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car-a2a

Agent2Agent (A2A) v1.0 bridge for Common Agent Runtime.

What it does

Exposes a CAR runtime as an A2A-compliant agent so peer agents — built on any A2A SDK (Python, JavaScript, Java, Go, .NET, or one of the Rust crates) — can discover the runtime via an Agent Card and submit work as A2A Tasks.

A2A is the Linux Foundation's open protocol for agent-to-agent interoperability. The full spec lives at https://a2a-protocol.org/latest/specification/.

Mapping

A2A	CAR
Agent Card	Manifest of registered tools + host metadata
Task	One-shot wrapper around an `ActionProposal`
Message (parts)	Free-form input from the peer; structured `data` parts become tool calls
Artifact	One per `ActionResult` returned by the runtime
`TaskState` SUBMITTED → WORKING → COMPLETED/FAILED	Lifecycle of `Runtime::execute`

Surface

Item	Purpose
`A2aDispatcher`	Transport-neutral JSON-RPC dispatcher for the 11 A2A methods
`AgentCardSource`	Trait the dispatcher calls to (re)build the agent card on demand
`TaskStore` / `InMemoryTaskStore`	Pluggable task persistence
`message_to_proposal`	Compile an A2A `Message` into a CAR `ActionProposal`
`action_results_to_artifacts`	Convert a `ProposalResult` into A2A `Artifact`s

Methods supported

message/send
tasks/get
tasks/list
tasks/cancel
tasks/pushNotificationConfig/{set,get,list,delete}
agent/getAuthenticatedExtendedCard

Out of scope for this prototype:

message/stream and tasks/resubscribe (Server-Sent Events)
gRPC and HTTP+JSON/REST transport bindings
Outbound A2A client calls — for those, use a2a-protocol-client or a2a-client from crates.io.

Status

Prototype. The dispatcher is exercised by unit tests against an in-process Runtime and InMemoryTaskStore. It is not yet wired into car-server-core::handler::run_dispatch or fronted by an HTTP listener — embedders pick the transport.

Example

use std::sync::Arc;
use car_a2a::{A2aDispatcher, AgentCard, InMemoryTaskStore};
use car_engine::Runtime;

let runtime = Arc::new(Runtime::new());
let store = Arc::new(InMemoryTaskStore::new());
let card_factory = Arc::new(|| AgentCard {
    name: "CAR runtime".into(),
    // ...fill in skills from runtime.tool_schemas() at refresh time
    # version: "1.0.0".into(),
    # description: String::new(),
    # url: String::new(),
    # provider: car_a2a::types::AgentProvider { organization: "you".into(), url: None },
    # capabilities: car_a2a::AgentCapabilities::default(),
    # default_input_modes: vec!["text".into()],
    # default_output_modes: vec!["text".into()],
    # skills: vec![],
    # additional_interfaces: vec![],
    # preferred_transport: Some("JSONRPC".into()),
    # protocol_version: "1.0".into(),
    # security_schemes: Default::default(),
    # supports_authenticated_extended_card: false,
    # security_requirements: vec![],
    # signatures: vec![],
});

let dispatcher = A2aDispatcher::new(runtime, store, card_factory);

// A peer's JSON-RPC request:
let result = dispatcher
    .dispatch("agent/getAuthenticatedExtendedCard", serde_json::Value::Null)
    .await
    .unwrap();