Ask Claude to Diagnose CORS Errors

CORS errors are some of the most frustrating in web development — the browser message tells you almost nothing useful, and the fix can be in half a dozen different places. Claude can diagnose them fast if you give it the right context.

Paste your error, your request headers, and your server config together:

I'm getting this CORS error in Chrome:
"Access to fetch at 'https://api.example.com/data' from origin 'https://app.example.com'
has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present"

My Nginx config:
[paste config]

My Laravel CORS config (config/cors.php):
[paste config]

The request is a POST with Content-Type: application/json and an Authorization header.

Claude will identify whether the issue is in your server config, your framework middleware, a missing preflight handler, or a proxy stripping headers upstream.

It'll also flag common traps — like CORS being handled correctly but a 500 error on the preflight OPTIONS request making it look like a CORS problem.

If you're in a browser and can copy the network request, paste the full request and response headers from DevTools:

Here are the request and response headers from the failed OPTIONS preflight.
What's missing and where should I add it?

The fix is almost always one line — the hard part is knowing which line and where.