CORS Preflight Failed Fix

CORS preflight failures happen before your frontend receives a normal response. Debug the OPTIONS request and response headers first.

Symptoms

  • Browser console mentions CORS policy or preflight request failed.
  • The OPTIONS request fails or lacks allow headers.
  • curl works but browser fetch fails.

Likely causes

  • Server does not handle OPTIONS requests.
  • Access-Control-Allow-Origin, Methods or Headers are missing.
  • Credentials are enabled with a wildcard origin.

Fix steps

  1. Inspect the OPTIONS request in DevTools Network.
  2. Return the correct Access-Control-Allow-* headers from the API or proxy.
  3. Avoid wildcard origin when credentials are required.

Verify the fix

  • Reload with cache disabled.
  • Test the browser request again, not only curl.
  • Check CDN or reverse proxy rules if headers vanish.

FAQ

Can frontend JavaScript fix CORS?

No. The responding server or proxy must send the correct CORS headers.

Why does curl work?

curl does not enforce browser CORS rules.

Related tools and guides

HTTP HTTP Headers Checker

Analyze pasted HTTP response headers for cache, redirects, security headers and content-type issues without making server-side requests.

 cURL cURL Command Builder

Build readable cURL commands for API requests with method, headers, JSON body and safe auth placeholders.

 JS JavaScript Error Explainer

Paste JavaScript browser or Node.js errors and get likely causes, fixes and debugging steps for common failure patterns.

 cURL cURL API Debugging Workflow for Developers

cURL is the simplest way to separate API behavior from SDK, framework or frontend code. Build a minimal request first, then compare it with the failing implementation.

Last updated: May 18, 2026