AP2 Merchant Agent Implementation

Before writing code

Fetch live docs:

1. Fetch https://ap2-protocol.org/specification/ for Merchant Endpoint responsibilities
2. Web-search site:github.com google-agentic-commerce AP2 samples roles merchant_agent for reference implementation
3. Web-search site:github.com google-agentic-commerce AP2 merchant cart mandate for Cart Mandate creation
4. Fetch SDK docs for merchant-side agent patterns

Conceptual Architecture

What the Merchant Agent Does

The Merchant Agent (ME) represents the seller in AP2 transactions:

1. Receives Intent Mandates from Shopping Agents
2. Searches product catalog for matching items
3. Creates Cart Mandates with specific product offers, prices, and totals
4. Signs Cart Mandates with the merchant entity's signature
5. Handles clarification requests when intent is ambiguous
6. Supports the payment flow by forwarding to Payment Processor

Agent Card

The Merchant Agent's Agent Card advertises:

• AP2 extension support with supported payment methods
• Skills related to product search, cart creation, order management
• Product categories and capabilities
• Authentication requirements for Shopping Agents

Key Responsibilities

Intent Processing

• Parse incoming Intent Mandates from Shopping Agents
• Match intent against product catalog
• Determine if the intent can be fulfilled within constraints
• Decide whether to proceed, request clarification, or reject

Cart Mandate Creation

• Select matching products from catalog
• Calculate prices, taxes, shipping, and totals
• Build the Cart Mandate with W3C Payment Request API structure
• Include all line items with individual pricing
• Sign the Cart Mandate with the merchant entity's key

Merchant Signature

The merchant signature is critical:

• It's an entity-level signature (the merchant organization, not the AI agent)
• It guarantees product availability at the stated prices
• It commits the merchant to fulfillment
• Key management must be handled at the organization level

Escalation Decisions

When facing ambiguous intent, the merchant can:

• Offer choices — Present multiple matching products for user selection
• Ask clarifications — Request more specific requirements
• Force human-present — Require the user to confirm directly
• Reject — Decline the intent if it can't be fulfilled

Catalog Integration

The Merchant Agent needs access to:

• Product catalog — Names, descriptions, SKUs, images
• Inventory — Real-time stock availability
• Pricing — Current prices, discounts, taxes
• Shipping — Available shipping methods, costs, timeframes
• Policies — Return policy, refund period, terms

Payment Method Advertisement

Cart Mandates include supported payment methods:

"method_data": [
  {
    "supportedMethods": "https://processor.example.com/pay",
    "data": { "merchant_id": "..." }
  }
]

Multi-Merchant Scenarios

A Shopping Agent may contact multiple merchants:

• Each merchant independently evaluates the Intent Mandate
• Each returns their own Cart Mandate
• The Shopping Agent compares offers on behalf of the user
• The merchant should respond quickly to be competitive

Best Practices

• Sign Cart Mandates at the entity level, not the agent level
• Include all items with clear descriptions and prices
• Respond to Intent Mandates promptly — the Shopping Agent may be comparing merchants
• Handle out-of-stock gracefully with alternatives or clear messaging
• Implement proper key management for merchant signing keys
• Validate incoming Intent Mandates for authenticity
• Keep catalog data fresh — stale prices cause disputes
• Log all Cart Mandate creation for audit trail

Fetch the specification for exact Merchant Agent requirements, Cart Mandate creation process, and signing formats before implementing.