FEP-844e: Capability discovery (#623)

Reviewed-on: https://codeberg.org/fediverse/fep/pulls/623 Co-authored-by: silverpill <silverpill@firemail.cc> Co-committed-by: silverpill <silverpill@firemail.cc>
2026-08-05 11:46:04 +00:00 · 2025-06-14 14:27:27 +02:00
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+{
+  "@context": {
+    "implements": {
+      "@id": "https://w3id.org/fep/844e/implements",
+      "@type": "@id",
+      "@container": "@list"
+    }
+  }
+}
@@ -0,0 +1,104 @@
+---
+slug: "844e"
+authors: silverpill <@silverpill@mitra.social>
+type: implementation
+status: DRAFT
+dateReceived: 2025-06-14
+discussionsTo: https://codeberg.org/silverpill/feps/issues
+---
+# FEP-844e: Capability discovery
+
+## Summary
+
+Capability discovery for [ActivityPub] applications.
+
+This document is based on the idea described in [FEP-aaa3: Listing Implemented Specifications on the Application Actor][FEP-aaa3].
+
+## Requirements
+
+The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC-2119].
+
+## Application object
+
+An application can advertise its capabilities using the `implements` property on an `Application` object.
+
+This object might not be an actor. The value of `implements` property MUST be an array of `Link` objects, each containing the following properties:
+
+- `href` (REQUIRED): the unique identifier of the implementable feature. The value MUST be an URI.
+- `name` (RECOMMENDED): the short description of the implementable feature.
+
+Example:
+
+```json
+{
+  "type": "Application",
+  "id": "https://social.example/server",
+  "implements": [
+    {
+      "href": "https://datatracker.ietf.org/doc/html/rfc9421",
+      "name": "RFC-9421: HTTP Message Signatures"
+    }
+  ]
+}
+```
+
+## Discovery through an actor
+
+An `Application` object can be linked to an actor using the [`generator`][generator] property. The value of this property SHOULD be a partial object containing the `implements` property. That object MAY be anonymous (without an identifier).
+
+Example:
+
+```json
+{
+  "id": "https://social.example/actors/1",
+  "type": "Person",
+  "inbox": "https://social.example/actors/1/inbox",
+  "outbox": "https://social.example/actors/1/outbox",
+  "generator": {
+    "type": "Application",
+    "implements": [
+      "href": "https://datatracker.ietf.org/doc/html/rfc9421",
+      "name": "RFC-9421: HTTP Message Signatures"
+    ]
+  }
+}
+```
+
+## Discovery through a well-known endpoint
+
+The mechanism of discovering an `Application` object using WebFinger is described in [FEP-d556].
+
+Capability discovery through NodeInfo endpoint is discouraged.
+
+## Intended use
+
+The reliance on the mechanism described in this document might increase implementation complexity and hinder interoperability.
+
+Implementers MUST NOT use it in cases where capabilities can be inferred from properties and types of objects.
+
+## Registry of implementable features
+
+(This section is non-normative.)
+
+| Name | Identifier |
+| ---  | ---        |
+| RFC-9421: HTTP Message Signatures     | https://datatracker.ietf.org/doc/html/rfc9421      |
+
+## References
+
+- Christine Lemmer-Webber, Jessica Tallon, Erin Shepherd, Amy Guy, Evan Prodromou, [ActivityPub], 2018
+- S. Bradner, [Key words for use in RFCs to Indicate Requirement Levels][RFC-2119], 1997
+- Helge, [FEP-aaa3: Listing Implemented Specifications on the Application Actor][FEP-aaa3], 2024
+- Steve Bate, [FEP-d556: Server-Level Actor Discovery Using WebFinger][FEP-d556], 2024
+
+[ActivityPub]: https://www.w3.org/TR/activitypub/
+[RFC-2119]: https://datatracker.ietf.org/doc/html/rfc2119.html
+[FEP-aaa3]: https://codeberg.org/helge/fep/src/commit/e1b2a16707b542ea5ea0cfb390ac1abce89f05bb/fep/aaa3/fep-aaa3.md
+[FEP-d556]: https://codeberg.org/fediverse/fep/src/branch/main/fep/d556/fep-d556.md
+[generator]: https://www.w3.org/TR/activitystreams-vocabulary/#dfn-generator
+
+## Copyright
+
+CC0 1.0 Universal (CC0 1.0) Public Domain Dedication
+
+To the extent possible under law, the authors of this Fediverse Enhancement Proposal have waived all copyright and related or neighboring rights to this work.