AP2 as an A2A Extension

Before writing code

Fetch live docs:

1. Fetch https://ap2-protocol.org/specification/ for the A2A extension specification
2. Fetch https://ap2-protocol.org/topics/ap2-a2a-and-mcp/ for protocol integration details
3. Web-search site:github.com google-agentic-commerce AP2 a2a extension for reference implementation
4. Web-search site:github.com google-agentic-commerce AP2 samples a2a for A2A-based samples

Conceptual Architecture

AP2 Extends A2A

AP2 is designed as a direct extension of the A2A protocol. It adds payment-specific capabilities to A2A's general-purpose agent-to-agent communication:

• A2A provides: tasks, messages, parts, streaming, agent cards
• AP2 adds: mandates, payment flows, VDCs, cryptographic signing, risk signals

Why A2A is Required

As the documentation states: "A2A is required to standardize intra-agent communication — as soon as you have more than one agent you need A2A." AP2 adds payment standardization on top.

How AP2 Rides on A2A

Agent Cards with AP2 Capabilities

AP2 agents declare payment support in their Agent Card:

{
  "name": "MerchantAgent",
  "capabilities": {
    "extensions": [
      {
        "description": "AP2 Payment Extension",
        "required": true,
        "uri": "https://ap2-protocol.org/extension/v1"
      }
    ]
  },
  "skills": [
    {
      "id": "checkout",
      "name": "Cart and Payment",
      "description": "Handles product search and cart creation",
      "tags": ["ap2", "payment", "commerce"]
    }
  ],
  "security": [...],
  "securitySchemes": {...}
}

Mandates as A2A DataParts

AP2 mandates are transmitted as DataParts within A2A messages:

• Intent Mandate → DataPart in message from Shopping Agent to Merchant
• Cart Mandate → DataPart in response from Merchant to Shopping Agent
• Payment Mandate → DataPart in payment flow messages

A2A Tasks for Payment Flows

Each phase of the AP2 flow maps to A2A tasks:

• Product search → A2A task (SA → Merchant)
• Payment method query → A2A task (SA → CP)
• Payment processing → A2A task (Merchant → MPP)
• Challenge handling → A2A messages within existing tasks

Message Structure

A2A messages carrying AP2 mandates include:

{
  "role": "user",
  "parts": [
    {
      "type": "text",
      "text": "I want to buy a coffee maker"
    },
    {
      "type": "data",
      "data": {
        "intent_mandate": { ... }
      }
    }
  ]
}

Protocol Stack

┌─────────────────────────────┐
│ AP2 (Mandates, VDCs, Signing)│
├─────────────────────────────┤
│ A2A (Tasks, Messages, Parts) │
├─────────────────────────────┤
│ HTTP / JSON-RPC 2.0          │
└─────────────────────────────┘

Discovery

Shopping Agents discover AP2-capable merchants by:

1. Fetching Agent Cards (from .well-known/agent-card.json or registries)
2. Checking for AP2 extension in capabilities.extensions[]
3. Examining skills for payment-related capabilities
4. Verifying authentication requirements

Watch Log

The reference samples include a watch.log that captures:

• Raw HTTP data (methods, URLs, request/response bodies)
• A2A message data (instructions and DataParts)
• AP2 protocol data (IntentMandate, CartMandate, PaymentMandate objects)

This is valuable for debugging the A2A ↔ AP2 integration.

Best Practices

• Declare AP2 extension in Agent Cards for discoverable payment capabilities
• Use A2A DataParts (not TextParts) for mandate transmission
• Follow A2A task lifecycle for payment flow state management
• Implement A2A streaming for real-time payment status updates
• Use A2A authentication alongside AP2 mandate signing
• Test the full A2A + AP2 integration with the official samples
• Log both A2A and AP2 protocol data for debugging

Fetch the specification for exact A2A extension schema, DataPart structures for mandates, and Agent Card AP2 capability declaration before implementing.