Building With Medusa

Medusa Backend Development

Comprehensive backend development guide for Medusa applications. Contains patterns across 6 categories covering architecture, type safety, business logic placement, and common pitfalls.

When to Apply

Load this skill for ANY backend development task, including:

• Creating or modifying custom modules and data models
• Implementing workflows for mutations
• Building API routes (store or admin)
• Defining module links between entities
• Writing business logic or validation
• Querying data across modules
• Implementing authentication/authorization

Also load these skills when:

• building-admin-dashboard-customizations: Building admin UI (widgets, pages, forms)
• building-storefronts: Calling backend API routes from storefronts (SDK integration)

CRITICAL: Load Reference Files When Needed

The quick reference below is NOT sufficient for implementation. You MUST load relevant reference files before writing code for that component.

Load these references based on what you're implementing:

• Creating a module? → MUST load reference/custom-modules.md first
• Creating workflows? → MUST load reference/workflows.md first
• Creating API routes? → MUST load reference/api-routes.md first
• Creating module links? → MUST load reference/module-links.md first
• Querying data? → MUST load reference/querying-data.md first
• Adding authentication? → MUST load reference/authentication.md first

Minimum requirement: Load at least 1-2 reference files relevant to your specific task before implementing.

Critical Architecture Pattern

ALWAYS follow this flow - never bypass layers:

Module (data models + CRUD operations)
  ↓ used by
Workflow (business logic + mutations with rollback)
  ↓ executed by
API Route (HTTP interface, validation middleware)
  ↓ called by
Frontend (admin dashboard/storefront via SDK)

Key conventions:

• Only GET, POST, DELETE methods (never PUT/PATCH)
• Workflows are required for ALL mutations
• Business logic belongs in workflow steps, NOT routes
• Query with query.graph() for cross-module data retrieval
• Query with query.index() (Index Module) for filtering across separate modules with links
• Module links maintain isolation between modules

Rule Categories by Priority

Quick Reference

1. Architecture Violations (CRITICAL)

• arch-workflow-required - Use workflows for ALL mutations, never call module services from routes
• arch-layer-bypass - Never bypass layers (route → service without workflow)
• arch-http-methods - Use only GET, POST, DELETE (never PUT/PATCH)
• arch-module-isolation - Use module links, not direct cross-module service calls
• arch-query-config-fields - Don't set explicit fields when using req.queryConfig

2. Type Safety (CRITICAL)

• type-request-schema - Pass Zod inferred type to MedusaRequest<T> when using req.validatedBody
• type-authenticated-request - Use AuthenticatedMedusaRequest for protected routes (not MedusaRequest)
• type-export-schema - Export both Zod schema AND inferred type from middlewares
• type-linkable-auto - Never add .linkable() to data models (automatically added)
• type-module-name-camelcase - Module names MUST be camelCase, never use dashes (causes runtime errors)

3. Business Logic Placement (HIGH)

• logic-workflow-validation - Put business validation in workflow steps, not API routes
• logic-ownership-checks - Validate ownership/permissions in workflows, not routes
• logic-module-service - Keep modules simple (CRUD only), put logic in workflows

4. Import & Code Organization (HIGH)

• import-top-level - Import workflows/modules at file top, never use await import() in route body
• import-static-only - Use static imports for all dependencies
• import-no-dynamic-routes - Dynamic imports add overhead and break type checking
• import-zod-framework - Import Zod from @medusajs/framework/zod, never from zod directly. Medusa is on Zod v4 (since v2.14.0): use z.email()/z.url()/z.uuid() instead of z.string().email() etc., .extend(other.shape) instead of .merge(other), z.strictObject()/z.looseObject() instead of .strict()/.passthrough(), z.enum(MyEnum) instead of z.nativeEnum(), and z.record(z.string(), value) instead of z.record(value)

5. Data Access Patterns (MEDIUM)

• data-price-format - CRITICAL: Prices are stored as-is in Medusa (49.99 stored as 49.99, NOT in cents). Never multiply by 100 when saving or divide by 100 when displaying
• data-query-method - Use query.graph() for retrieving data; use query.index() (Index Module) for filtering across linked modules
• data-query-graph - Use query.graph() for cross-module queries with dot notation (without cross-module filtering)
• data-query-index - Use query.index() when filtering by properties of linked data models in separate modules
• data-list-and-count - Use listAndCount for single-module paginated queries
• data-linked-filtering - query.graph() can't filter by linked module fields - use query.index() or query from that entity directly
• data-no-js-filter - Don't use JavaScript .filter() on linked data - use database filters (query.index() or query the entity)
• data-same-module-ok - Can filter by same-module relations with query.graph() (e.g., product.variants)
• data-auth-middleware - Trust authenticate middleware, don't manually check req.auth_context

6. File Organization (MEDIUM)

• file-workflow-steps - Recommended: Create steps in src/workflows/steps/[name].ts
• file-workflow-composition - Composition functions in src/workflows/[name].ts
• file-middleware-exports - Export schemas and types from middleware files
• file-links-directory - Define module links in src/links/[name].ts

Workflow Composition Rules

The workflow function has critical constraints:

// ✅ CORRECT
const myWorkflow = createWorkflow(
  "name",
  function (input) { // Regular function, not async, not arrow
    const result = myStep(input) // No await
    return new WorkflowResponse(result)
  }
)

// ❌ WRONG
const myWorkflow = createWorkflow(
  "name",
  async (input) => { // ❌ No async, no arrow functions
    const result = await myStep(input) // ❌ No await
    if (input.condition) { /* ... */ } // ❌ No conditionals
    return new WorkflowResponse(result)
  }
)

Constraints:

• No async/await (runs at load time)
• No arrow functions (use function)
• No conditionals/ternaries (use when())
• No variable manipulation (use transform())
• No date creation (use transform())
• Multiple step calls need .config({ name: "unique-name" }) to avoid conflicts

Common Mistakes Checklist

Before implementing, verify you're NOT doing these:

Architecture:

• Calling module services directly from API routes
• Using PUT or PATCH methods
• Bypassing workflows for mutations
• Setting fields explicitly with req.queryConfig
• Skipping migrations after creating module links

Type Safety:

• Forgetting MedusaRequest<SchemaType> type argument
• Using MedusaRequest instead of AuthenticatedMedusaRequest for protected routes
• Not exporting Zod inferred type from middlewares
• Adding .linkable() to data models
• Using dashes in module names (must be camelCase)

Business Logic:

• Validating business rules in API routes
• Checking ownership in routes instead of workflows
• Manually checking req.auth_context?.actor_id when middleware already applied

Imports:

• Using await import() in route handler bodies
• Dynamic imports for workflows or modules

Data Access:

• CRITICAL: Multiplying prices by 100 when saving or dividing by 100 when displaying (prices are stored as-is: $49.99 = 49.99)
• Filtering by linked module fields with query.graph() (use query.index() or query from other side instead)
• Using JavaScript .filter() on linked data (use query.index() or query the linked entity directly)
• Not using query.graph() for cross-module data retrieval
• Using query.graph() when you need to filter across separate modules (use query.index() instead)

Validating Implementation

CRITICAL: Always run the build command after completing implementation to catch type errors and runtime issues.

When to Validate

• After implementing any new feature
• After making changes to modules, workflows, or API routes
• Before marking tasks as complete
• Proactively, without waiting for the user to ask

How to Run Build

Detect the package manager and run the appropriate command:

npm run build      # or pnpm build / yarn build

Handling Build Errors

If the build fails:

1. Read the error messages carefully
2. Fix type errors, import issues, and syntax errors
3. Run the build again to verify the fix
4. Do NOT mark implementation as complete until build succeeds

Common build errors:

• Missing imports or exports
• Type mismatches (e.g., missing MedusaRequest<T> type argument)
• Incorrect workflow composition (async functions, conditionals)

Linting

Since Medusa v2.16.0, projects can install @medusajs/eslint-plugin, which catches violations of Medusa conventions (API routes, subscribers, scheduled jobs, admin customizations, module patterns) that a type check won't.

• If the project has an eslint.config.* with @medusajs/eslint-plugin, medusa build and medusa develop run linting by default and fail on lint errors. Fix lint errors rather than passing --no-lint.
• Run linting explicitly with npx medusa lint (supports --fix and --quiet). Run it after implementing a feature, alongside the build.
• If the project has no ESLint config, suggest adding it (@medusajs/eslint-plugin, eslint, and jiti as dev dependencies, then an eslint.config.ts exporting defineConfig([...medusa.configs.recommended])) — but don't add it unprompted.

Next Steps - Testing Your Implementation

After successfully implementing a feature, always provide these next steps to the user:

1. Start the Development Server

If the server isn't already running, start it:

npm run dev      # or pnpm dev / yarn dev

2. Access the Admin Dashboard

Open your browser and navigate to:

• Admin Dashboard: http://localhost:9000/app

Log in with your admin credentials to test any admin-related features.

3. Test API Routes

If you implemented custom API routes, list them for the user to test:

Admin Routes (require authentication):

• POST http://localhost:9000/admin/[your-route] - Description of what it does
• GET http://localhost:9000/admin/[your-route] - Description of what it does

Store Routes (public or customer-authenticated):

• POST http://localhost:9000/store/[your-route] - Description of what it does
• GET http://localhost:9000/store/[your-route] - Description of what it does

Testing with cURL example:

# Admin route (requires authentication)
curl -X POST http://localhost:9000/admin/reviews/123/approve \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer YOUR_TOKEN" \
  --cookie "connect.sid=YOUR_SESSION_COOKIE"

# Store route
curl -X POST http://localhost:9000/store/reviews \
  -H "Content-Type: application/json" \
  -d '{"product_id": "prod_123", "rating": 5, "comment": "Great product!"}'

4. Additional Testing Steps

Depending on what was implemented, mention:

• Workflows: Test mutation operations and verify rollback on errors
• Subscribers: Trigger events and check logs for subscriber execution
• Scheduled jobs: Wait for job execution or check logs for cron output

Format for Presenting Next Steps

Always present next steps in a clear, actionable format after implementation:

## Implementation Complete

The [feature name] has been successfully implemented. Here's how to test it:

### Start the Development Server
[server start command based on package manager]

### Access the Admin Dashboard
Open http://localhost:9000/app in your browser

### Test the API Routes
I've added the following routes:

**Admin Routes:**
- POST /admin/[route] - [description]
- GET /admin/[route] - [description]

**Store Routes:**
- POST /store/[route] - [description]

### What to Test
1. [Specific test case 1]
2. [Specific test case 2]
3. [Specific test case 3]

How to Use

For detailed patterns and examples, load reference files:

reference/custom-modules.md    - Creating modules with data models
reference/workflows.md          - Workflow creation and step patterns
reference/api-routes.md         - API route structure and validation
reference/module-links.md       - Linking entities across modules
reference/querying-data.md      - Query patterns and filtering rules
reference/authentication.md     - Protecting routes and accessing users
reference/error-handling.md     - MedusaError types and patterns
reference/scheduled-jobs.md     - Cron jobs and periodic tasks
reference/subscribers-and-events.md - Event handling
reference/troubleshooting.md    - Common errors and solutions

Each reference file contains:

• Step-by-step implementation checklists
• Correct vs incorrect code examples
• TypeScript patterns and type safety
• Common pitfalls and solutions

When to Use This Skill vs MedusaDocs MCP Server

⚠️ CRITICAL: This skill should be consulted FIRST for planning and implementation.

Use this skill for (PRIMARY SOURCE):

• Planning - Understanding how to structure Medusa backend features
• Architecture - Module → Workflow → API Route patterns
• Best practices - Correct vs incorrect code patterns
• Critical rules - What NOT to do (common mistakes and anti-patterns)
• Implementation patterns - Step-by-step guides with checklists

Use MedusaDocs MCP server for (SECONDARY SOURCE):

• Specific method signatures after you know which method to use
• Built-in module configuration options
• Official type definitions
• Framework-level configuration details

Why skills come first:

• Skills contain opinionated guidance and anti-patterns MCP doesn't have
• Skills show architectural patterns needed for planning
• MCP is reference material; skills are prescriptive guidance

Integration with Frontend Applications

⚠️ CRITICAL: Frontend applications MUST use the Medusa JS SDK for ALL API requests

When building features that span backend and frontend:

For Admin Dashboard:

1. Backend (this skill): Module → Workflow → API Route
2. Frontend: Load building-admin-dashboard-customizations skill
3. Connection:
   • Built-in endpoints: Use existing SDK methods (sdk.admin.product.list())
   • Custom API routes: Use sdk.client.fetch("/admin/my-route")
   • NEVER use regular fetch() - missing auth headers will cause errors

For Storefronts:

1. Backend (this skill): Module → Workflow → API Route
2. Frontend: Load building-storefronts skill
3. 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 will cause 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 respective frontend skills for complete integration patterns.