ucp-checkout-a2a

Agent Building

Implement UCP Checkout over the A2A (Agent-to-Agent) binding — enable autonomous agent-to-agent commerce using Agent Cards and structured message parts. Use when building agent-to-agent commerce flows.

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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

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

Key Data Part Keys

Key	Direction	Purpose
`a2a.ucp.checkout`	Both	Checkout session data
`a2a.ucp.checkout.payment_data`	Platform → Business	Payment credentials for completion
`a2a.ucp.checkout.risk_signals`	Platform → Business	Optional risk signals
`ap2.merchant_authorization`	Business → Platform	Merchant's JWS detached content signature for AP2 mandate flow
`ap2.checkout_mandate`	Platform → Business	SD-JWT+kb credential proving user-authorized agent checkout

Message Structure

Messages use A2A's standard format:

messageId: Unique ID (used for idempotency)
contextId: Session/conversation ID
kind: "message"
role: "user" (from platform) or "agent" (from business)
parts: Array of TextPart and DataPart objects

Checkout data is carried in DataPart with the appropriate key.

Idempotency

The Business agent uses messageId to detect duplicate requests. Task tracking uses taskId with contextId.

When to Use A2A

Fully autonomous agent-to-agent commerce (no human in the loop for most steps)
Multi-agent architectures where commerce is one capability among many
When you want the Business to maintain its own agent logic (not just expose tools)

Implementation Guidance

Before implementing, fetch the latest sample A2A Business Agent from https://github.com/Universal-Commerce-Protocol/samples (look for the a2a/ directory) to see the reference pattern.