426 Upgrade Required
This is an educational reference page about HTTP 426 Upgrade Required. The page itself is served as 200 OK so it can be indexed as HTTP documentation.
The client should switch to a different protocol such as TLS/1.3, given in the Upgrade header field.
Specification
Registered in the IANA HTTP Status Code Registry as Upgrade Required. Defined in RFC 9110, Section 15.5.22.
Header and content rules
What RFC 9110 says about a 426 response, quoted from the RFC. The 4xx class rules also apply.
- MUST (server; Upgrade): Send Upgrade naming the required protocol or protocols. Table row
The server MUST send an Upgrade header field in a 426 response to indicate the required protocol(s) (Section 7.8). RFC 9110 §15.5.22
The same rule is in the response obligations table and its JSON.