Sunset Header

The Sunset HTTP header field (RFC 8594) is an informational IETF specification that allows servers to communicate to clients that a URI is likely to become unresponsive at a specified future point in time. Published in May 2019, it provides a standardized mechanism for API deprecation signaling, enabling clients to plan migrations before service retirement. The header value is an HTTP-date timestamp following RFC 7231 syntax. A complementary sunset link relation type allows resources to reference documentation covering the sunset policy, migration guidance, and alternatives. RFC 9745 (Deprecation HTTP Header Field) extends this pattern by providing a two-phase deprecation + sunset lifecycle where Deprecation marks the start of deprecation and Sunset marks the end-of-life. These two headers are widely adopted in REST API governance practices and API lifecycle management tooling.