CORS Error Diagnostic

The CORS Error Diagnostic takes a cross origin error from your console and tells you the likely cause and the server side fix. A CORS error means the browser blocked the request because the response did not carry the right headers, not because the network failed, so the server may have answered perfectly. Common root causes are a missing allow origin header, a preflight that was not accepted, or a credentials setting that does not line up. Use it when a fetch that works in Postman fails in the browser, and follow the suggested change, or use the CORS Header Generator to produce the exact configuration you need. A cross origin resource sharing error is one of the most misleading messages in web development, because it looks like a network failure while the server may have answered perfectly well. The browser is the one enforcing the block, and it does so after receiving the response, on the grounds that the response did not carry headers permitting your origin to read it. Recognising that the enforcement point is the browser reframes the whole diagnosis, since the fix is always a change to server configuration rather than to client code. The common causes are a missing allow origin header, a preflight request that was not answered or answered with the wrong methods or headers, and a credentials setting that does not line up between the request and the response, which matters because a wildcard origin is not permitted together with credentialed requests. A diagnostic that reads the error and points at the likely cause turns an opaque message into a specific change, and the practical test that separates the two cases is whether the same call succeeds from a tool outside the browser, since that proves the endpoint works and the problem is header configuration.

📘 Related guide: Debug an API request in the browser (including CORS) →