Prefer

Expresses an optional client preference about request processing or the response.

Use preferences such as return=minimal or respond-async only when the server documents their behavior; preferences do not change the method semantics.

httpheaderapiasync

Reference (http)

Prefer: return=minimal, respond-async

Use preferences such as return=minimal or respond-async only when the server documents their behavior; preferences do not change the method semantics.

Common mistakes

Assuming the server must honor every Prefer value.

IANA registry: http-fields/field-names

Registry reference: RFC 7240: Prefer Header for HTTP

Sources

MDN HTTP Reference
IANA HTTP Field Name Registry