The HTTP 426 Upgrade Required client error response code indicates that the server refuses to perform the request using the current protocol but might be willing to do so after the client upgrades to a different protocol.
The server sends an Upgrade header with this response to indicate the required protocol(s).
426 Upgrade Required
HTTP/1.1 426 Upgrade Required Upgrade: HTTP/2.0 Connection: Upgrade Content-Length: 53 Content-Type: text/plain This service requires use of the HTTP/2.0 protocol
| Specification |
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| HTTP Semantics # status.426 |
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https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/426