Why am I getting CORS errors even though my server returns 200?

The browser blocks the response if CORS headers are missing or incorrect. The server must return Access-Control-Allow-Origin and handle OPTIONS preflight requests.

How do I fix CORS on Express?

Install the cors package and add app.use(cors({ origin: 'your-frontend.com' })). CORSFixer generates the exact config based on your live preflight response.

Can I use Access-Control-Allow-Origin: * with credentials?

No — browsers reject this combination per the CORS spec. You must specify an explicit origin when using credentials: true.

Paste your URL and origin. CORSFixer sends a live OPTIONS preflight and generates middleware config for Express, Nginx, FastAPI, Django, Laravel, and Cloudflare Workers.

Tests your real CORS configuration with a live OPTIONS preflight. Generates fixes for Express, Nginx, FastAPI, Django, Laravel, and Cloudflare Workers.

Target URL

Request Origin

Your URL never leaves your browser — this is a live client-side preflight

Tip: curl -sI -X OPTIONS https://api.your-domain.com -H "Origin: https://your-frontend.com"

Request origin (for fix generation)

Detected framework:

About CORSFixer

What does CORSFixer check?

CORSFixer sends a live OPTIONS preflight request from your browser to your API endpoint and reads the real CORS response headers. It checks for missing Access-Control-Allow-Origin, incorrect wildcard usage with credentials, missing preflight handlers, and misconfigured Access-Control-Allow-Methods and Access-Control-Allow-Headers.

Why does CORS fail with credentials?

When a request includes credentials (cookies or Authorization headers), browsers reject responses with Access-Control-Allow-Origin: *. You must specify the exact origin. This is the most common CORS misconfiguration in FastAPI, Express, Django, and Nginx setups.

Which frameworks does CORSFixer generate fixes for?

CORSFixer generates middleware config for Express, Nginx, FastAPI, Django, Laravel, and Cloudflare Workers. The fix is tailored to the framework you select — not a generic header snippet.

What is a CORS preflight request?

A preflight is an OPTIONrequest the browser sends before the actual request to check whether the server allows the cross-origin call. If the server does not respond correctly to the OPTIONS request, the browser blocks the main request before it is sent.