version: "1.0.0" name: tdd description: > Test-driven development with red-green-refactor loop. Use when user wants to build features or fix bugs using TDD, mentions "red-green-refactor", wants integration tests, or asks for test-first development. allowed-tools: Read, Write, Edit, Bash, Grep, Glob

Test-Driven Development

Tests verify behavior through public interfaces, not implementation details. Always use vertical slices (one test -> one implementation -> repeat), never horizontal slices.

Additional resources

• For TDD philosophy and anti-patterns, see reference/philosophy.md
• For good/bad test examples, see examples/tests.md
• For mocking guidelines, see examples/mocking.md
• For deep module design, see reference/deep-modules.md
• For interface testability, see reference/interface-design.md
• For refactor candidates, see reference/refactoring.md

Workflow

1. Planning

• [ ] Identify what interface changes are needed
• [ ] Identify which behaviors to test (prioritize)
• [ ] Design interfaces for testability (accept deps, return results, small surface)
• [ ] List the behaviors to test (not implementation steps)

2. Tracer Bullet

Write ONE test that confirms ONE thing about the system:

RED:   Write test for first behavior -> test fails
GREEN: Write minimal code to pass -> test passes

3. Incremental Loop

For each remaining behavior:

RED:   Write next test -> fails
GREEN: Minimal code to pass -> passes

• One test at a time
• Only enough code to pass current test
• Don't anticipate future tests

4. Refactor

After all tests pass:

• [ ] Extract duplication
• [ ] Deepen modules (move complexity behind simple interfaces)
• [ ] Run tests after each refactor step

Never refactor while RED. Get to GREEN first.

Checklist Per Cycle

• [ ] Test describes behavior, not implementation
• [ ] Test uses public interface only
• [ ] Test would survive internal refactor
• [ ] Code is minimal for this test
• [ ] No speculative features added

Completion Criteria

• [ ] All planned behaviors have a corresponding test
• [ ] All tests pass (GREEN state)
• [ ] Refactor pass completed — no duplication, modules are deep
• [ ] No speculative code — every line justified by a test

Gotchas

• Iterate this section: After each TDD session, append new gotchas here