Building Storefronts Agent Skill

Medusa Storefront Development

Frontend integration guide for building storefronts with Medusa. Covers SDK usage, React Query patterns, and calling custom API routes.

When to Apply

Load this skill for ANY storefront development task, including:

• Calling custom Medusa API routes from the storefront
• Integrating Medusa SDK in frontend applications
• Using React Query for data fetching
• Implementing mutations with optimistic updates
• Error handling and cache invalidation

Also load building-with-medusa when: Building the backend API routes that the storefront calls

CRITICAL: Load Reference Files When Needed

The quick reference below is NOT sufficient for implementation. You MUST load the reference file before writing storefront integration code.

Load this reference when implementing storefront features:

• Calling API routes? → MUST load references/frontend-integration.md first
• Using SDK? → MUST load references/frontend-integration.md first
• Implementing React Query? → MUST load references/frontend-integration.md first

Rule Categories by Priority

| Priority | Category | Impact | Prefix |

|----------|----------|--------|--------|

| 1 | SDK Usage | CRITICAL | sdk- |

| 2 | React Query Patterns | HIGH | query- |

| 3 | Data Display | HIGH (includes CRITICAL price rule) | display- |

| 4 | Error Handling | MEDIUM | error- |

Quick Reference

1. SDK Usage (CRITICAL)

• sdk-always-use - ALWAYS use the Medusa JS SDK for ALL API requests - NEVER use regular fetch()
• sdk-existing-methods - For built-in endpoints, use existing SDK methods (sdk.store.product.list(), sdk.admin.order.retrieve())
• sdk-client-fetch - For custom API routes, use sdk.client.fetch()
• sdk-required-headers - SDK automatically adds required headers (publishable API key for store, auth for admin) - regular fetch() missing these headers causes errors
• sdk-no-json-stringify - NEVER use JSON.stringify() on body - SDK handles serialization automatically
• sdk-plain-objects - Pass plain JavaScript objects to body, not strings
• sdk-locate-first - Always locate where SDK is instantiated in the project before using it

2. React Query Patterns (HIGH)

• query-use-query - Use useQuery for GET requests (data fetching)
• query-use-mutation - Use useMutation for POST/DELETE requests (mutations)
• query-invalidate - Invalidate queries in onSuccess to refresh data after mutations
• query-keys-hierarchical - Structure query keys hierarchically for effective cache management
• query-loading-states - Always handle isLoading, isPending, isError states

3. Data Display (HIGH)

• display-price-format - CRITICAL: Prices from Medusa are stored as-is ($49.99 = 49.99, NOT in cents). Display them directly - NEVER divide by 100

4. Error Handling (MEDIUM)

• error-on-error - Implement onError callback in mutations to handle failures
• error-display - Show error messages to users when mutations fail
• error-rollback - Use optimistic updates with rollback on error for better UX

Critical SDK Pattern

ALWAYS pass plain objects to the SDK - NEVER use JSON.stringify():

// ✅ CORRECT - Plain object
await sdk.client.fetch("/store/reviews", {
  method: "POST",
  body: {
    product_id: "prod_123",
    rating: 5,
  }
})

// ❌ WRONG - JSON.stringify breaks the request
await sdk.client.fetch("/store/reviews", {
  method: "POST",
  body: JSON.stringify({  // ❌ DON'T DO THIS!
    product_id: "prod_123",
    rating: 5,
  })
})

Why this matters:

• The SDK handles JSON serialization automatically
• Using JSON.stringify() will double-serialize and break the request
• The server won't be able to parse the body

Common Mistakes Checklist

Before implementing, verify you're NOT doing these:

SDK Usage:

• [ ] Using regular fetch() instead of the Medusa JS SDK (causes missing header errors)
• [ ] Not using existing SDK methods for built-in endpoints (e.g., using sdk.client.fetch("/store/products") instead of sdk.store.product.list())
• [ ] Using JSON.stringify() on the body parameter
• [ ] Manually setting Content-Type headers (SDK adds them)
• [ ] Hardcoding SDK import paths (locate in project first)
• [ ] Not using sdk.client.fetch() for custom routes

React Query:

• [ ] Not invalidating queries after mutations
• [ ] Using flat query keys instead of hierarchical
• [ ] Not handling loading and error states
• [ ] Forgetting to disable buttons during mutations (isPending)

Data Display:

• [ ] CRITICAL: Dividing prices by 100 when displaying (prices are stored as-is: $49.99 = 49.99, NOT in cents)

Error Handling:

• [ ] Not implementing onError callbacks
• [ ] Not showing error messages to users
• [ ] Not handling network failures gracefully

How to Use

For detailed patterns and examples, load reference file:

references/frontend-integration.md - SDK usage, React Query patterns, API integration

The reference file contains:

• Step-by-step SDK integration patterns
• Complete React Query examples
• Correct vs incorrect code examples
• Query key best practices
• Optimistic update patterns
• Error handling strategies

When to Use MedusaDocs MCP Server

Use this skill for (PRIMARY SOURCE):

• How to call custom API routes from storefront
• SDK usage patterns (sdk.client.fetch)
• React Query integration patterns
• Common mistakes and anti-patterns

Use MedusaDocs MCP server for (SECONDARY SOURCE):

• Built-in SDK methods (sdk.admin.*, sdk.store.*)
• Official Medusa SDK API reference
• Framework-specific configuration options

Why skills come first:

• Skills contain critical patterns like "don't use JSON.stringify" that MCP doesn't emphasize
• Skills show correct vs incorrect patterns; MCP shows what's possible
• Planning requires understanding patterns, not just API reference

Integration with Backend

⚠️ CRITICAL: ALWAYS use the Medusa JS SDK - NEVER use regular fetch()

When building features that span backend and frontend:

• Backend (building-with-medusa skill): Module → Workflow → API Route
• Storefront (this skill): SDK → React Query → UI Components
• Connection:
• Built-in endpoints: Use existing SDK methods (sdk.store.product.list())
• Custom API routes: Use sdk.client.fetch("/store/my-route")
• NEVER use regular fetch() - missing publishable API key causes errors

Why the SDK is required:

• Store routes need x-publishable-api-key header
• Admin routes need Authorization and session headers
• SDK handles all required headers automatically
• Regular fetch() without headers → authentication/authorization errors

See building-with-medusa for backend API route patterns.