CORS & API Contracts: Preparing Your Backend for Frontend Collaboration

What is CORS? When a React frontend on localhost:3000 tries to call an API on localhost:8000, the browser blocks it. That's CORS protection and it's a good thing. But your own frontend should be allowed through. That's what CORS configuration does. Enabling CORS in FastAPI from fastapi.middleware.cors import CORSMiddleware app . add_middleware ( CORSMiddleware , allow_origins = [ " http://localhost:3000 " , " http://localhost:5173 " ], allow_credentials = True , allow_methods = [ " GET " , " POST " , " PUT " , " DELETE " , " OPTIONS " ], allow_headers = [ " * " ], ) One middleware call, every response now includes the correct CORS headers automatically. API Contract : camelCase for Frontend Backend Python uses snake_case by default. But JavaScript frontends expect camelCase. Instead of: created_at -> use createdAt category_id -> use categoryId access_token -> use accessToken Small change, big impact on frontend collaboration. The Consistent Response Shape Every endpoint follows the sam