ap2-dev-patterns

AP2 Development Patterns

Before writing code

Fetch live docs:

1. Fetch https://ap2-protocol.org/specification/ for the latest protocol details
2. Fetch https://ap2-protocol.org/topics/ap2-and-ucp/ for UCP integration patterns
3. Fetch https://ap2-protocol.org/topics/ap2-and-x402/ for x402 crypto integration
4. Web-search site:github.com google-agentic-commerce AP2 samples for reference architectures

Conceptual Architecture

Multi-Agent Payment Architecture

The standard AP2 architecture involves four agents communicating via A2A:

┌──────────────────────────────────────────────────┐
│                    User                           │
│          (trusted device surface)                 │
└──────────┬───────────────────────────────────────┘
           │ intent + signatures
┌──────────▼───────────────────────────────────────┐
│            Shopping Agent (SA)                     │
│    Orchestrator — coordinates the full flow        │
└──┬──────────────┬────────────────────────────────┘
   │              │
   │ A2A          │ A2A
   │              │
┌──▼──────┐  ┌───▼──────────┐
│ Merchant │  │ Credentials  │
│ Agent    │  │ Provider     │
│          │  │              │
│ catalog  │  │ payment      │
│ cart     │  │ methods      │
│ signing  │  │ tokenization │
└────┬─────┘  └──────────────┘
     │
     │ (Merchant → MPP)
     │
┌────▼─────────────┐
│ Merchant Payment  │
│ Processor (MPP)   │
│                   │
│ authorization     │
│ settlement        │
│ Payment Mandate   │
│ challenges        │
└────────┬──────────┘
         │
    ┌────▼──────────┐
    │ Network/Issuer │
    │ (Visa, MC,     │
    │  banks)        │
    └────────────────┘

Important: The SA communicates with the Merchant and the CP via A2A. The Merchant communicates with the MPP. The SA does not directly communicate with the MPP.

UCP Integration Pattern

UCP (Universal Commerce Protocol) operationalizes AP2:

• UCP's Checkout object maps to a commerce-level checkout flow
• UCP's checkout_mandate is distinct from (but related to) AP2's Cart Mandate — checkout_mandate is a UCP-specific concept that wraps the commerce checkout authorization, while AP2's Cart Mandate is the cryptographically signed VDC binding user consent to specific transaction terms
• UCP's /complete_checkout API = AP2's payment authorization step
• PaymentMandate (constructed by the MPP) proves payment authorization for the network/issuer

Use UCP when you need the full commerce checkout flow (catalog, cart, checkout, orders) with AP2's payment security.

x402 Crypto Payment Pattern

AP2 supports emerging digital payment methods via x402:

• AP2 is payment-method agnostic — supports cards and crypto
• x402 represents a specific crypto payment standard
• AP2's VDC framework provides trust infrastructure for any payment method
• Reference: github.com/google-agentic-commerce/a2a-x402

Testing with Mock Providers

Development testing without real money:

• Set up internal environments with mock payment methods
• Use the official samples as test harnesses (no real payment dependencies)
• Mock Credentials Provider returns fake DPANs
• Mock Payment Processor approves without real authorization
• OTP test value: "123" (from samples)
• Use "verbose" mode for full mandate payload visibility

Framework Flexibility

AP2 agents can be built with any framework:

• Google ADK — Reference implementation framework
• LangGraph — LangChain-based agent graphs
• CrewAI — Crew-based multi-agent orchestration
• AG2 — Microsoft's agent framework
• Custom — Any Python/JS framework with A2A support

The protocol doesn't mandate a specific framework — only A2A compliance.

Multi-Merchant Comparison

Shopping Agents can query multiple merchants:

1. Send Intent Mandate to Merchant A, B, C in parallel
2. Receive Cart Mandates from each
3. Compare offers (price, shipping, availability)
4. Present best options to user
5. User selects preferred offer
6. Proceed with selected merchant

Subscription and Recurring Payments

On the V1.x roadmap:

• Standardized recurring payment flows
• Subscription management
• Recurring Intent Mandates with TTL-based reauthorization

Production Deployment Considerations

• Key management — HSMs for merchant signing keys, secure hardware for user keys
• PCI compliance — Credentials Provider must meet PCI DSS requirements
• Logging — Complete audit trail for all mandates and transactions
• Monitoring — Track mandate creation, signing, and payment success rates
• Scaling — Each agent role scales independently
• Trust establishment — Start with allowlists, evolve to certificate-based trust

Security Checklist

• Shopping Agent cannot access raw payment credentials
• User signatures are hardware-backed
• Merchant signatures are entity-level (not agent-level)
• All mandates are stored with signatures for audit
• Challenge flows redirect to trusted surfaces
• Risk signals are captured at transaction time
• PCI data stays within Credentials Provider boundary
• Agent-to-agent communication is encrypted (HTTPS)

Best Practices

• Start with the official samples as a foundation
• Use mocked payment providers for development
• Test the full multi-agent flow end-to-end
• Implement proper role separation from the beginning
• Build with the UCP integration pattern for full commerce flows
• Monitor the AP2 roadmap for new capabilities (push payments, subscriptions)
• Follow Google ADK patterns for the reference implementation approach
• Engage with the AP2 community on GitHub for updates

Fetch the latest specification, roadmap, and sample implementations for current capabilities and patterns before implementing.