claudekit/skills/backend-frameworks/SKILL.md

name, description

name	description
backend-frameworks	Use when building REST APIs or web servers with FastAPI, Django, NestJS, or Express — including routing, middleware, dependency injection, Pydantic models, serializers, controllers, services, guards, pipes, app.get, app.post, APIRouter, class-based views, or framework-specific patterns.

Backend Frameworks

When to Use

• Building REST APIs with FastAPI, Django REST Framework, NestJS, or Express
• Configuring middleware, routing, authentication, or request validation
• Setting up dependency injection, services, or module structure
• Integrating with databases via ORMs (SQLAlchemy, Django ORM, TypeORM, Prisma)
• WebSocket servers, microservices, or GraphQL resolvers

When NOT to Use

• Frontend development — use frontend
• Database-specific queries without framework context — use databases
• API design/documentation — use openapi

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Quick Reference

Framework	Reference	Language	Key features
FastAPI	references/fastapi.md	Python	Pydantic, async, APIRouter, Depends(), OpenAPI auto-docs
Django	references/django.md	Python	ORM, admin, DRF serializers, class-based views, migrations
NestJS	references/nestjs.md	TypeScript	Modules, DI, guards, pipes, interceptors, Prisma/TypeORM
Express	references/express.md	TypeScript	Middleware, Router, error handling, helmet, rate limiting

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Best Practices

1. Use validation models for all request/response data. Pydantic (FastAPI), class-validator DTOs (NestJS), Zod (Express), serializers (Django).
2. Separate business logic from routes/controllers. Route handlers handle HTTP; services handle domain logic.
3. Organize routes by resource and version. APIRouter (FastAPI), module structure (NestJS), Router (Express), URL conf (Django).
4. Return proper HTTP status codes. 201 for creation, 204 for deletion, 202 for accepted-but-not-done, 409 for conflicts.
5. Use async all the way down. Never mix sync blocking calls in async routes (especially FastAPI).
6. Configure settings from environment variables. pydantic-settings (FastAPI), django-environ (Django), dotenv (Express/NestJS).
7. Use select_related/prefetch_related for every query touching relations (Django).
8. Use transaction.atomic() for multi-step writes (Django).

Common Pitfalls

1. Blocking I/O in async routes. requests.get(), time.sleep() in async def routes starves the event loop (FastAPI).
2. Missing response_model / leaking internal fields (FastAPI).
3. N+1 queries from missing eager loading (Django select_related, NestJS relations).
4. Circular imports/dependencies. Use forwardRef() (NestJS), restructure modules (Django/FastAPI).
5. Forgetting asyncHandler — unhandled promise rejections crash the process (Express).
6. Error handler not registered last (Express).
7. Putting business logic in controllers (NestJS).
8. Not using whitelist: true on ValidationPipe (NestJS).

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Related Skills

• databases — Database queries, schema design, migrations
• openapi — API specification and documentation
• error-handling — Exception handling and API error responses
• authentication — Auth flows for web applications