CORS Checker / Explainer

CORS (Cross-Origin Resource Sharing) is a browser mechanism that decides whether JavaScript from one site is allowed to read a response from a server on a different domain. This tool walks through, step by step, whether a specific request will succeed and which Access-Control-* headers it needs.

How to use it

• Provide the method, the request's origin, and the server's response headers (Access-Control-Allow-Origin, etc.) and the tool determines whether the browser will let the request through.
• The "simple" vs. "preflight" classification shows whether the browser will first send an OPTIONS preflight request before the actual one.
• The summary gives the exact reason for a failure when a request would be blocked — faster than parsing the browser console's error message.

Common uses

• Debugging the classic "has been blocked by CORS policy" error with a step-by-step explanation of what's missing, instead of guessing.
• Checking a backend's CORS configuration before the frontend and API end up on separate domains in production.
• Understanding why a request with credentials: include is rejected even though Access-Control-Allow-Origin looks correct.

Things to keep in mind

CORS protects the user's browser, not the server — the restriction only applies to requests made from page JavaScript in a browser, not to requests from curl, Postman, or server-to-server calls.

Access-Control-Allow-Origin: * is incompatible with credentials: include — for requests carrying cookies or authorization, the server must return a specific origin instead of a wildcard.