# Development Standards

## Core Philosophy

**TDD is non-negotiable.** Every line of production code must be written in response to a failing test. No exceptions.

**Model Strategy:** Use `opusplan` - Opus for planning/architecture, Sonnet for code execution.

## Quick Reference

### Languages & Tools
| Language | Test Framework | Linter | Formatter |
|----------|---------------|--------|-----------|
| Python | pytest + pytest-asyncio | Ruff | Ruff |
| TypeScript | Vitest | ESLint | ESLint |
| Rust | cargo test | clippy | rustfmt |
| Terraform | terraform validate | tflint | terraform fmt |

### Commands by Language
```bash
# Python
pytest                          # Run tests
pytest --cov=src --cov-report=term-missing  # With coverage
ruff check . && ruff format .   # Lint and format

# TypeScript
npm test                        # Vitest
npm run lint                    # ESLint
npm run typecheck               # tsc --noEmit

# Rust
cargo test                      # Run tests
cargo clippy -- -D warnings     # Lint (deny warnings)
cargo fmt --check               # Check formatting

# Terraform
terraform validate              # Validate
terraform fmt -check -recursive # Check formatting
terraform plan                  # Preview changes
```

## TDD Workflow (RED-GREEN-REFACTOR)

1. **RED**: Write a failing test FIRST
   - Test must fail for the right reason
   - NO production code until test fails

2. **GREEN**: Write MINIMUM code to pass
   - Only what's needed for current test
   - No "while you're there" additions

3. **REFACTOR**: Assess improvements (if tests green)
   - Commit BEFORE refactoring
   - Only refactor if it adds value
   - All tests must still pass

## Coverage Requirements

| Layer | Target | Notes |
|-------|--------|-------|
| Domain/Business Logic | 90%+ | Critical path |
| API Routes | 80%+ | Validation paths |
| Infrastructure/DB | 70%+ | Integration points |
| UI Components | 80%+ | Behavior testing |

**Exceptions** (document in code):
- Generated code (migrations, types)
- Third-party thin wrappers
- Debug utilities

## Type Safety

### Python
- Pydantic v2 for validation at boundaries
- Type hints on all public functions
- `mypy --strict` compliance

### TypeScript
- Strict mode always (`"strict": true`)
- No `any` types - use `unknown` if truly unknown
- Zod schemas at trust boundaries
- `type` for data, `interface` for behavior contracts only

### Rust
- Leverage the type system fully
- Use `Result<T, E>` for fallible operations
- Prefer `thiserror` for error types

## Testing Principles

### Test Behavior, Not Implementation
```typescript
// BAD - tests implementation
it('should call validateAmount', () => {
  const spy = jest.spyOn(validator, 'validateAmount');
  processPayment(payment);
  expect(spy).toHaveBeenCalled();
});

// GOOD - tests behavior
it('should reject negative amounts', () => {
  const payment = getMockPayment({ amount: -100 });
  const result = processPayment(payment);
  expect(result.success).toBe(false);
});
```

### Factory Functions (No `let`/`beforeEach`)
```typescript
const getMockPayment = (overrides?: Partial<Payment>): Payment => ({
  amount: 100,
  currency: 'GBP',
  ...overrides,
});
```

## Security (Zero Tolerance)

- **NEVER** hardcode secrets, passwords, API keys
- **NEVER** commit `.env` files with real values
- Use AWS Secrets Manager or environment variables
- Validate ALL user input at boundaries
- Parameterized queries only (no string concatenation)

## Code Style

- **Immutable by default** - no mutations
- **Pure functions** where possible
- **Early returns** over nested conditionals
- **Options objects** for 3+ parameters
- **No comments** - code should be self-documenting
- **DRY (Don't Repeat Knowledge)** - eliminate duplicate business logic, but don't abstract merely similar code

### DRY Clarification
DRY means don't duplicate **knowledge**, not code. Structural similarity is fine.

```typescript
// NOT duplication - different business rules
const validatePaymentLimit = (amount: number) => amount <= 10000;
const validateTransferLimit = (amount: number) => amount <= 10000;
// These will evolve independently - keep separate

// IS duplication - same business rule in multiple places
// BAD: FREE_SHIPPING_THRESHOLD defined in 3 files
// GOOD: Single constant imported where needed
```

> "Duplicate code is far cheaper than the wrong abstraction."

## Monorepo Patterns

```
project/
├── apps/
│   ├── backend/          # Python FastAPI
│   └── frontend/         # React TypeScript
├── packages/
│   └── shared/           # Shared types/utils
├── infrastructure/
│   ├── terraform/        # IaC
│   └── ansible/          # Config management
└── tests/
    ├── backend/
    └── frontend/
```

## Git Workflow

- Conventional commits: `feat:`, `fix:`, `refactor:`, `test:`, `docs:`
- One logical change per commit
- Tests and implementation in same commit
- PR must pass all checks before merge

## Chrome/Browser Testing

For quick verification during development:
1. Use Claude's Chrome MCP tools for rapid testing
2. Take screenshots to verify UI state
3. Record GIFs for complex interactions

For CI/permanent tests:
1. Write Playwright tests for E2E flows
2. Use React Testing Library for component behavior
3. Test accessibility with axe-core

## When to Use Which Model

**Opus (via plan mode or explicit switch):**
- Architecture decisions
- Complex debugging
- Code review
- Multi-file refactoring plans

**Sonnet (default execution):**
- Writing code
- Running tests
- Simple fixes
- File operations

## Skills (Auto-Loaded)

Skills are automatically loaded when relevant context is detected. Available skills:

**Languages:** Python, TypeScript, Rust, Go, Java, C#

**Infrastructure:** AWS, Azure, GCP, Terraform, Ansible, Docker/Kubernetes, Database/Migrations, CI/CD

**Testing:** TDD workflow, UI Testing, Browser Testing (Playwright + Chrome MCP)

**Patterns:** Monorepo, API Design, Observability (logging, metrics, tracing)

## Agents (Invoke with @)

- `@tdd-guardian` - TDD enforcement and guidance
- `@code-reviewer` - Comprehensive PR review
- `@security-scanner` - Vulnerability detection
- `@refactor-scan` - Refactoring assessment (TDD step 3)
- `@dependency-audit` - Package security/updates

## Quality Gates (Before Every Commit)

- [ ] All tests pass
- [ ] Coverage meets threshold
- [ ] No linting errors
- [ ] Type checking passes
- [ ] No secrets in code
- [ ] TDD compliance verified