What does HTTP status 400 (Bad Request) mean?

HTTP_STATUS_BAD_REQUEST

Typical causes include malformed syntax, invalid message framing, inconsistent headers, or deceptive routing information. The status does not identify which field was wrong unless the response body or logs provide details.

Compare the raw request with the endpoint contract: method, URL encoding, host, content type, framing, and body schema. Do not retry unchanged requests automatically.

RFC 9110: HTTP Semantics

Operational meaning of HTTP 400 for HTTP_STATUS_BAD_REQUEST

HTTP 400 (this result) is a client-error response. AllStat records its meaning as “400 Bad 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

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

For telemetry, record HTTP 400, 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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