# Test Driven Development

> Use when: implement a change with a TDD loop — failing test first, minimal fix, verify, refactor.

- Skill: `kimtth/test-driven-development` (Agent Skill)
- Install (CLI): `npx skillmds@latest add kimtth/test-driven-development`
- Raw SKILL.md: https://api.skillmd.com/api/skills/kimtth/test-driven-development/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: kimtth (https://skillmd.com/u/kimtth)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/kimtth/test-driven-development

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Goal: drive behavior with tests so code is correct by construction.

Use for:
- new behavior with a clear input/output contract
- bug fixes that need a regression guard
- code where you keep writing tests that only pass trivially

Workflow:
1. Write one failing test that describes the desired behavior.
2. Run it; confirm it fails for the right reason.
3. Write the minimal code to make it pass.
4. Run the suite; confirm green.
5. Refactor with tests staying green.
6. Repeat for the next behavior.

Test quality:
- assert observable behavior, not implementation detail
- one reason to fail per test
- cover edge cases and error paths, not just the happy path

Rules:
- never write production code without a failing test first
- do not test through mocks what you can test for real
- keep the red-green-refactor steps separate
- a test that cannot fail is not a test