RDF Protocol Documents — an A2A Extension

Extension URI: https://w3id.org/jeswr/a2a-rdf/v1 (w3id redirect pending) Spec: index.html (unofficial draft) Reference codec: @jeswr/solid-a2a

An extension to the Linux Foundation Agent2Agent (A2A) protocol — defined through A2A's own extension mechanism (spec §4.6; no core changes) — that lets two agents negotiate once in natural language and then crystallise the routine into a hash-pinned Protocol Document whose body is a SHACL shape. Thereafter every exchange under that protocol is a SHACL-validated RDF graph carried in ordinary A2A message Parts: machine-checkable, vocabulary-grounded, zero LLM inference on the hot path, with a fail-closed no-silent-downgrade rule for security-bearing exchanges.

The design carries the AGORA hash-pinned protocol-document result (the editor is an AGORA co-author) one step further, making the protocol document RDF/SHACL-native so validated intents compose directly with ODRL policies and Agent Authorization Credentials.

What the spec defines

1. Extension identity + Agent Card declaration — the extension URI, the AgentExtension entry (uri/description/required/params), and a params schema (formats, nlNegotiation, protocolDocuments) that lets an agent pre-declare the protocols it already speaks.
2. Activation — standard A2A A2A-Extensions negotiation plus per-message tagging via the core Message.extensions field.
3. Protocol Documents — the a2a:ProtocolDocument data model (request/response SHACL shapes + Dublin Core metadata) and content addressing: sha256: over the RDFC-1.0 canonical N-Quads of the full graph, verified on every fetch.
4. The upgrade handshake — upgrade-offer / upgrade-response payloads (JSON and RDF forms) and the normative no-silent-downgrade rule: an unrelated/absent/malformed response never authorises NL fallback, and a declined required offer aborts the exchange.
5. Message-content binding — Part profiles for handshake, intent, response, and authorization payloads (text/turtle / application/ld+json / application/vp), with extension-namespaced Part.metadata keys and mandatory pre-action SHACL validation.
6. Authorization composition — how an intent references the ODRL policy it exercises and travels with an AgentAuthorizationCredential presentation (by reference to the ODRL Agent-Delegation Profile and the AAC spec — composed, not restated).
7. Security considerations — untrusted-RDF hardening, strict security-flag parsing, downgrade resistance, SSRF-safe PD fetching, replay, validation-cost bounds.
8. An honest implemented-vs-proposed inventory and a worked end-to-end example whose payloads were generated and validated with the reference codec.

Status

Unofficial draft — nothing has been submitted to the A2A project. This document is a candidate proposal prepared per A2A's extension governance (proposal = an issue on a2aproject/A2A; then maintainer sponsorship → an experimental-ext-* repo). Submission is maintainer-gated: the editor posts, the drafting agent posts nothing externally.

AI-assisted draft (Claude Fable 5), verified against the A2A v1.0.0 specification, the A2A extensions topic guide + governance doc, and the @jeswr/solid-a2a codec on 2026-07-03. Design decisions and their alternatives are recorded in DECISIONS.md.

Viewing

index.html is a ReSpec document — open it in a browser (it fetches the ReSpec script from w3.org) or view the rendered copy at https://jeswr.github.io/a2a-rdf-extension/ once Pages is enabled.