What does HTTP status 421 (Misdirected Request) mean?

HTTP_STATUS_MISDIRECTED_REQUEST

This status is especially relevant to reused HTTP/2 or HTTP/3 connections: a server may receive a request for an authority it is not configured to serve on that connection.

Clients can retry on a new connection to the target origin. Server operators should check TLS certificate coverage, virtual-host configuration, authority headers, and connection coalescing behavior.

RFC 9110: HTTP Semantics

Operational meaning of HTTP 421 for HTTP_STATUS_MISDIRECTED_REQUEST

HTTP 421 (HTTP_STATUS_MISDIRECTED_REQUEST) is a client-error response. AllStat records its meaning as “421 Misdirected Request”. For HTTP_STATUS_MISDIRECTED_REQUEST, interpret it with the request method, selected representation, response headers, and the specification or product that defines the code; the number alone does not prove that application work completed.

Client and server handling for HTTP_STATUS_MISDIRECTED_REQUEST

• Treat HTTP_STATUS_MISDIRECTED_REQUEST as a condition visible at the request boundary; inspect authentication, syntax, preconditions, routing, and resource state before retrying.
• For HTTP_STATUS_MISDIRECTED_REQUEST, preserve the response body and challenge or retry-related headers because they often carry the actionable reason.
• For HTTP_STATUS_MISDIRECTED_REQUEST, automatic retry is safe only when the method and status contract allow it and the request has been corrected.

For telemetry, record HTTP 421, method, authority, route template rather than secrets, response headers relevant to caching or retry, latency, upstream identity, and whether the response originated at the application or an intermediary.

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