Reason

Reason: invalid token 'xyz' in CORS header 'Access-Control-Allow-Headers'

What went wrong?

The response to theCORS request that was sent by the server includesanAccess-Control-Allow-Headers header which includes at least oneinvalid header name.

TheAccess-Control-Allow-Headers header is sent by the server in responseto apreflight request; it lets the client know whichHTTP headers are permitted in CORS requests.If the clientuser agent finds among the comma-delineated valuesprovided by the header any header name it does not recognize, this error occurs.

This is a problem that most likely can only be fixed on the server side, by modifyingthe server's configuration to no longer send the invalid or unknown header name with theAccess-Control-Allow-Headers header. It may also be worth checking toensure that the user agent or HTTP library you're using on the client is up-to-date.

See also

• CORS errors
• Glossary:CORS
• CORS introduction
• HTTP headers