4xx Client errors
431 Request Header Fields Too Large
The request's headers are too big.
Common causes
- Too many or too large cookies, often after years of adding tracking cookies.
How to fix it
- As a visitor: clear cookies for the site.
- As the owner: trim cookies, or raise the server's header size limit.
Find out the moment your site returns 431
Spot Downtime monitors count this as down and alert you, since visitors see an error. (The public “Is it down?” checker treats 4xx answers from big sites as online, because many refuse automated requests.)