Definition
api-catalog (RFC 9727)
api-catalog is an IETF standard, RFC 9727, that defines /.well-known/api-catalog as the single place a site publishes a machine-readable list of the APIs it offers.
RFC 9727 became a Proposed Standard in June 2025. The document at the well-known path is a Linkset, served as `application/linkset+json`, whose entries link to each API's own description: an OpenAPI document, documentation, a status page, terms of service. The point is one predictable starting URL, so a client does not have to guess or scrape a developer portal.
Adoption is close to zero. The only original measurement we could find anywhere is a single blog post from May 2026 counting four API providers that publish a real one. For a standard that is trivial to implement and directly useful to agents, that gap is the interesting fact about it.
For a site with a public API, publishing one is an afternoon of work and it is checkable: request the path, confirm the content type, then resolve the links. Our readiness audit reports it because near-zero adoption makes it a cheap differentiator, not because engines reward it today.
Frequently asked questions
What format does api-catalog use?
A Linkset, per RFC 9264, served as application/linkset+json. Each entry links to an API description or related resource with a declared relation type.
Is api-catalog worth publishing if almost nobody does?
For a site with a public API, yes, on cost grounds: it is one static document at a fixed path. Just do not expect measurable traffic from it, because nothing consumes it at scale yet.