CORS Tester tools let you check if an API or site has valid CORS headers by simulating cross-origin requests. See how they work and where to find them.
A CORS Tester is a free, web-based diagnostic tool that checks whether a server or API has Cross-Origin Resource Sharing configured correctly for a given origin, HTTP method, and set of headers. It automates the same preflight and actual request checks a browser performs, then reports the resulting CORS response headers in a readable format.
CORS-Tester is not one single branded product; it describes a category served by multiple independent tools, including offerings from TestingBot, Corsfix, EaseCloud, CodeHappy, ToolDock, TestMu AI (formerly LambdaTest), WebBrowserTools, and U2Tool, all of which are free and require no account.
A typical CORS tester asks for a target URL, the origin that should be allowed to call it, the HTTP method the real request would use, and any custom headers the request would send. It then issues an OPTIONS preflight request, and often the actual follow-up request, exactly as a browser would.
The tool reads back headers such as Access-Control-Allow-Origin, Access-Control-Allow-Methods, Access-Control-Allow-Headers, and Access-Control-Allow-Credentials, then compares them against what the browser would require to allow the request, returning a pass or fail result with an explanation of any mismatch.
The most frequent issue these tools surface is a missing or incorrect Access-Control-Allow-Origin header, where the server either omits the header entirely or lists an origin that does not match the calling frontend's domain.
Other common findings include a server that allows a wildcard origin while also trying to allow credentialed requests, which browsers block outright, missing methods in Access-Control-Allow-Methods for non-GET requests, and custom request headers that are not included in Access-Control-Allow-Headers, all of which a CORS tester can identify without the developer needing to reproduce the error manually in a browser.
It checks whether a target URL returns the correct Cross-Origin Resource Sharing headers, such as Access-Control-Allow-Origin, -Methods, -Headers, and -Credentials, for a given calling origin, HTTP method, and set of request headers.
Yes, the known implementations, including those from TestingBot, Corsfix, EaseCloud, CodeHappy, ToolDock, TestMu AI, WebBrowserTools, and U2Tool, are free to use with no registration required.
Enter the target URL in the endpoint field, set the origin to the domain your frontend runs on, select the HTTP method you plan to use, optionally add custom headers, then run the test to see whether the request passes or fails and why.
CORS blocks a cross-origin request when the server's response does not include an Access-Control-Allow-Origin header matching the calling origin, or is missing required allowed methods or headers; a CORS Tester identifies exactly which header is the problem.
Support varies by tool; many free CORS testers are built for simple public endpoint checks and may have limited support for cookies, tokens, or other authenticated request scenarios.
It can be, particularly when combined with credentialed requests (cookies or authorization headers), which browsers block by design; several CORS testers flag this specific misconfiguration.
Most run the test request from the tool's own server rather than the user's browser, which avoids the browser's own CORS restrictions interfering with the diagnostic test itself.
No, it describes a category of similar free tools built by different companies and individual developers, rather than one single branded product.