4xx Client Error
426 Upgrade Required
The server refuses this protocol version and names an alternative in `Upgrade`.
What causes it
- A server that requires TLS, or requires HTTP/2.
How to fix it
- Follow the `Upgrade` header. This is also how some APIs enforce a minimum client version.
At a glance
- Class
- 4xx Client Error
- Reference
- RFC 9110 §15.5.22
- Standardised
- Yes
- Retry the request?
- No — fix the request
Other 4xx client error codes
- 400 Bad Request
- 401 Unauthorized
- 402 Payment Required
- 403 Forbidden
- 404 Not Found
- 405 Method Not Allowed
- 406 Not Acceptable
- 407 Proxy Authentication Required
- 408 Request Timeout
- 409 Conflict
- 410 Gone
- 411 Length Required
- 412 Precondition Failed
- 413 Content Too Large
- 414 URI Too Long
- 415 Unsupported Media Type
- 416 Range Not Satisfiable
- 417 Expectation Failed
- 418 I'm a teapot
- 421 Misdirected Request
- 422 Unprocessable Content
- 423 Locked
- 424 Failed Dependency
- 425 Too Early
- 428 Precondition Required
- 429 Too Many Requests
- 431 Request Header Fields Too Large
- 451 Unavailable For Legal Reasons
- 499 Client Closed Request