The Expires HTTP header — caching, RFC 9111, declared by 20 providers in the API Evangelist catalog.
Expires
Caching IANA permanent response 2 spellings
A reference entry. The registry facts and catalog measurements below are complete; the written guidance for this header has not been authored yet.
Expires is caching — Headers that tell an intermediary what it may store and for how long. The oldest, best-specified corner of HTTP, and the one API providers most routinely ignore.
The registry
Listed in the IANA HTTP Field Name Registry as a permanent entry. Defined in RFC 9111, Section 5.3: HTTP Caching.
In the catalog
Declared by 20 providers across 1,413 published specification files in the API Evangelist catalog, where it appears as a response header — sent by the server.
It is spelled 2 different ways across those contracts — Expires, expires. HTTP field names are case-insensitive (RFC 9110, §5.1), so every one of these is the same header. They are not the same string, which is why generated clients disagree about it.
Governed by these rules
Machine-enforceable governance rules from rules.apievangelist.com that apply to this header when it appears in an OpenAPI.
OpenAPI Components Headers Error error
Utilizing the headers object in the centralized OpenAPI components library helps make headers reusable across API requests and responses
Guidance: Rate Limits →OpenAPI Components Headers Info info
Utilizing the headers object in the centralized OpenAPI components library helps make headers reusable across API requests and responses
Guidance: Rate Limits →OpenAPI Headers Hyphenated Pascal Case error
HTTP headers should follow Hyphenated-Pascal-Case naming convention for consistency and readability, such as Content-Type, X-Request-Id, or Accept-Language.
Guidance: Naming →