426
4xx Client ErrorUpgrade Required
The client should switch to a different protocol.
426 Upgrade Required means the server refuses to keep talking over the current protocol. The client has to switch to the one named in the response's Upgrade header and try again. It's the spec's way to force a protocol upgrade on the same port, though in the wild it's rare. Some services use it to turn away ancient TLS or HTTP versions.
What 426 looks like on the wire
GET /api/data HTTP/1.1 Host: example.com HTTP/1.1 426 Upgrade Required Upgrade: HTTP/3.0 Connection: Upgrade
About 4xx Client Error
The request has a problem the client needs to fix, like a bad URL, missing auth, or invalid input.
How to fix 426 Upgrade Required
- Read the Upgrade header in the response to see which protocol the server wants.
- Update the client, library, or TLS stack to a version the server accepts.
Where it's defined
426 Upgrade Required is defined in RFC 9110 §15.5.22. MDN's 426 reference covers browser behavior and examples.