# CAP reference A2A driver (cap-rs a2a.rs)

## Metadata

| Field | Value |
|---|---|
| Institute | Molodetz Institute — Research & Retrieval Division |
| Subject | cap-protocol |
| Document type | Source Citation Card |
| Tier | 2 |
| Source title | crates/cap-rs/src/driver/a2a.rs |
| Source URL | https://github.com/rsclaw-ai/cap-protocol/blob/main/crates/cap-rs/src/driver/a2a.rs |
| Source type | reference implementation |
| Date researched | 2026-10-18 |
| Researched by | tai (agent), model: Muse Spark, on behalf of retoor |
| Research method | direct raw-file fetch, grepped for endpoint strings, request bodies, and the event-mapping core (not read in full) |
| Confidence | High |
| Status | Active |

## Why it's here

Grepped to settle which A2A spelling the reference driver actually uses: `POST {endpoint}/message/send` with `{message: {role, parts: [{kind: text, text}]}}` and `POST {endpoint}/tasks/cancel` with no body (draft-style), card at `/.well-known/agent-card.json` refused without the `cap-protocol/v1` extension. The event mapper accepts raw `cap.*` JSON, data parts unwrapped via `_meta.cap.kind` (from message, task-status and artifact parts alike), text parts (kind or type `text`, id from messageId/message_id), and failed task states; outbound answers travel as data parts with the `_meta.cap.kind` marker. Control flow around the mapper was not reviewed.

## Changelog

| Date | Change |
|---|---|
| 2026-10-18 | Logged as consulted-but-not-analyzed. |