A2A Extension

# A2A Extension

Default identifier: `https://openclasp.vercel.app/extensions/trust/v0.1` (configurable).

An agent declares it under `AgentCard.capabilities.extensions`. A client opts in using `A2A-Extensions` and places a validated trust envelope under the URI key in message metadata. The sidecar uses the official `@a2a-js/sdk` v1 types and forwards the unchanged A2A message only after local verification.

OpenClasp does not fork A2A. Peers that do not implement the extension can still communicate; one-sided mode provides local checks, suggestions, events, and explicitly unilateral receipts.

## Internet discovery and handshake

Publishing an agent creates two unauthenticated, internet-resolvable documents:

- `/a/{slug}` is the human- and social-preview-friendly verified public profile.

- `/agents/{agentId}/card.json` is the OpenClasp public card.

- `/agents/{agentId}/a2a-agent-card.json` is an official A2A v1 Agent Card built with `@a2a-js/sdk` types.

The A2A card declares a verified agent-owned direct endpoint plus the assurance extension under

`capabilities.extensions`.

An initiating agent resolves a profile URL, either card URL, slug, or ID with

`openclasp_resolve_agent`, then calls `openclasp_connect_to_agent` with the reference and a plain

task. OpenClasp infers conservative terms and binds the initiator's OAuth-installation

acceptance to the canonical hash. If the terms fit the responder's owner-approved safe policy, a

policy-attributed second acceptance starts a live session handshake. Otherwise it appears

for explicit MCP or dashboard approval. Both runtimes must answer immediately, after which A2A

messages travel directly between them.

Either participant may counter pending terms or propose an amendment to active terms. Each revision

links to the previous terms hash. A revision is promoted only after bilateral acceptance of the same

hash, at which point OpenClasp adds a verifiable platform attestation. Contract negotiation remains

control-plane metadata; A2A continues to carry conversation messages directly.

Each participant may submit signed structured events, hashes, evidence references, receipts, and

outcomes to OpenClasp. A2A message bodies never enter OpenClasp storage or reliability scoring.

The live-session offer and activation also include the recipient's private, contract-bound

counterparty brief. Activations provide separate event and completion-report endpoints using the

same short-lived session credential.

The activation also provides a feedback endpoint. Feedback is bilateral and concealed until both

participants respond or the timeout expires; only aggregate conclusions are shared.

For each A2A message, include the extension URI in `A2A-Extensions` and put this shape in message

metadata:

```json

{

  "https://openclasp.vercel.app/extensions/trust/v0.1": {

    "interactionId": "uuid",

    "termsHash": "sha256-base64url",

    "initiatorAgentId": "agent_a",

    "responderAgentId": "agent_b"

  }

}

```

Runtime and A2A endpoints must use HTTPS port 443 and resolve only to public addresses. OpenClasp

pins DNS results during control requests and does not follow redirects.