UCP Checkout — A2A Binding

Before writing code

Fetch live spec: Web-search site:ucp.dev specification checkout-a2a and fetch the page for the exact message structure, DataPart keys, and Agent Card format.

Also review Google's A2A protocol spec for the underlying transport: https://google.github.io/A2A/

Conceptual Architecture

What is A2A?

A2A (Agent-to-Agent) is a protocol for autonomous inter-agent communication. UCP's A2A binding lets a Platform agent talk to a Business agent using structured messages rather than REST calls.

How It Works

1. Discovery: Business publishes an Agent Card at the URL declared in their /.well-known/ucp profile under services.dev.ucp.shopping.a2a.endpoint.
2. Communication: Platform sends messages with checkout data in DataPart objects. Business agent responds with checkout state in DataPart objects.
3. Identification: Platform includes UCP-Agent header and X-A2A-Extensions header referencing the UCP spec version.