polyc-a2a 2026.8.3

polychrome-a2a — an A2A v1.0 edge that puts an agent-to-agent peer in front of a polychrome conversation.

A2A (Agent2Agent) is the open protocol agents use to discover and call one another. The wire is the v1.0 (lf.a2a.v1) ProtoJSON dialect ([types]) — PascalCase methods, SCREAMING_SNAKE enums, field-presence unions — mirroring the canonical Rust SDK (a2aproject/a2a-rs) so the edge interops with any v1.0 peer.

Both halves live here:

• Server — serves a domain-signed Agent Card at /.well-known/agent-card.json ([card]) and implements the capability-ungated method set over JSON-RPC ([rpc]): SendMessage maps a message onto a single polychrome turn ([task]) and records the resulting task durably ([store]); GetTask/CancelTask/ListTasks read it back. The record lives in the state plane and this edge reaches it the same way it reaches a turn: an RPC into the control plane.
• Client ([client]) — fetches + verifies a peer's card and drives SendMessage/GetTask against it.

The load-bearing mapping

A2A's input-required task state and polychrome's human-in-the-loop approval gate are the same idea: a task cannot proceed until a human decides. A turn that pauses on an approval ([polyc_rpc_client::TurnEvent::ApprovalPending]) becomes a TASK_STATE_INPUT_REQUIRED task ([task::outcome_from_events] → [rpc]), so an A2A peer is told, in its own protocol, that the work is blocked on a decision.

Identity provisioning (follow-up)

For this foundation slice the signing key is derived from a configured seed (the convention polyc_crypto's signers use for dev/test). Wiring the deployment principal to a secret store — so the card is signed by the same durable key that backs the rest of the deployment's provenance — is a follow-up.