WebFinger

WebFinger, defined in RFC 7033, is a discovery protocol that answers a simple question: given an identifier like acct:[email protected], where do I find machine-readable information about it? A client makes an HTTPS request to a well-known endpoint — /.well-known/webfinger — with the identifier as a query, and the server returns a JSON Resource Descriptor (JRD) listing links and properties: a profile page, an avatar, an authentication endpoint, or a federation inbox.

• URI-addressed entities - Works over acct: identifiers and ordinary URIs, so people, accounts, and services can all be looked up.
• The /.well-known/ convention - A fixed, predictable path means a client never has to guess where discovery lives.
• JSON Resource Descriptor - Responses are JRD documents — a list of typed links and key-value properties about the subject.
• Federation backbone - It is the bootstrap step that lets independent servers find and talk to each other.

WebFinger is the quiet plumbing under federated systems: it is how the fediverse resolves a handle to a real actor before ActivityPub takes over, and how some identity providers advertise their OpenID Connect configuration. For API operations it is a lightweight, standards-based discovery pattern — turn a human-friendly identifier into the concrete endpoints a client needs. That same well-known-endpoint instinct is now showing up across agent-facing discovery, where a machine needs a predictable place to ask “who is this, and how do I reach them?”