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CORS Tester

Test cross-origin requests from your browser — exactly what real clients see

About the Cors Tester

CORS (Cross-Origin Resource Sharing) controls which websites can make requests to your API. When it breaks, the browser blocks the response with a confusing error message. The fix usually involves adding the right Access-Control-Allow-Origin header — but you need to see what the server actually returns to know what is missing.

This tester makes a real cross-origin request from your browser to the target URL and shows exactly which CORS headers came back: Access-Control-Allow-Origin, Allow-Methods, Allow-Headers, and more. Because it runs in the browser, you see the same behavior your users see — not a server-side approximation.

How to use

  1. Enter the API URL you want to test.
  2. Choose the HTTP method (GET, POST, PUT, etc.).
  3. Click Test — the request runs from your browser.
  4. Review the CORS headers that were returned.

Frequently Asked Questions

Why did my CORS request fail? ▾

The server did not return Access-Control-Allow-Origin for your origin. The fix is to add the header server-side: Access-Control-Allow-Origin: https://yourapp.com (or * for public APIs).

What is a CORS preflight request? ▾

For non-simple requests (custom headers, PUT/DELETE methods), the browser first sends an OPTIONS request to check if the server allows the actual request. The server must respond with Access-Control-Allow-Methods and Access-Control-Allow-Headers.

Is this tool free to use? ▾

Yes! This tool is 100% free. No signup, no credit card, no limits. It's part of Formatho's privacy-first developer toolkit.

Is my data safe? Does this tool send data to a server? ▾

Absolutely. This tool runs entirely in your browser. Your data never leaves your device. Zero server-side processing, zero tracking.