Comprehensive guide for Medusa backend architecture, workflows, and critical implementation rules.

• Covers six rule categories (architecture, type safety, business logic placement, imports, data access, file organization) with specific anti-patterns and enforcement checks
• Enforces strict layer separation: Module → Workflow → API Route → Frontend, with workflows required for all mutations and only GET/POST/DELETE HTTP methods allowed
• Includes critical data handling rules: prices stored as-is (not in cents), query.graph() for cross-module retrieval, query.index() for filtering across linked modules
• Provides reference files for custom modules, workflows, API routes, module links, querying, authentication, error handling, scheduled jobs, and subscribers
• Requires build validation after implementation and includes testing checklist with cURL examples for API route verification