Trace a blocked CORS preflight request back to its missing header

Your fetch call throws a network error in the console, but the API itself works fine everywhere else you've tried it.

IntermediateRead 5 minDoing: about twenty minutes5 steps

▶ Start

What you'll have at the end

The exact preflight request and response for one blocked call, with the missing header identified by name

Before you start

You need

A cross-origin fetch call that fails only when it runs in a browser, plus a way to send the identical request from outside one.

Not covered

Editing your server's own CORS configuration to add the missing method or header, once you know which one it is.

Leans on

5 steps

Where to go next

A CORS error blocks your frontend from calling this brand new API

once you know which method or header the server is missing and you're the one who owns the API, that recipe covers adding it to a FastAPI backend's own CORS setup

Checked 18 Aug 2026

Part of the HTTP cookbook