# A2A a2a.proto (wire shapes)

## Metadata

| Field | Value |
|---|---|
| Institute | Molodetz Institute — Research & Retrieval Division |
| Subject | cap-protocol |
| Document type | Source Dossier |
| Tier | 1 |
| Source title | A2A specification/a2a.proto |
| Source URL | https://github.com/a2aproject/A2A/blob/main/specification/a2a.proto |
| Source type | schema |
| Source date (published) | Unknown, not stated (living file) |
| Date researched | 2026-10-18 |
| Researched by | tai (agent), model: Muse Spark, on behalf of retoor |
| Research method | direct raw-file fetch, read selectively (TaskState, TaskStatus, Part, TaskStatusUpdateEvent, AgentExtension) |
| Confidence | High |
| Status | Active |

## Summary

The machine-readable A2A schema behind the template-generated spec tables: 9-state TaskState enum, TaskStatus (state + message + timestamp), Part oneof (text/raw/url/data + metadata + filename + media_type), status/artifact update events, AgentExtension (uri/description/required/params).

## Key findings

- `TASK_STATE_INPUT_REQUIRED = 6` exists, validating CAP's C.4 mapping row; JSON form is lowercase (`working`, `input-required`, ...).
- Parts are a oneof: exactly one of text/raw/url/data, with media_type on all variants; CAP's TextPart/DataPart map to the text/data arms.
- TaskStatusUpdateEvent carries task_id + context_id + full TaskStatus; artifact updates are separate events, matching CAP's split mapping.
- context_id is REQUIRED on updates: multi-turn correlation key tai's driver must persist per session.

## Pros / strengths

- Unambiguous field shapes with presence/required annotations; no prose interpretation needed.
- Confirms CAP's event mapping (C.4) is implementable against real A2A frames.

## Cons / limitations / caveats

- Read selectively (5 messages of ~40); security/push/video-part messages not reviewed.
- Proto is the schema, not the JSON mapping; JSON enum casing confirmed only via the reference driver's test fixtures.

## Relevance to tai

Schema source for tai's A2A driver frame parsing and expose-server synthesis.

## Changelog

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