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.

Key Data Part Keys#

KeyDirectionPurposea2a.ucp.checkoutBothCheckout session dataa2a.ucp.checkout.payment_dataPlatform → BusinessPayment credentials for completiona2a.ucp.checkout.risk_signalsPlatform → BusinessOptional risk signalsap2.merchant_authorizationBusiness → PlatformMerchant's JWS detached content signature for AP2 mandate flowap2.checkout_mandatePlatform → BusinessSD-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.