<overview>
TDD is about design quality, not coverage metrics. The red-green-refactor cycle forces you to think about behavior before implementation, producing cleaner interfaces and more testable code.

**Principle:** If you can describe the behavior as `expect(fn(input)).toBe(output)` before writing `fn`, TDD improves the result.
</overview>

<detection>
## When TDD Improves Quality

**TDD candidates (add `tdd="true"`, include `<test-first>`):**
- Business logic with defined inputs/outputs
- API endpoints with request/response contracts
- Data transformations, parsing, formatting
- Validation rules and constraints
- Algorithms with testable behavior
- State machines and workflows
- Utility functions with clear specifications

**Skip TDD (standard `type="auto"`):**
- UI layout, styling, visual components
- Configuration changes
- Glue code connecting existing components
- One-off scripts and migrations
- Simple CRUD with no business logic
- Exploratory prototyping

**Heuristic:** Can you write `expect(fn(input)).toBe(output)` before writing `fn`?
→ Yes: TDD will help
→ No: Implement first, add tests after if needed
</detection>

<execution_flow>
## Red-Green-Refactor Cycle

**RED - Write failing test:**
1. Create test file following project conventions
2. Write test describing expected behavior (from `<test-first>` element)
3. Run test - it MUST fail
4. If test passes: feature exists or test is wrong. Investigate.
5. Commit: `test: add failing test for [feature]`

**GREEN - Implement to pass:**
1. Write minimal code to make test pass
2. No cleverness, no optimization - just make it work
3. Run test - it MUST pass
4. Commit: `feat: implement [feature]`

**REFACTOR (if needed):**
1. Clean up implementation if obvious improvements exist
2. Run tests - MUST still pass
3. Only commit if changes made: `refactor: clean up [feature]`

**Atomic commits:** Each TDD task produces 2-3 commits following this pattern.
</execution_flow>

<test_quality>
## Good Tests vs Bad Tests

**Test behavior, not implementation:**
- Good: "returns formatted date string"
- Bad: "calls formatDate helper with correct params"
- Tests should survive refactors

**One concept per test:**
- Good: Separate tests for valid input, empty input, malformed input
- Bad: Single test checking all edge cases with multiple assertions

**Descriptive names:**
- Good: "should reject empty email", "returns null for invalid ID"
- Bad: "test1", "handles error", "works correctly"

**No implementation details:**
- Good: Test public API, observable behavior
- Bad: Mock internals, test private methods, assert on internal state
</test_quality>

<framework_setup>
## Test Framework Setup (If None Exists)

When a TDD task exists but no test framework is configured:

**1. Detect project type:**
```bash
# JavaScript/TypeScript
if [ -f package.json ]; then echo "node"; fi

# Python
if [ -f requirements.txt ] || [ -f pyproject.toml ]; then echo "python"; fi

# Go
if [ -f go.mod ]; then echo "go"; fi

# Rust
if [ -f Cargo.toml ]; then echo "rust"; fi
```

**2. Install minimal framework:**
| Project | Framework | Install |
|---------|-----------|---------|
| Node.js | Jest | `npm install -D jest @types/jest ts-jest` |
| Node.js (Vite) | Vitest | `npm install -D vitest` |
| Python | pytest | `pip install pytest` |
| Go | testing | Built-in |
| Rust | cargo test | Built-in |

**3. Create config if needed:**
- Jest: `jest.config.js` with ts-jest preset
- Vitest: `vitest.config.ts` with test globals
- pytest: `pytest.ini` or `pyproject.toml` section

**4. Verify setup:**
```bash
# Run empty test suite - should pass with 0 tests
npm test  # Node
pytest    # Python
go test ./...  # Go
cargo test    # Rust
```

**5. Create first test file:**
Follow project conventions for test location:
- `*.test.ts` / `*.spec.ts` next to source
- `__tests__/` directory
- `tests/` directory at root
</framework_setup>

<error_handling>
## Error Handling

**Test doesn't fail in RED phase:**
- Feature may already exist - investigate
- Test may be wrong (not testing what you think)
- Fix before proceeding

**Test doesn't pass in GREEN phase:**
- Debug implementation
- Don't skip to next task
- Keep iterating until green

**Tests fail in REFACTOR phase:**
- Undo refactor
- Commit was premature
- Refactor in smaller steps

**Unrelated tests break:**
- Stop and investigate
- May indicate coupling issue
- Fix before proceeding
</error_handling>

<commit_pattern>
## Commit Pattern for TDD Tasks

Each TDD task produces atomic commits:

```
test: add failing test for email validation

feat: implement email validation

refactor: extract regex to constant (optional)
```

This pattern:
- Creates clean git history
- Each commit is independently revertable
- Shows TDD discipline in commit log
</commit_pattern>