Toronto SEO
All HTTP status codes
4xx Client Error
RFC 9110

426 Upgrade Required

The client must switch to a different protocol to complete the request.

What it means

HTTP 426 Upgrade Required signals that the server refuses to perform the request using the current protocol but might be willing to do so after the client upgrades. The Upgrade response header must be present.

When it's used

Forcing HTTPS upgrade, requiring HTTP/2, or mandating WebSocket protocol.

Common causes
  • Server requires TLS but client used plain HTTP
  • API requires HTTP/2 features unavailable in HTTP/1.1
  • Forced WebSocket upgrade for real-time endpoints
How to fix it
  • Switch to the protocol indicated in the Upgrade header
  • Use https:// instead of http://
  • Update HTTP client library to support the required version
Example response
HTTP/1.1 426 Upgrade Required
Upgrade: TLS/1.3, HTTP/1.1
Connection: Upgrade
SEO impact

Should not occur for crawlers — Googlebot supports modern protocols. Use 301 to https:// for HTTP→HTTPS instead.