# A2A protocol specification (current)

## Metadata

| Field | Value |
|---|---|
| Institute | Molodetz Institute — Research & Retrieval Division |
| Subject | cap-protocol |
| Document type | Source Dossier |
| Tier | 1 |
| Source title | Agent2Agent (A2A) Protocol Specification |
| Source URL | https://github.com/a2aproject/A2A/blob/main/docs/specification.md |
| Source type | specification |
| Source date (published) | Unknown, not stated (living LF doc) |
| Date researched | 2026-10-18 |
| Researched by | tai (agent), model: Muse Spark, on behalf of retoor |
| Research method | direct raw-file fetch, read selectively: full outline, §3.1 ops, §5.3 method mapping, §5.4 errors, discovery sections via grep |
| Confidence | High |
| Status | Active |

## Summary

Current A2A: JSON-RPC PascalCase methods (`SendMessage`, `SendStreamingMessage`, `CancelTask`...) or colon-REST equivalents (`POST /message:send`), AgentCard at `/.well-known/agent-card.json`, cursor-paginated task listing, extension declarations with required flags. Critically, its method names do NOT match what CAP v1 cites (`message/send`, `tasks/cancel`).

## Key findings

- Method mapping (§5.3): JSON-RPC `SendMessage` / REST `POST /message:send`; streaming `SendStreamingMessage` / `POST /message:stream`; cancel `CancelTask` / `POST /tasks/{id}:cancel`. CAP's slash-forms match the pre-1.0 A2A draft, not current A2A.
- Stream patterns: message-only (exactly one Message then close) or task-lifecycle (Task first, then status/artifact updates, close at terminal state).
- Terminal states: completed/failed/canceled/rejected; interrupted: input-required (6), auth-required (8).
- AgentCard discovery via well-known URI; extensions carry uri/description/required/params, matching CAP's `cap-protocol/v1` extension check.
- A2A error codes (-32001..-32009) collide numerically with CAP's registry but with different meanings; bindings must not mix the two registries.

## Pros / strengths

- Normative LF spec with explicit binding-equivalence rules, giving tai a stable target for the A2A driver and expose-server.
- The extension mechanism is exactly what CAP's compatibility check needs.

## Cons / limitations / caveats

- Markdown tables are template-generated (`proto_to_table`), so the .md alone omits field shapes; the .proto file is the real schema.
- Large surface (push notifications, OAuth flows, mTLS) far beyond what CAP's binding needs; tai implements the message/stream/cancel/card subset only.
- Read selectively (operations, mappings, discovery); auth and push sections not read.

## Relevance to tai

Forces the dual-spelling decision in the naming-drift conclusion: tai's A2A driver must speak CAP-literal draft endpoints to CAP peers and current endpoints to real A2A agents.

## Changelog

| Date | Change |
|---|---|
| 2026-10-18 | Initial dossier created. |