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

Priority	Category	Impact	Prefix
1	Architecture Violations	CRITICAL	`arch-`
2	Type Safety	CRITICAL	`type-`
3	Business Logic Placement	HIGH	`logic-`
4	Import & Code Organization	HIGH	`import-`
5	Data Access Patterns	MEDIUM (includes CRITICAL price rule)	`data-`
6	File Organization	MEDIUM	`file-`

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

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:

Read the error messages carefully
Fix type errors, import issues, and syntax errors
Run the build again to verify the fix
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)

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

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:

Backend (this skill): Module → Workflow → API Route
Frontend: Load building-admin-dashboard-customizations skill
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:

Backend (this skill): Module → Workflow → API Route
Frontend: Load building-storefronts skill
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.

building-with-medusa