The client should switch to a different protocol using the Upgrade header.
Quick facts
When does it occur
The client should switch to a different protocol using the Upgrade header.
It belongs to the 4xx Client Error class of responses, defined in RFC7231#6.5.15.
Browse all codes in this class: 4xx Client Error
How to fix it
Retry the request over the upgraded protocol advertised in the Upgrade header.
Common causes
How to fix it
Double-check the URL, return to the homepage and search again, or retry later.
Retry the request over the upgraded protocol advertised in the Upgrade header.
Review access logs and permission rules for the affected path, and confirm routing and security policies.
Frequently asked questions
426 belongs to the 4xx Client Error class and is generally not cached for long; behavior is controlled by Cache-Control.
4xx Client Error: The client should switch to a different protocol using the Upgrade header.
Sources & standards
Updated 2026-09-13
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