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426
Client Error

426 Upgrade Required

The client should switch to a different protocol, as indicated by the Upgrade header field.

Description

426 Upgrade Required indicates that the server refuses to perform the request using the current protocol but may accept the request after the client upgrades to a different protocol. The server must include an Upgrade header specifying the required protocol(s).

Commonly used to enforce TLS (HTTPS) connections or to require WebSocket upgrade.

Common Causes

  • HTTP request to a server that requires HTTPS
  • WebSocket handshake requiring protocol upgrade
  • Legacy API enforcing minimum TLS version
  • HTTP/1.1 client connecting to an HTTP/2-only endpoint

How to Fix

  • Switch to the protocol specified in the Upgrade response header
  • Ensure all connections use HTTPS (implement HSTS)
  • For WebSockets, send a proper Upgrade request
  • Update client to support the required protocol version

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Related Status Codes

400
400 Bad Request
The server cannot process the request due to malformed syntax.
 401
401 Unauthorized
Authentication is required and has failed or not been provided.
 402
402 Payment Required
Payment is required to access this resource.
 403
403 Forbidden
The server understood the request but refuses to authorize it.
 404
404 Not Found
The server cannot find the requested resource.
 405
405 Method Not Allowed
The HTTP method used is not allowed for this resource.

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