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Implement the UCP Discount extension — discount code application, validation, allocation tracking, and error handling. Use when adding promo code or discount logic to a UCP checkout.

Implement UCP payment handlers — configure Google Pay, Shop Pay, or custom payment methods with tokenization, credential flow, and instrument schemas. Use when integrating payment processing into a UCP checkout.

Implement commerce-specific WebMCP tools — product search, cart management, checkout, returns, subscriptions, and support. Use when building agentic shopping experiences on e-commerce websites.

Set up and use the MCP-B polyfill — vanilla JS and React packages that implement navigator.modelContext for browsers without native WebMCP. Use when developing for browsers that don't yet support WebMCP natively.

Build and consume WooCommerce REST API v3 endpoints — authentication, custom endpoints, extending existing resources, webhooks, and batch operations. Use when creating custom API endpoints or integrating external systems with WooCommerce.

Build with Spree's event bus and Webhooks 2.0 — `Spree::Events` publication, `Spree::Subscriber` DSL with `subscribes_to` and `on`, wildcard matching, lifecycle events (`{model}.created/.updated/.deleted` via `publishes_lifecycle_events`), the canonical event catalog (order.*, payment.*, shipment.*, product.*), Webhooks 2.0 endpoints, HMAC-SHA256 signing (`X-Spree-Webhook-Signature`), exponential-

Author custom UCP schemas and extensions — create capability schemas, extension schemas, and type definitions using JSON Schema 2020-12 composition. Use when extending UCP with custom capabilities or building domain-specific extensions.

Build Spree extensions as Rails engines — gem scaffolding, `bin/rails g spree:extension`, mounting routes/migrations/assets, the modern `prepend` decorator pattern (`*_decorator.rb` with `self.prepended(base)`), generators (`spree:model_decorator`, `spree:controller_decorator`), the four customization surfaces in preference order (Events > Webhooks > Dependencies > Decorators), Spree::Dependencies

Work with Spree's legacy v2 APIs — JSON:API-style Storefront API at `/api/v2/storefront/*` and Platform API at `/api/v2/platform/*`, Doorkeeper OAuth2 (password grant for storefront, client_credentials + admin scope for platform), the `spree_legacy_api_v2` gem (required in v5+ for backward compatibility), and migration patterns to API v3. Use when maintaining a v2 client during a migration window,

Integrate payment gateways with Spree — PaymentMethod model, the v5.4+ PaymentSession provider-agnostic checkout flow, Stripe via `spree_stripe` (Apple/Google Pay, Link, Connect for marketplaces), Adyen via `spree_adyen`, PayPal via `spree_paypal_checkout`, StoreCredit / GiftCard as payment methods, refunds, payment state machine, and authoring a custom gateway. Use when wiring a payment integrati

Build storefronts and integrations using `@spree/sdk` — the official TypeScript SDK for Spree's API v3 (v5.4+). Covers installation, the resource-builder pattern (`spree.products.list`, `spree.checkout.create`, etc.), Zod schema validation, server-only auth (httpOnly JWTs, never exposing API keys), error handling, typing customizations, and pinning SDK versions to backend Spree releases. Use when

Upgrade a Spree application — version-to-version migration paths (v4 → v5 is a major rewrite; v5.x → v5.y are simpler), Gemfile pinning strategy, decorator brittleness across versions, deprecated gems to remove (`spree_auth_devise`, `deface`, `spree_backend`), API v2 → v3 migration via `spree_legacy_api_v2`, admin Bootstrap → Tailwind transition, the upgrade guide workflow, and rollback strategy.