Checkout Capability - A2A Binding

This document specifies the Agent2Agent Protocol (A2A) binding for Checkout Capability.

Transport Discovery

Businesses that support A2A transport must specify the agent card endpoint as part of services in UCP Profile at /.well-known/ucp. This allows capable platforms to interact with the business services over A2A Protocol.

{
  "ucp": {
    "version": "2026-01-11",
    "services": {
      "dev.ucp.shopping": [
        {
          "version": "2026-01-11",
          "spec": "https://ucp.dev/2026-01-11/specification/overview",
          "transport": "a2a",
          "endpoint": "https://example-business.com/.well-known/agent-card.json"
        }
      ]
    }
  }
}

Shopping Agent Profile Advertisement

Shopping platforms interacting with the business agent must send their profile URI as UCP-Agent request headers with every request.

UCP-Agent: profile="https://agent.example/profiles/v2025-11/shopping-agent.json"
Content-Type: application/json

Header Mapping Reference

Header Name	Description
`UCP-Agent`	Shopping platform application profile URI.
`X-A2A-Extensions`	UCP Extension URI (specified below).

A2A Interactions

The A2A Protocol provides a strong foundation for inter-agent communication. A2A extensions enable communication between agents with structured data types. This enables businesses to build AI applications to leverage UCP data types for communication with platforms.

The URI for UCP A2A extension: https://ucp.dev/2026-01-11/specification/reference

Businesses supporting UCP must advertise the extension and any optional capabilities in their A2A Agent Card to allow platforms to activate the extension.

An example:

{
  "extensions": [
    {
      "uri": "https://ucp.dev/2026-01-11/specification/reference",
      "description": "Business agent supporting UCP",
      "params": {
        "capabilities": {
          "dev.ucp.shopping.checkout": [
            {"version": "2026-01-11"}
          ],
          "dev.ucp.shopping.fulfillment": [
            {
              "version": "2026-01-11",
              "extends": "dev.ucp.shopping.checkout"
            }
          ]
        }
      }
    }
  ]
}

Agent2Agent Negotiation

The business agents can leverage A2A Message objects for allowing interaction with shopping agents/platforms. The A2A Message object returned by the agent will return structured data in DataPart objects within the message. Platforms must pass the business agent generated contextId for subsequent turns in a session to preserve the current context.

Business agents may also leverage A2A Task objects for scenarios where applicable. In such scenarios, the business agent will return Task objects with appropriate payload for interaction with the platforms. Platforms must pass the server generated taskId along with the contextId for subsequent turns until the task is completed.

Request Idempotency

Business agents must leverage the messageId sent as part of the A2A Message to detect duplicate messages from platform retries.

Checkout Functionality

The Checkout capability allows consumers to manage items in a checkout session and complete the purchase process. The checkout object MUST be returned as part of a DataPart object with key a2a.ucp.checkout.

Request format: Agentic applications can accept natural language input from users interacting with the agent to identify the user’s intent, negotiate with the user to capture any required information and then invoke the appropriate tools to perform the operation.

Examples:

Natural language input

{
  "message": {
    "role": "user",
    "parts": [
      {
        "type": "text",
        "text": "add Pixel 10 Pro to my checkout"
      }
    ],
    "messageId": "69da8f87-991b-479e-80dc-ed92fcb57cbe",
    "kind": "message",
    "contextId": "aad14abc-4082-4748-84ca-4afff85aedfa"
  }
}

Structured inputs on user actions

{
  "message": {
    "role": "user",
    "parts": [
      {
        "type": "data",
        "data": {
          "action": "add_to_checkout",
          "product_id": "PIXEL-10-PRO",
          "quantity": 1
        }
      }
    ],
    "messageId": "e94a8c10-69f4-4c4c-b988-21a298302da6",
    "kind": "message",
    "contextId": "aad14abc-4082-4748-84ca-4afff85aedfa"
  }
}

Checkout Completion

When a user is ready to make a payment, payment must be submitted to the business agent to complete the checkout process. payment is a structured data type specified as part of UCP. When processing a payment to complete the checkout, payment must be submitted to the business agent as a DataPart with attribute name a2a.ucp.checkout.payment. Any associated risk signals should be sent with attribute name a2a.ucp.checkout.risk_signals.

Upon completion of the checkout process, the business agent must return the checkout object containing an order attribute with id and permalink_url.

AP2 based Checkout Completion

Business agents can implement AP2 mandates extension that enables secure exchange of user intents and authorizations for Agent-to-Agent payment interactions. Businesses that support AP2 mandates extension for UCP must specify this in the UCP discovery document and the A2A agent card. The AP2 mandates extension is considered implicitly active when a platform and business agent advertise AP2 mandates extension in their respective profiles.

When AP2 mandates extension is enabled, the business agent must create a detached JWS for the checkout object and must return the generated signature as part of the DataPart as ap2.merchant_authorization.

When the user confirms the payment on a platform, the user signed checkout and payment mandate objects must be sent as DataParts to the business agent for completing checkout. The payment which includes the payment mandate must be submitted as part of a DataPart with attribute name a2a.ucp.checkout.payment. Signed checkout mandate must be specified in the DataPart as ap2.checkout_mandate. Refer to AP2 Mandates Extension documentation for more details about verification and processing of the mandates to complete the checkout.