CORS preflight failed fix for browser API calls
Browsers send OPTIONS before non-simple requests. Preflight fails when Allow-Origin, Allow-Methods, or Allow-Headers do not match what the app sends. API keys in custom headers always trigger preflight.
Common causes
- OPTIONS returns 404 or 401.
- Credentials without Access-Control-Allow-Credentials.
- Wildcard origin with credentials.
How to fix
- Fix server OPTIONS handler and headers.
- Use CORS Checker against the real URL.
- Align allowed headers with client fetch options.
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