Checkout Capability - MCP Binding

Checkout Capability - MCP Binding

This document specifies the Model Context Protocol (MCP) binding for the Checkout Capability.

Protocol Fundamentals

Discovery

Businesses advertise MCP transport availability through their UCP profile at /.well-known/ucp.

{
  "ucp": {
    "version": "2026-01-11",
    "services": {
      "dev.ucp.shopping": [
        {
          "version": "2026-01-11",
          "spec": "https://ucp.dev/2026-01-11/specification/overview",
          "transport": "mcp",
          "schema": "https://ucp.dev/2026-01-11/services/shopping/mcp.openrpc.json",
          "endpoint": "https://business.example.com/ucp/mcp"
        }
      ]
    },
    "capabilities": {
      "dev.ucp.shopping.checkout": [
        {
          "version": "2026-01-11",
          "spec": "https://ucp.dev/2026-01-11/specification/checkout",
          "schema": "https://ucp.dev/2026-01-11/schemas/shopping/checkout.json"
        }
      ]
    }
  }
}

Request Metadata

MCP clients MUST include a meta object in every request containing protocol metadata:

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "tools/call",
  "params": {
    "name": "create_checkout",
    "arguments": {
      "meta": {
        "ucp-agent": {
          "profile": "https://platform.example/profiles/shopping-agent.json"
        },
        "idempotency-key": "550e8400-e29b-41d4-a716-446655440000"
      },
      "checkout": { ... }
    }
  }
}

The meta["ucp-agent"] field is required on all requests to enable capability negotiation. The complete_checkout and cancel_checkout operations also require meta["idempotency-key"] for retry safety.

Tools

UCP Capabilities map 1:1 to MCP Tools.

Tool	Operation	Description
create_checkout	Create Checkout	Create a checkout session.
get_checkout	Get Checkout	Get a checkout session.
update_checkout	Update Checkout	Update a checkout session.
complete_checkout	Complete Checkout	Place the order.
cancel_checkout	Cancel Checkout	Cancel a checkout session.

create_checkout

Maps to the Create Checkout operation.

Input Schema

• checkout (Checkout): Required. Contains the initial checkout session data and optional extensions.
  • Extensions (Optional):
    • dev.ucp.shopping.buyer_consent: Buyer Consent
    • dev.ucp.shopping.fulfillment: Fulfillment
    • dev.ucp.shopping.discount: Discount
    • dev.ucp.shopping.ap2_mandate: AP2 Mandates

Output Schema

• Checkout object.

complete_checkout

Maps to the Complete Checkout operation.

Input Schema

• meta (Object): Required. Request metadata containing:
  • ucp-agent (Object): Required. Platform agent identification.
  • idempotency-key (String, UUID): Required. Unique key for retry safety.
• id (String): Required. The ID of the checkout session.
• checkout (Checkout): Required. Contains payment credentials and other finalization data to execute the transaction.

Error Handling

UCP distinguishes between protocol errors and business outcomes.

• Protocol errors: Transport-level failures (authentication, rate limiting, unavailability) that prevent request processing. Returned as JSON-RPC error with code -32000 (or -32001 for discovery errors).
• Business outcomes: Application-level results from successful request processing, returned as JSON-RPC result with UCP envelope and messages.

Message Signing

Platforms SHOULD authenticate agents when using MCP transport. When using HTTP Message Signatures, all checkout operations follow the Message Signatures specification.

Conformance

A conforming MCP transport implementation MUST:

1. Implement JSON-RPC 2.0 protocol correctly.
2. Provide all core checkout tools defined in this specification.
3. Return errors per the Core Specification.
4. Return business outcomes as JSON-RPC result with UCP envelope and messages array.
5. Validate tool inputs against UCP schemas.
6. Support HTTP transport with streaming.