What the payload actually is. Second-most-declared header in the catalog and a routine source of interoperability failure.
Content-Type
Representation Metadata IANA permanent response 3 spellings
Content-Type tells the recipient how to parse the bytes that follow. It is one of the oldest headers in HTTP and it is still, reliably, a source of broken integrations — because a media type is not just a label, it is a contract about structure, and providers are casual about it.
The failure modes are consistent: sending application/json for something that is actually application/problem+json; omitting charset and letting the recipient guess; declaring application/json in the contract while the server returns text/plain on errors. That last one is the expensive one, because it only shows up when something has already gone wrong.
The registry
Listed in the IANA HTTP Field Name Registry as a permanent entry. Defined in RFC 9110, Section 8.3: HTTP Semantics.
In the catalog
Declared by 353 providers across 15,186 published specification files in the API Evangelist catalog, where it appears as a response header — sent by the server.
It is spelled 3 different ways across those contracts — Content-Type, content-type, Content-type. 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.
Using it
Use the most specific registered media type you can. If you return RFC 9457 problem details on errors, say application/problem+json and mean it. If you accept more than one representation, declare all of them in the contract rather than documenting one and quietly accepting three.
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 →