HTTP Diagnostic

How to Fix "No 'Access-Control-Allow-Origin' header is present on the requested resource" CORS Error

The browser blocks cross-origin fetch/XHR requests because the target HTTP server did not return an Access-Control-Allow-Origin response header matching the requesting frontend domain.

No Access-Control-Allow-Origin

Quick Fix Checklist

  1. Install and mount the official CORS middleware on your backend framework (e.g., app.use(cors()))
  2. Ensure the Access-Control-Allow-Origin header matches your frontend origin exactly if sending credentials
  3. Ensure HTTP OPTIONS preflight requests return HTTP 200 or 204 with allowed headers and methods
  4. For local development, configure a Vite or Next.js proxy to avoid cross-origin domain boundaries entirely