426 Upgrade Required means the server refuses to perform the request with the current protocol but might do so after the client upgrades to a different one. The server must send an Upgrade header naming the required protocol or protocols.
The most common case is a WebSocket endpoint that receives an ordinary HTTP request: many WebSocket servers answer 426 to say that the client should send a WebSocket handshake. 426 has also been used to require TLS (RFC 2817) or a newer HTTP version, but for HTTPS a redirect is the usual approach.
Example response
HTTP/1.1 426 Upgrade Required
Date: Fri, 06 Nov 2026 10:15:32 GMT
Upgrade: websocket
Connection: Upgrade
Content-Type: text/plain
Content-Length: 49
This endpoint only accepts WebSocket connections.SEO and redirect checking
A URL that returns 426 cannot be indexed, but such URLs are usually WebSocket or API endpoints that should not be crawled anyway. If a redirect check shows 426 for a normal page, the server or proxy is routing that URL to a WebSocket backend.
Common causes and use cases
- A plain HTTP
GETwas sent to a WebSocket endpoint. - A proxy in front of the server does not pass on the
Upgradeheader, so the backend sees an ordinary request. - The server requires a protocol version that the client did not use.