided', () => {
+      const server = new CodeAssistServer({} as never);
+      const url = server.getMethodUrl('testMethod');
+      expect(url).toBe(
+        'https://generativelanguage.googleapis.com/v1beta:testMethod',
+      );
+    });
+  });
+
+  describe('loadCodeAssist project hijacking prevention', () => {
+    it('should strip cloudaicompanionProject from response when no projectId is set', async () => {
+      const mockRequest = vi.fn();
+      const client = { request: mockRequest } as unknown as OAuth2Client;
+      const server = new CodeAssistServer(client, undefined, {}, undefined);
+
+      mockRequest.mockResolvedValue({
+        data: {
+          cloudaicompanionProject: 'ghost-project-id',
+          currentTier: { id: 'standard-tier' },
+        },
+      });
+
+      const result = await server.loadCodeAssist({
+        metadata: { ideType: 'IDE_UNSPECIFIED' },
+      });
+
+      expect(result.cloudaicompanionProject).toBeUndefined();
+    });
+
+    it('should preserve cloudaicompanionProject when projectId is explicitly set', async () => {
+      const mockRequest = vi.fn();
+      const client = { request: mockRequest } as unknown as OAuth2Client;
+      const server = new CodeAssistServer(
+        client,
+        'explicit-project',
+        {},
+        undefined,
+      );
+
+      mockRequest.mockResolvedValue({
+        data: {
+          cloudaicompanionProject: 'server-project-id',
+          currentTier: { id: 'standard-tier' },
+        },
+      });
+
+      const result = await server.loadCodeAssist({
+        metadata: { ideType: 'IDE_UNSPECIFIED' },
+      });
+
+      expect(result.cloudaicompanionProject).toBe('server-project-id');
+    });
   });
 
   it('should call the generateContentStream endpoint and parse SSE', async () => {
diff --git a/packages/core/src/code_assist/server.ts b/packages/core/src/code_assist/server.ts
index 4ed8328f3d..7ae5498e96 100644
--- a/packages/core/src/code_assist/server.ts
+++ b/packages/core/src/code_assist/server.ts
@@ -264,10 +264,29 @@ export class CodeAssistServer implements ContentGenerator {
     req: LoadCodeAssistRequest,
   ): Promise<LoadCodeAssistResponse> {
     try {
-      return await this.requestPost<LoadCodeAssistResponse>(
+      const res = await this.requestPost<LoadCodeAssistResponse>(
         'loadCodeAssist',
         req,
       );
+
+      /**
+       * FIX: Prevent automatic project hijacking for personal users.
+       * For Google One AI Premium subscribers, the backend may return a
+       * "shadow" project ID (e.g., master-impulse-jrkws) in
+       * cloudaicompanionProject. If adopted, this forces the CLI to route
+       * through the Enterprise endpoint (cloudcode-pa.googleapis.com),
+       * which rejects personal OAuth tokens with 403 PERMISSION_DENIED.
+       *
+       * By stripping this field when no explicit projectId was provided,
+       * we preserve the Personal/Standard flow routing.
+       * See: https://github.com/google-gemini/gemini-cli/issues/25189
+       * See: https://github.com/google-gemini/gemini-cli/issues/24517
+       */
+      if (res.cloudaicompanionProject && !this.projectId) {
+        delete res.cloudaicompanionProject;
+      }
+
+      return res;
     } catch (e) {
       if (isVpcScAffectedUser(e)) {
         return {
@@ -508,6 +527,18 @@ export class CodeAssistServer implements ContentGenerator {
   }
 
   private getBaseUrl(): string {
+    /**
+     * FIX: Endpoint fallback logic.
+     * If no explicit projectId is provided via CLI flags or config, default
+     * to the standard Generative Language API. This prevents accidental
+     * routing to the Enterprise Cloud Code PA endpoint for personal users.
+     * See: https://github.com/google-gemini/gemini-cli/issues/25189
+     * See: https://github.com/google-gemini/gemini-cli/issues/24517
+     */
+    if (!this.projectId) {
+      return 'https://generativelanguage.googleapis.com/v1beta';
+    }
+
     const endpoint =
       process.env['CODE_ASSIST_ENDPOINT'] ?? CODE_ASSIST_ENDPOINT;
     const version =