CORS Preflight Request Explainer and Fix Checklist Generator
Diagnose browser CORS errors by entering your frontend origin, API URL, request method, request headers, content type, credentials setting and console error. Generate a plain-English explanation, preflight prediction, required response headers, safe server checklist and frontend fetch notes.
- Check if preflight is expected.
- Find missing CORS headers.
- Generate server header examples.
- Avoid unsafe fixes like no-cors or exposed secrets.
Explain a CORS or preflight error
Enter the browser request details and console error. The tool will decide whether a preflight is likely, which headers the server must return, what the browser is blocking, and how to debug it safely.
Your CORS explanation will appear here
Enter your browser request details and generate a CORS/preflight fix checklist.
What this CORS preflight explainer does
CORS errors are confusing because the browser often blocks the response before your JavaScript can read the real backend error. A request can work perfectly in Postman, cURL, PHP or a server-side job, but still fail in browser fetch. That is because CORS is enforced by browsers. The server must explicitly tell the browser which origins, methods and headers are allowed.
This tool helps you understand that browser decision. Enter the frontend origin, API URL, method, content type, custom request headers, credentials mode and console error. The generator predicts whether a preflight request is expected, explains why the browser is blocking the request, lists the required response headers, creates a safe server-side checklist and gives frontend fetch notes.
The tool does not send a request, bypass CORS or change your server. It creates a debugging plan. CORS must be fixed on the server that owns the API response. Frontend workarounds such as mode: "no-cors" usually hide the response from your JavaScript and do not solve real API integration problems.
CORS and preflight basics
| Part | Meaning | Common mistake |
|---|---|---|
| Origin | The scheme, host and port of the frontend page making the browser request. | Allowing the wrong origin or forgetting that ports matter. |
| Access-Control-Allow-Origin | Server response header that tells the browser which origin may read the response. | Missing it, setting the wrong origin, or using wildcard with credentials. |
| Preflight OPTIONS | A browser check before the actual request for certain methods, headers or content types. | Backend route handles POST but not OPTIONS. |
| Access-Control-Allow-Methods | Response header listing methods allowed for the actual request. | Forgetting PUT, PATCH, DELETE or POST. |
| Access-Control-Allow-Headers | Response header listing request headers the browser may send. | Forgetting Authorization, Content-Type, X-API-Key or X-Requested-With. |
| Access-Control-Allow-Credentials | Allows cookies or credentials when the frontend request uses credentials. | Using credentials with wildcard origin. |
| Access-Control-Max-Age | Lets the browser cache the preflight result for a time. | Setting it too high while still testing unstable CORS rules. |
Why the browser sends a preflight request
A preflight request is a browser safety check. Before sending the actual request, the browser sends an OPTIONS request to ask the server whether the real request is allowed. The preflight includes the origin, requested method and sometimes requested headers. The server must respond with matching CORS headers. If the preflight fails, the browser never sends the actual POST, PUT, PATCH or DELETE request.
Example preflight request: OPTIONS /v1/orders HTTP/1.1 Origin: https://app.example.com Access-Control-Request-Method: POST Access-Control-Request-Headers: authorization, content-type Example preflight response: HTTP/1.1 204 No Content Access-Control-Allow-Origin: https://app.example.com Access-Control-Allow-Methods: GET, POST, OPTIONS Access-Control-Allow-Headers: Authorization, Content-Type Access-Control-Max-Age: 600
Many developers only build the actual API route and forget the OPTIONS route. That is why the browser console may show a CORS preflight error even though the real API endpoint works from Postman.
Common CORS errors and what they usually mean
If the console says there is no Access-Control-Allow-Origin header, the browser did not see permission for the frontend origin to read the response. If the console says a request header is not allowed, the server probably forgot to include that header name in Access-Control-Allow-Headers. If the console says the method is not allowed, the server probably forgot the method in Access-Control-Allow-Methods. If credentials are included, the server must handle cookies/auth carefully and cannot simply use wildcard origin for credentialed browser requests.
A redirect during preflight can also break the request. Some servers redirect HTTP to HTTPS, add trailing slashes, redirect to login, or send OPTIONS to a default route. Preflight should usually return a direct 200 or 204 response with the correct CORS headers.
Safe server-side CORS fix pattern
The safest CORS fix is not “allow everything forever.” The safer pattern is to allow the exact frontend origin, allow only the methods your API needs, allow only the request headers your frontend actually sends, and enable credentials only when cookies or browser credentials are truly required.
Safer CORS response idea: Access-Control-Allow-Origin: https://app.example.com Access-Control-Allow-Methods: GET, POST, PUT, PATCH, DELETE, OPTIONS Access-Control-Allow-Headers: Authorization, Content-Type, X-API-Key Access-Control-Allow-Credentials: true Access-Control-Max-Age: 600 Vary: Origin
For public APIs that do not use cookies or credentials, wildcard origin can be acceptable in some cases. For private APIs, dashboards, admin panels, paid APIs, or APIs using browser cookies, use explicit allowlists and avoid exposing secrets in frontend code.
Frontend mistakes that do not really fix CORS
One common mistake is adding mode: "no-cors" to fetch. This often creates an opaque response that JavaScript cannot read. The request may appear to stop showing the same error, but the frontend still cannot access the JSON response. Another mistake is moving a private API key into browser JavaScript just to make a direct frontend request work. Browser code can be inspected, so private API keys do not belong there.
When a third-party API does not allow browser origins, the correct solution is often a backend route, server-side proxy, WordPress HTTP API request or serverless function that keeps secrets on the server. Use the cURL to JavaScript, PHP and WordPress Converter when documentation gives you cURL and you need a safer server-side PHP or WordPress example.
CORS troubleshooting workflow
Start by checking where the request works. If it works in Postman or cURL but fails in browser, the backend may be fine from a raw HTTP perspective but missing browser CORS permission. Next, inspect the browser Network tab. Look for an OPTIONS request. Check its status code, response headers, request method and requested headers. Then check whether the actual request was sent after preflight.
If the OPTIONS request returns 401 or 403, your authentication middleware may be blocking preflight before the API route can answer. If it returns 404, the OPTIONS route may not exist. If it returns 405, the route exists but does not allow OPTIONS. If it redirects, the browser may reject the preflight chain. If it returns 200 or 204 but still fails, compare the exact Origin, requested method and requested headers against the server’s allow headers.
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FAQ
What is a CORS error?
A CORS error happens when the browser blocks frontend JavaScript from reading a response from a different origin because the API response does not include the required CORS permission headers.
What is a preflight request?
A preflight request is an OPTIONS request sent by the browser before certain cross-origin requests. It checks whether the server allows the requested method and headers from the frontend origin.
Why does the API work in Postman but not in browser fetch?
Postman and cURL are not blocked by browser CORS rules in the same way. The API may work as raw HTTP but still fail in a browser if the server does not return the correct CORS headers.
Can frontend JavaScript fix CORS?
Usually no. CORS permission must come from the API server response. Frontend code can reduce unnecessary preflight triggers, but it cannot force the browser to accept a response that the server did not allow.
Should I use mode no-cors in fetch?
Usually no for API work. It often creates an opaque response that JavaScript cannot read, so it hides the problem instead of giving your app usable JSON.
Can I use Access-Control-Allow-Origin star?
Wildcard origin can be acceptable for some public, non-credentialed resources. It is not a safe default for private APIs, cookie-based requests, admin APIs or APIs that need credentials.
Why does Authorization header trigger preflight?
Custom or non-safelisted headers such as Authorization usually cause the browser to ask the server first with a preflight request. The server must allow that header in Access-Control-Allow-Headers.
Why does application/json trigger preflight?
JSON requests commonly trigger preflight because application/json is not one of the simple content types used by simple CORS requests. The server must handle OPTIONS and allow Content-Type.
How do I fix CORS for a third-party API?
If the third-party API does not allow browser origins, you usually need to call it from your backend, serverless function or WordPress/PHP server code instead of exposing private keys in the browser.
Does this tool send a real preflight request?
No. It runs in your browser and generates a diagnostic checklist from the details you enter. Use your browser Network tab or server logs to inspect the real request.
Final practical note
CORS is not a random frontend bug. It is a browser-enforced permission check. The fix is usually on the API response: correct origin, methods, headers, credentials behavior and OPTIONS handling. Use this tool to identify what the browser is asking for and what the server must return.
