How to configure CORS in FastAPI for frontend integration

CORS Configuration in FastAPI

CORS (Cross-Origin Resource Sharing) allows browsers to call your API from a different domain. Without it, requests from your frontend domain will be blocked by the browser.

Adding CORSMiddleware

from fastapi import FastAPI
from fastapi.middleware.cors import CORSMiddleware

app = FastAPI()

app.add_middleware(
    CORSMiddleware,
    allow_origins=["https://myapp.com", "http://localhost:3000"],
    allow_credentials=True,
    allow_methods=["*"],
    allow_headers=["*"],
)

The middleware intercepts preflight OPTIONS requests and responds with the correct headers before the actual request is processed.

Configuration rules

In development, add http://localhost:3000 (or your Vite/CRA port). In production, list only exact allowed origins. Never use allow_origins=["*"] with allow_credentials=True — browsers block this combination because it would allow any site to send credentialed requests to your API.

# Development only — do not use in production with credentials
app.add_middleware(
    CORSMiddleware,
    allow_origins=["*"],
    allow_credentials=False,
    allow_methods=["GET", "POST"],
    allow_headers=["*"],
)

If your frontend sends cookies or Authorization headers, you need allow_credentials=True and an exact origin list — wildcards are disallowed by the CORS spec in that case.