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