What does HTTP 426 mean?
The client must switch to a different protocol.
Common causes and when you see it
The server requires a newer TLS version or HTTP version.
How to fix or handle it
Upgrade the client and follow the Upgrade header.
Test it yourself
Send a request with the online API tester, inspect the response with the HTTP header checker, or host an endpoint that returns 426 with the mock API generator to see how your app reacts.
HTTP 426 questions
The client must switch to a different protocol.
The server requires a newer TLS version or HTTP version.
Upgrade the client and follow the Upgrade header.