Protocol explainer

OpenAPI: APIs an agent can actually call.

An OpenAPI description is a machine-readable contract for an API: every endpoint, parameter and response, in a format an agent can read once and then call correctly. For agents it is the difference between an API that exists and an API that can be used — no documentation page to parse, no guesswork about field names.

Most of the web’s OpenAPI descriptions are unpublished or undiscoverable. The convention this index tests for is simple: a spec at a predictable URL (/openapi.json), or linked from an RFC 9727 API catalog at /.well-known/api-catalog.

Status in this index: OpenAPI verification lands with our next crawl (September 2026). Each verified entry will record where the spec was found, whether it parses, and the date — alongside the site’s other probe-verified capabilities.

How Known-Good verifies OpenAPI

The probe fetches /openapi.json and the specs an API catalog references, checks each parses as OpenAPI, and records the version and endpoint count with the date. A spec that returns an HTML error page fails — a 200 that lies is worse than a clean 404. Auth-gated APIs are recorded as keyed doors: real, present, and marked as needing credentials.

Publishing yours

/openapi.json                     the spec itself, at a predictable URL
/.well-known/api-catalog          an RFC 9727 catalog linking every spec you publish

Then request a probe. Placement is decided by the probe, never by the request — and every claim on your report carries the date we tested it.