Transport Bindings

Overview

UCP supports multiple transport protocols for different integration scenarios:

TransportUse CaseDescription
RESTHTTP-based integrationRESTful API over HTTPS
MCPAI Agent integrationModel Context Protocol via JSON-RPC
A2AAgent-to-AgentAgent2Agent Protocol for inter-agent communication
EmbeddedEmbedded checkoutEmbedded Checkout Protocol for iframe/webview integration

Transport Bindings

REST Binding

The REST binding provides standard HTTP-based integration for platforms and businesses.

  • Checkout REST Binding: REST API mapping for Checkout capability
  • Base URL discovered via UCP Profile
  • Uses JSON request/response over HTTPS
  • Supports standard HTTP methods (POST, GET, PUT, DELETE)

MCP Binding

The MCP binding enables AI agents to interact with UCP using the Model Context Protocol.

  • Checkout MCP Binding: MCP tools mapping for Checkout capability
  • Uses JSON-RPC 2.0 format
  • Tools map 1:1 to UCP operations
  • Platform profile included in _meta.ucp structure

A2A Binding

The A2A binding enables agent-to-agent communication using the Agent2Agent Protocol.

  • Checkout A2A Binding: A2A protocol mapping for Checkout capability
  • Uses A2A Message and Task objects
  • UCP extension: https://ucp.dev/specification/reference?v=2026-01-11
  • Checkout data returned in a2a.ucp.checkout DataPart

Embedded Protocol

The Embedded Checkout Protocol (ECP) enables checkout embedding with native UI delegation.

  • Embedded Checkout Protocol: ECP specification
  • Uses JSON-RPC 2.0 over postMessage
  • Supports payment and address delegation
  • Handshake via ec.ready message

Transport Selection

When to use REST

  • Standard web server integration
  • Payment gateway or PSP integration
  • Platform with existing HTTP infrastructure
  • Simple request/response patterns

When to use MCP

  • AI agent or LLM integration
  • Model Context Protocol infrastructure
  • Tool-based agent interactions
  • Streaming responses required

When to use A2A

  • Agent-to-agent communication
  • A2A Protocol infrastructure
  • Multi-agent workflows
  • Task-based agent coordination

When to use Embedded

  • Native mobile app checkout
  • Embedded iframe checkout
  • UI delegation requirements
  • Platform-controlled payment flows

Security Considerations

All transport bindings MUST:

  • Use HTTPS with minimum TLS 1.3
  • Implement proper authentication
  • Validate all inputs against UCP schemas
  • Follow transport-specific security requirements

Canonical Sources