4xx Client errors
426 Upgrade Required
The client must switch to another protocol, given in the Upgrade header.
Common causes
- The server requires a newer protocol, like TLS or HTTP/2.
How to fix it
- Update the client or use the protocol the server asks for.
Find out the moment your site returns 426
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.)