--- layout: default ---

Prefer

IANA permanent request RFC 7240 reached by regulation 2 spellings

Declared by 29 providers across 520 published specification files in the APIs.io network.

What this header is for, and how to use it →

A count here is providers whose published contract declares this header — not providers who send it. Response headers set at the edge rarely appear in an OpenAPI at all, so a low number can measure documentation practice rather than deployment.

Spelled 2 ways

HTTP field names are case-insensitive (RFC 9110 §5.1), so every spelling below is the same header on the wire. A contract is not the wire: generated clients key off the string, and a developer reading two of these sees two different headers.

Preferprefer

Reached by regulation

Basis: mandated. The law, or a technical standard the law makes binding, names this header. Visible in a published contract, which is what is counted here.

cms-interoperability-prior-authorization 21st-century-cures-act

Carried by the fhir-bulk-data standard.

The registry

Status	permanent
Reference	RFC 7240: Prefer Header for HTTP

Providers declaring it (29)

advancedmd aptible athena-health cdk-global cms-gov cognite gc-ai gonitro insforge jefferson-health microsoft-azure microsoft-dynamics-365 microsoft-dynamics-365-sales microsoft-power-apps morningstar nextgen-healthcare oracle-weblogic paypal postman quinyx refinitiv replicate resemble-ai specterops supabase temple-health vastdata visier wmo

Explore

All headers Async & Long-Running Guidance on headers.apievangelist.com