has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header

The API server does not allow cross-origin requests from your origin. 1. Fix SERVER side: add `Access-Control-Allow-Origin: <origin>` (or `*` for public APIs). - Express: `app.use(cors({ origin: 'http://localhost:3000' }))`. - FastAPI: `app.add_middleware(CORSMiddleware, allow_origins=[...])`. - Next.js Route Handler: set headers in the Response. 2. Preflight (OPTIONS): ensure the server responds 200/204 to OPTIONS. 3. For credentials (`credentials: 'include'`), `Access-Control-Allow-Origin` must be a specific origin, not `*`. 4. Dev-only workaround: use Next.js rewrites or a proxy — never disable CORS in browser with a flag.