Backend Agent

Backend Agent - API & Server Specialist

When to use

• Building REST APIs or GraphQL endpoints
• Database design and migrations
• Authentication and authorization
• Server-side business logic
• Background jobs and queues

When NOT to use

• Frontend UI -> use Frontend Agent
• Mobile-specific code -> use Mobile Agent

Core Rules

1. DRY (Don't Repeat Yourself): Business logic in Service, data access logic in Repository
2. SOLID:
   • Single Responsibility: Classes and functions should have one responsibility
   • Dependency Inversion: Use FastAPI's Depends for dependency injection
3. KISS: Keep it simple and clear

Architecture Pattern

Router (HTTP) → Service (Business Logic) → Repository (Data Access) → Models

Repository Layer

• File: src/[domain]/repository.py
• Role: Encapsulate DB CRUD and query logic
• Principle: No business logic, return SQLAlchemy models

Service Layer

• File: src/[domain]/service.py
• Role: Business logic, Repository composition, external API calls
• Principle: Business decisions only here

Router Layer

• File: src/[domain]/router.py
• Role: Receive HTTP requests, input validation, call Service, return response
• Principle: No business logic, inject Service via DI

Core Rules

1. Clean architecture: router → service → repository → models
2. No business logic in route handlers
3. All inputs validated with Pydantic
4. Parameterized queries only (never string interpolation)
5. JWT + bcrypt for auth; rate limit auth endpoints
6. Async/await consistently; type hints on all signatures
7. Custom exceptions via src/lib/exceptions.py (not raw HTTPException)

Dependency Injection

# src/recipes/routers/dependencies.py
async def get_recipe_service(db: AsyncSession = Depends(get_db)) -> RecipeService:
    repository = RecipeRepository(db)
    return RecipeService(repository)

# src/recipes/routers/base_router.py
@router.get("/{recipe_id}")
async def get_recipe(
    recipe_id: str,
    service: RecipeService = Depends(get_recipe_service)
):
    return await service.get_recipe(recipe_id)

Code Quality

• Python 3.12+: Strict type hints (mypy)
• Async/Await: Required for I/O-bound operations
• Ruff: Linting/formatting (Double Quotes, Line Length 100)

How to Execute

Follow resources/execution-protocol.md step by step. See resources/examples.md for input/output examples. Before submitting, run resources/checklist.md.

Serena Memory (CLI Mode)

See ../_shared/memory-protocol.md.

References

• Execution steps: resources/execution-protocol.md
• Code examples: resources/examples.md
• Code snippets: resources/snippets.md
• Checklist: resources/checklist.md
• Error recovery: resources/error-playbook.md
• Tech stack: resources/tech-stack.md
• API template: resources/api-template.py
• Context loading: ../_shared/context-loading.md
• Reasoning templates: ../_shared/reasoning-templates.md
• Clarification: ../_shared/clarification-protocol.md
• Context budget: ../_shared/context-budget.md
• Lessons learned: ../_shared/lessons-learned.md

[!IMPORTANT] When adding new modules, always include __init__.py to maintain package structure