How do I confirm an error is actually CORS?
Read the browser console. A CORS error specifically mentions cross-origin or Access-Control headers. Other errors during a fetch, like network errors or 404s, are not CORS even though they happen during a request.
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Cross-check this information using these trusted sources:
More FAQs in A CORS Debugging Checklist for React Developers
Confirm it is CORS in the console, identify your frontend origin, check the server response headers in the Network tab, verify preflight is handled, check credentials if relevant, apply the right fix on the server or via a proxy, and test from the browser.
Look for Access-Control-Allow-Origin set to your origin or a wildcard. If it is missing or set to a different origin, the server is not allowing you. Also check that OPTIONS preflight requests are handled with the right headers.
Because with cookies or auth headers, the server must set Access-Control-Allow-Credentials to true and must not use a wildcard origin. It must specify your exact origin, or the browser blocks the response.
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