CORS Debugger
Enter your frontend URL and API URL and this tool explains whether CORS applies, whether the browser will send a preflight, and the exact Access-Control-* response headers your API must return — including the common credentials-plus-wildcard mistake. A rules-based explainer (it can't send the request, since the browser would block it).
Free · No sign-up · Updated August 2026
This is a cross-origin request (https://app.example.com → https://api.example.com), so CORS applies. The browser will first send a preflight OPTIONS request because this isn't a “simple” request.
Access-Control-Allow-Origin: https://app.example.com (or *) Access-Control-Allow-Methods: POST Access-Control-Allow-Headers: Content-Type, Authorization, X-Custom-Header
Explains what a CORS failure requires from the server — it can't actually send the request (the browser would block it), so it works from the rules, not a live call.
🔒 This tool runs entirely in your browser. Your data is never uploaded.
Frequently asked questions
Can it actually send the request?＋
No — if there were a CORS problem the browser would block it anyway. Instead it explains, from the rules, exactly which response headers your API needs.
What's the most common CORS mistake?＋
Using Access-Control-Allow-Origin: * together with credentials. With cookies/credentials the header must echo the exact origin and include Access-Control-Allow-Credentials: true.