A2aEngine (a2a_engine v0.1.0)

A2A (Agent-to-Agent) protocol engine — pure spec library.

Implements Google's A2A protocol types, JSON-RPC 2.0 codec, SSE codec, and a transport behaviour. Stateless. No broker. No DB. No HTTP server.

Host applications pull this in to speak A2A over any transport they choose.

Layout

• A2aEngine.Types.* — spec data types (Task, Message, Part, Artifact, AgentCard, PushNotificationConfig, lifecycle events, …)
• A2aEngine.LifecycleState — task state enum
• A2aEngine.Codec.JsonRpc — JSON-RPC 2.0 request/response envelopes
• A2aEngine.Codec.SSE — Server-Sent Events encoder/decoder with Last-Event-ID resumption
• A2aEngine.Codec.Keys — camelCase ↔ snake_case key conversion (A2A wire format is camelCase; Elixir is snake_case)
• A2aEngine.Errors — A2A-specific JSON-RPC error codes

Checkpoint-in-envelope convention

In the "checkpoint-in-envelope" pattern we use for ephemeral-worker resumption, the worker stashes its opaque resumable state in a DataPart with metadata:

%{
  "type" => "checkpoint",
  "owner" => "<producer-agent-name>",
  "session" => "<conv-engine-session-id>"
}

The A2A spec has no opinion on metadata contents; this convention lives in application code. Brokers MUST route DataParts tagged as checkpoints back to their producer only, never to the counter-party.