What it means
The request reached a server that is not configured to answer for that host, often because an HTTP/2 connection was reused for a different domain.
Common causes
- Connection coalescing across domains that share a certificate
- TLS SNI and Host header mismatch
- Load balancer routing to the wrong virtual host
How to fix it
As a client or visitor
Browsers retry on a new connection automatically. In custom clients, open a fresh connection for that host.
As the site or API owner
Make sure each certificate only covers hosts the server can serve, and that SNI and Host routing agree.
Example response
HTTP/1.1
HTTP/1.1 421 Misdirected Request
Date: Tue, 07 Oct 2025 09:30:00 GMT
Server: nginx
Content-Length: 0Specification
FAQ
› Why do I see 421 only on HTTP/2?
HTTP/2 can reuse one connection for several hosts covered by a certificate; if the server cannot serve one of them, it answers 421.
› Is 421 a client error?
It is in the 4xx class, but usually caused by server or certificate configuration.