---
alwaysApply: true
---
# Implementing New Features

## Checklist Before Coding

1. Identify which layer the feature belongs to
2. Define interfaces/contracts before implementations
3. Start with the domain if business rules are involved
4. Write tests alongside the implementation

## Implementation Order

```
1. Domain (entities, services)          → Unit tests
2. Application (interfaces, use cases)  → Tests with mocks
3. Infrastructure (repositories, APIs)  → Integration tests
4. Presentation (UI, formatters)        → Visual verification
```

## Patterns to Follow

- Single responsibility per class/function
- Prefer composition over inheritance
- Use discriminated unions to represent states
- Inject dependencies — never instantiate services internally
- Side-effect-free code in the domain

## What to Avoid

- Classes with more than 200 lines
- Functions with more than 5 parameters (use a config object)
- Circular imports between layers
- Complex conditional logic (extract into strategy/policy)
- Magic strings (use enums or constants)
- Comments explaining "what" — the code should be clear