Localhost request debugger

CORS Header Analyzer

Compare a browser request with the response headers that decide whether JavaScript may read it.

Analysis is local. This page does not fetch the target URL.

Request and response

Use the values shown in your browser’s Network panel.

CORS verdict

Waiting for response headers

CORS failure checklist

  1. Confirm it is actually cross-origin

    Scheme, hostname, and port all belong to the origin. Changing only 5173 to 3000 makes a different origin.

  2. Inspect the OPTIONS request

    If the browser sends a preflight, the preflight response must approve the intended method and non-safelisted request headers.

  3. Configure the API, not the frontend response

    CORS permission is returned by the server being called. Adding response headers in frontend JavaScript cannot repair a rejected browser response.

  4. Keep credentials and wildcard origins separate

    Credentialed CORS requests need an explicit allowed origin and credential permission; a wildcard origin is not valid for that case.

Related localhost diagnosis

If the API never responds, solve reachability before CORS: see Vite on port 5173, localhost:3000 troubleshooting, or generate an HTTP header check with the Port Command Generator.

Reference

The analyzer follows the browser CORS model defined by the WHATWG Fetch Standard. Server frameworks may expose these settings under different option names.