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Why does my React app get a CORS error the moment it tries to call my FastAPI backend?
What you'll have at the end
A fetch from your frontend's own origin that succeeds instead of showing a CORS error
You need
A backend already running, and a frontend on a different origin, its own dev server or a deployed build, whose calls to it are already failing in the browser console with a CORS error.
Not covered
Setting up the login cookie itself, its SameSite or Secure flags, or CORS handling done at a reverse proxy or CDN sitting in front of the API.
Checked 18 Aug 2026
Part of the FastAPI cookbook