What does CORS actually protect?

It controls whether your JavaScript may read a cross-origin response — not whether the request happened. The server did receive the request; the browser blocked your page from reading the reply.

Can I fix it with Access-Control-Allow-Origin: *?

You can, and you should not. A wildcard — or reflecting whatever Origin arrives — means any website can read your API's responses on behalf of your logged-in users. Use an explicit allow-list of your own origins.

Why does it fail only for some requests?

Requests with custom headers or a JSON content type become preflights, and many servers never answer OPTIONS. Simple GETs pass while the rest fail.

I listed my origin and it still fails.

Origin matching is exact. A trailing slash or a protocol mismatch counts as a different origin. Sending credentials also forbids a wildcard, so the server must name the origin explicitly.

Error explained3 min readUpdated 2026-08-03

"Blocked by CORS policy" — what it means and how to fix

CORS is not in your way by accident. The one-line fix people reach for removes the protection entirely.

What it means

Your page on one origin asked for a resource on another, and the server did not say that your origin is allowed to read the response. The browser blocked it. Note the server did receive the request — CORS controls whether your JavaScript may read the reply, not whether the request happened.

Why it happens in AI-built apps

Frontend and API are on different domains or ports, and the API has no CORS configuration at all.
The request became a preflight (custom headers or a JSON content type) and the server does not answer OPTIONS.
The origin is listed with a trailing slash or a protocol mismatch — matching is exact.
Credentials are being sent, which forbids a wildcard origin; the server must name the origin explicitly.

Is this error actually a problem?

Check it yourself

See what your API actually replies to a cross-origin request

curl -sI -X OPTIONS https://YOUR-API.com/endpoint \
  -H "Origin: https://example.com" \
  -H "Access-Control-Request-Method: POST" | grep -i access-control

Fix it

Copy this into your AI coding tool

I am getting "blocked by CORS policy" calling my own API.

1. Show me the current CORS configuration and explain precisely which
   check is failing — origin, method, headers, or the preflight.
2. Fix it with an explicit allow-list of my own origins. Do not use "*"
   and do not reflect the incoming Origin header back.
3. Make sure OPTIONS preflight requests are answered with the right
   Access-Control-Allow-Methods and Access-Control-Allow-Headers.
4. If credentials are involved, set Access-Control-Allow-Credentials: true
   with a named origin, never a wildcard — the two are incompatible.
5. Add Vary: Origin so caches do not serve one origin's response to
   another.

Show me the final configuration and list exactly which origins can now
read responses.

Common questions

What does CORS actually protect?: It controls whether your JavaScript may read a cross-origin response — not whether the request happened. The server did receive the request; the browser blocked your page from reading the reply.
Can I fix it with Access-Control-Allow-Origin: *?: You can, and you should not. A wildcard — or reflecting whatever Origin arrives — means any website can read your API's responses on behalf of your logged-in users. Use an explicit allow-list of your own origins.
Why does it fail only for some requests?: Requests with custom headers or a JSON content type become preflights, and many servers never answer OPTIONS. Simple GETs pass while the rest fail.
I listed my origin and it still fails.: Origin matching is exact. A trailing slash or a protocol mismatch counts as a different origin. Sending credentials also forbids a wildcard, so the server must name the origin explicitly.

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