The Tool Desk
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What AnotherExample does
According to Arthur G’s DEV Community article introducing the tool, AnotherExample is free and compares a failing request with a similar request sent to a known-working test endpoint. The point of the comparison is to show where the two requests differ, so you know where to look next. In the author’s words: “The comparison helps narrow down where to investigate next.”
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The same article calls the tool a work in progress and invites feedback. It is a diagnostic aid. It does not change a remote server’s policy, and the article does not document the exact test coverage, privacy behavior, or the browsers and CORS scenarios it supports. Treat its output as a lead to check against the browser’s own messages, not as a verdict on the server.
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Browsers enforce the same-origin policy: by default a page can only read responses from the same origin, meaning the same scheme, host, and port. CORS (Cross-Origin Resource Sharing) is the mechanism a server uses to relax that rule. The server sends response headers that name which other origins may read the resource, and the browser checks them before handing the response to your code.
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A CORS error therefore has one of two broad causes. Either the server intentionally does not allow the requesting origin, or the response does not satisfy the browser’s CORS checks, for example because a header is missing, mismatched, or sent in the wrong form. In both cases the server controls the outcome. Your page can only change the request it sends.
Read the browser’s own diagnosis first
JavaScript does not receive the detailed reason for a CORS rejection. A failed fetch() typically surfaces as a generic network error, so the browser’s developer tools are where the specific reason appears.
- Open developer tools. In Chrome, Edge, or Firefox, press F12 on Windows or Linux, or Cmd+Option+I on macOS.
- Select the Console tab, reload the page, and trigger the request. Chromium-based browsers print messages such as “has been blocked by CORS policy” and name the missing or mismatched header. Firefox uses different wording, so read the full message rather than matching a phrase.
- Select the Network tab, click the failing request, and open Headers to inspect the request headers and the response headers.
- If the request was not a simple one, look for a preceding request with method
OPTIONS. This is the preflight, and its status and response headers matter as much as the main request’s.
Compare the failing request with a working one
Whether you use AnotherExample or a manual comparison, the useful unit of analysis is the pair of requests. Check the same axes on each.
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|---|---|---|
| Requesting origin | Scheme, host, and port of the page making the call | Page served over http while the API expects https, or a different port, so the origin is not in the allow-list |
| URL and redirects | The final URL after any redirects | A redirect to another host, where the target’s responses lack the expected CORS headers |
| Method | GET, POST, PUT, PATCH, DELETE, and so on | A method other than GET, HEAD, or POST triggers a preflight that the server does not answer |
| Request headers | Authorization, custom headers, and Content-Type | A custom header or Content-Type such as application/json triggers a preflight, and the header is absent from Access-Control-Allow-Headers |
| Preflight response | Status and headers of the OPTIONS response | A non-success status, or missing Access-Control-Allow-Methods or Access-Control-Allow-Headers |
| Credentials | Whether cookies or HTTP authentication are sent | Credentials are included, but the server answers with a wildcard origin or omits Access-Control-Allow-Credentials |
| Response headers | Access-Control-Allow-Origin and related headers on the main response | The header is missing, or its value does not match the requesting origin exactly |
Where the working request differs from the failing one on one axis, you have a strong lead. Where they differ on several, change one variable at a time and retest.
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Fixing the most common causes
Missing Access-Control-Allow-Origin
If the console reports that the header is absent, the response did not grant your origin. If you control the server, return Access-Control-Allow-Origin with the value of the requesting origin, for example https://app.example.com. A wildcard * is acceptable only for public resources that do not need credentials. If the server answers different origins with different values, send Vary: Origin so caches do not serve one origin’s response to another.
Credentials and the wildcard
Credentialed requests are stricter. The client must opt in with credentials: 'include' in fetch(), or withCredentials = true on an XMLHttpRequest. The server must then send Access-Control-Allow-Credentials: true and must name the exact origin in Access-Control-Allow-Origin. A wildcard * is not permitted in this case, so a fix that works without credentials can break as soon as cookies are added.
Preflight failures
A preflight is a permission check the browser sends before the real request, for requests with non-simple methods, custom headers, or certain content types. The server must answer the OPTIONS request with a success status and the headers that cover the actual request:
Access-Control-Allow-Originmatching the requesting originAccess-Control-Allow-Methodsincluding the method you useAccess-Control-Allow-Headersincluding every custom header and Content-Type value you send- Optionally,
Access-Control-Max-Ageto let the browser cache the preflight result
If the API design allows it, you can also change the request shape, such as dropping an unnecessary custom header, so no preflight is needed. Do this only where the change keeps the API’s behavior correct.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.If you do not control the server
Your options are narrower. You can ask the service owner to add an allow-origin value for your site, which is the durable fix. Alternatively, you can send the request from a server you control. Server-to-server calls are not subject to the browser’s CORS checks, so your backend can forward the request and return the result to your page. Be cautious with third-party services that offer to bypass CORS for you: they see your request and any credentials or data in it.
Why no-cors is not a general fix
Setting mode: 'no-cors' in fetch() lets the request go out, but the response is opaque. JavaScript cannot read its status, headers, or body. That makes it suitable only when the page does not need to inspect the result, such as some fire-and-forget beacons or image and script loads. For an API call whose result your code uses, it hides the problem rather than solving it.
What the comparison can and cannot establish
A failing-versus-working comparison can show which header, method, or credential setting differs, and that is often enough to direct a fix. It cannot prove that a remote server’s policy is wrong, and it cannot confirm that a given server will accept your origin. Verify any finding against the browser console and the server’s response headers before you rely on it. Before pasting a request that contains tokens or personal data into any online tool, read the tool’s own page for its current data-handling terms, since the author’s article does not establish them.
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For the underlying rules, the MDN Web Docs pages on CORS and on CORS errors are the reference to check the header semantics against.
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