The Tool Desk
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A CORS error means the browser refused to let your page read a response, and the cause is almost always in the server’s response headers or in how the request was built. AnotherExample is a free, author-built tool that helps you narrow that down by comparing your failing request with a similar request to a known-working test endpoint. It can point you toward where to look. It cannot change a remote server’s policy, so the fix still has to happen on the server or in the request itself.
What a CORS error actually tells you
Cross-Origin Resource Sharing (CORS) is a browser mechanism. Response headers tell the browser which other origins may read a resource, and the server that controls the resource decides whether to grant that access. When the headers are missing, or they do not match what the browser expects for the request, the browser blocks the response from reaching your JavaScript. The page sees only a generic network failure.
That is why the browser console matters more than the error text in your application code. The console states the specific reason the request was rejected. JavaScript generally cannot see that detailed reason, so debugging starts in the developer tools, not in your fetch or XMLHttpRequest catch block.
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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsA CORS error has two broad origins, and they call for different fixes:
#1 Best Overall
- The server intentionally disallows your origin. The response does not grant access to the page’s origin, so the correct fix is a policy change, made by whoever runs the server.
- The server meant to allow you, but the response does not satisfy the browser’s checks. The header is present but has the wrong value, the preflight is mishandled, or the credentials settings conflict with the headers. This is usually a configuration bug.
What AnotherExample does, and what it does not
According to Arthur G, who built the tool and described it in a DEV Community article, AnotherExample is free. Its method is comparison: you give it a request that fails, and it sets that against a similar request to a known-working test endpoint. The stated purpose is to help narrow down where to investigate next. The creator’s own words are that “the comparison helps narrow down where to investigate next.”
That framing matters. A passing test endpoint shows you which parts of your request differ from one that the browser accepts. It does not prove that your target server will behave differently, and it does not replace reading the response headers yourself.
The same article describes the tool as still a work in progress and asks for input from readers. The available description does not document the exact test coverage, data handling, request retention, privacy behavior, or the browsers and CORS scenarios it supports. Check the tool directly for those details before relying on it for anything sensitive, and do not assume it covers every browser or endpoint type.
A troubleshooting sequence that works without the tool
Use these steps in order. The tool fits best at step 4, once you have the raw facts about the failing request.
Rank #3
- Reproduce the failure and copy the console message. Open the browser’s developer tools, reload the page, and trigger the request. Record the exact CORS message in the Console tab, along with the URL that failed.
- Inspect the network transaction. In the Network tab, select the failing request. Record the request headers (including
Origin), the response headers, and, if one was sent, theOPTIONSpreflight request and its response. Note whether the request was sent with credentials. - Check whether the server is meant to serve your origin. If the response has no
Access-Control-Allow-Originheader, the server has not granted your origin access. If you control the server, add a header whose value matches the requesting origin. If you do not control it, contact the service owner, or place a server-side proxy you control between the page and the API. - Compare the failing request with a known-working one. Use the axes in the table below. AnotherExample automates this comparison against a test endpoint, or you can do it by hand with two captured requests.
- Verify the fix in the Network tab. Reload, confirm the preflight now succeeds where one is sent, and confirm the main response carries the expected allow-origin value.
What to compare between a failing and a working request
A side-by-side comparison only helps if you record the same fields for both requests. The table lists the axes that most often differ.
| Axis | What to record | Typical mismatch |
|---|---|---|
| Requesting origin | The Origin header sent by the page |
The page runs on a different scheme, host, or port than the server expects |
| URL and redirects | The final URL after any redirect | A redirect moves the request to a host the server does not allow |
| Method | GET, POST, PUT, DELETE, and so on | A non-simple method triggers a preflight that the server does not answer |
| Request headers | Custom headers such as Authorization or Content-Type values |
A custom header is not listed in the preflight’s allowed headers |
| Preflight response | Status and Access-Control-Allow-Methods / Access-Control-Allow-Headers |
The preflight returns an error status or omits the required headers |
| Credentials mode | Whether cookies or authorization are sent | Credentials are sent but the response uses a wildcard origin or omits the credentials header |
| Main response headers | Access-Control-Allow-Origin and Access-Control-Allow-Credentials |
The header is missing, or its value does not match the requesting origin |
These axes come from the Mozilla Developer Network (MDN) CORS documentation, which lists them as common sources of mismatch.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Fixing the common cases
The Access-Control-Allow-Origin header is missing
This is the most common branch. The server returns a response without the header, so the browser has no permission to expose it to the page. On a server you control, return Access-Control-Allow-Origin with a value for the requesting origin. On a third-party API you do not control, the page cannot grant itself access. Your realistic options are to ask the provider, use an endpoint the provider documents for browser access, or proxy the request through a server you control.
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If the request includes credentials, do not return Access-Control-Allow-Origin: *. The response must name an allowed origin explicitly and meet the credentials requirements. Setting the wildcard is the most frequent cause of a failure that looks correct when you only read the headers quickly.
The preflight fails
Some methods, request headers, and content types cause the browser to send an OPTIONS preflight before the real request. The server must answer that preflight with the correct status and allow-list headers. If the server does not handle OPTIONS, or returns an error, the real request never goes out. Where the application can use a simpler request shape that is valid for its use case, changing the request may avoid the preflight. Do not change the request shape just to hide a server problem.
Do not use no-cors as a general fix
Setting mode: 'no-cors' on a request stops the console error, but it produces an opaque response. JavaScript cannot read its body or most of its headers. That is useful only when the caller does not need to inspect the response, such as some fire-and-forget requests. For any code that reads the data, it does not solve the problem.
Where the tool fits, and where it stops
AnotherExample is most useful when you have a failing request and no clear idea which axis differs. It gives you a concrete second request to compare against, which shortens the search. It does not establish that every browser, endpoint, or CORS scenario is covered, and it does not verify a server’s policy on your behalf. Treat its result as a lead, then confirm the final fix in the Network tab and on the server’s response headers.
Recommended Free Tools
No independent source supplied for this article measures how often the tool finds the cause, how quickly it does so, or how many people use it. Those claims should not be made about it until a published, attributable source provides them.
Quick Recap
Sources
- Arthur G, “I built AnotherExample: a free tool for troubleshooting CORS,” DEV Community.
- MDN Web Docs, “CORS errors,” in the HTTP documentation.
- MDN Web Docs, “Reason: CORS header ‘Access-Control-Allow-Origin’ missing,” in the HTTP documentation.
- MDN Web Docs, “Cross-Origin Resource Sharing (CORS),” in the HTTP documentation.
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