CORS No Access-Control-Allow-Origin Fix

This CORS error is enforced by the browser, but it is fixed on the API or proxy response, not in frontend fetch options alone.

Symptoms

  • Browser console mentions No Access-Control-Allow-Origin.
  • The same URL works in curl or Postman.
  • Requests fail only from a different domain or port.

Likely causes

  • API does not allow the frontend origin.
  • Preflight OPTIONS response is missing CORS headers.
  • Credentials are used with wildcard origins.
  • A proxy or error response drops CORS headers.

Fix steps

  1. Check the actual response headers in browser devtools.
  2. Handle OPTIONS preflight on the API or proxy.
  3. Return a specific origin when credentials are used.
  4. Make error responses include the same CORS policy.

Verify the fix

  • Test OPTIONS and real request separately.
  • Inspect Access-Control-Allow-Origin and credentials headers.
  • Confirm the failing response is not an HTML error page.

FAQ

Why does curl work?

curl is not a browser and does not enforce browser CORS rules.

Can mode no-cors fix it?

No. It creates an opaque response and usually hides the data you need.

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