UCP Payment Handlers

Before writing code

Fetch live docs:

• Google Pay handler: https://developers.google.com/merchant/ucp/guides/google-pay-payment-handler
• Shop Pay handler: Web-search site:shopify.dev UCP Shop Pay payment handler
• Payment architecture: https://ucp.dev/specification/overview/ (payment section)
• Reference types: https://ucp.dev/specification/reference/ (credential and instrument schemas)

Conceptual Architecture

Trust Triangle

Business <——> PSP <——> Credential Provider

• Credential Provider (Google Pay, Shop Pay): Issues encrypted payment tokens to the Platform
• PSP (Stripe, Adyen, etc.): Decrypts tokens, authorizes with card networks, settles funds
• Business: Configures which handlers/PSPs it accepts; receives credentials from Platform, forwards to PSP

Critical security rule: Credentials flow Platform → Business ONLY. Business MUST NEVER echo credentials back to the Platform.

Payment Handler Concept

A payment handler is a specification, not an entity. It defines:

• id: Unique identifier for this payment handler instance
• name: Reverse-domain identifier (e.g., com.google.pay, com.shopify.shop_pay)
• version: Date-based version
• spec: URI to the handler specification
• config: Handler-specific configuration (merchant ID, accepted card networks, tokenization params)
• config_schema: JSON Schema defining the structure of the config object for this handler
• instrument_schemas: Schemas defining the shape of payment instruments this handler produces

Handlers are declared in the Business's discovery profile and echoed in checkout responses.

Three Payment Processing Scenarios

1. Digital Wallet (Google Pay, Shop Pay): Platform acquires encrypted tokens from the wallet provider's API, sends to Business in complete_checkout.
2. Direct Tokenization: Platform calls PSP endpoint directly with Business's public key, gets network tokens.
3. Autonomous Agent (AP2): Agent generates cryptographically-signed mandates proving user authorization — no human interaction needed. See the ucp-ap2-mandates skill.

Credential Flow in Complete Checkout

When calling complete:

• payment_data.instrument: Describes the payment method (type, brand, last digits, billing address)
• payment_data.credential: The actual token/cryptogram (encrypted, handler-specific)

Implementation Guidance

Business side:

1. Configure your PSP (Stripe, Adyen, etc.) and get merchant credentials
2. Build payment handler config for your discovery profile — fetch the exact format from the live handler spec
3. On complete_checkout, extract the credential and forward it to your PSP for authorization
4. Map PSP responses back to UCP checkout status (completed, or error with messages)

Platform side:

1. Read payment.handlers from the checkout response
2. Use the handler's config to initialize the payment provider SDK (e.g., Google Pay JS API)
3. Acquire a payment credential from the user
4. Send it in the complete_checkout call

Always verify the exact handler config schema from the live spec — payment handler configurations change frequently.