CORS Tester & Checker
Test any URL for CORS headers, diagnose cross-origin errors, and get framework-specific fix suggestions. Enter a URL to test or leave it blank for a comprehensive CORS explainer.
How the CORS Tester & Checker Works
This free online CORS tester sends a real fetch request from your browser to any URL you specify, then inspects the response for standard CORS headers. It checks whether the server allows your origin, which HTTP methods are permitted, whether credentials can be sent, and how long the browser should cache the preflight response. If the request fails due to CORS restrictions, the tool explains exactly what went wrong and gives you copy-paste code to fix it in your specific backend framework.
How CORS Works
When your JavaScript makes a request to a different domain than the page it runs on, the browser performs a security check called Cross-Origin Resource Sharing (CORS). For simple GET requests, the browser adds an Origin header and checks the response for Access-Control-Allow-Origin. For complex requests (PUT, DELETE, or requests with custom headers), the browser first sends an OPTIONS preflight request to verify the server allows it before sending the actual request.
Why You Need a CORS Tester
CORS errors are among the most common and frustrating issues in web development. The browser console shows a generic "blocked by CORS policy" message that does not tell you which specific header is missing or misconfigured. This tool gives you a complete breakdown of every CORS header the server returns (or does not return), pinpoints exactly what needs to change, and provides ready-to-use code snippets for popular backend frameworks including Express, Laravel, Django, Rails, NestJS, and Spring Boot.
Common CORS Issues and Fixes
The most frequent CORS error is a missing Access-Control-Allow-Origin header. Other common problems include the origin not matching exactly (including protocol and port), the requested HTTP method not being listed in Access-Control-Allow-Methods, custom headers not being listed in Access-Control-Allow-Headers, and credentials being required but Access-Control-Allow-Credentials not being set to true. Each of these has a specific fix on the server side.
Understanding Preflight Requests
A preflight request is an automatic OPTIONS request the browser sends before certain cross-origin requests. It occurs when you use HTTP methods other than GET, HEAD, or POST, when you set custom headers, or when you send certain content types. The preflight checks if the server allows the actual request. If the preflight fails, the browser never sends the real request. The Access-Control-Max-Age header controls how long the browser caches the preflight result, reducing overhead for repeated requests.
CORS Explainer Mode
If you leave the URL field empty and click the button, the tool displays a comprehensive visual guide to how CORS works. This includes the request-response flow, the role of each CORS header, when preflight requests are triggered, and a comparison of simple versus complex requests. This is useful for learning or for sharing with team members who are new to cross-origin concepts.
Privacy and Security
All CORS testing happens directly from your browser. The fetch request goes from your machine to the target URL — no proxy server is involved. Your URLs, headers, and results are never stored or transmitted to any third party. This makes the tool safe for testing internal APIs, staging environments, and endpoints that require authentication.