# Tdd

> Apply test-driven development for features or fixes. Use when the user requests test-first work, red-green-refactor, or integration tests.

- Skill: `davidbasilefilho/tdd` (Agent Skill, multi-file: 3 files)
- Install (CLI): `npx skillmds@latest add davidbasilefilho/tdd`
- Raw SKILL.md: https://api.skillmd.com/api/skills/davidbasilefilho/tdd/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Integrations & APIs
- Author: davidbasilefilho (https://skillmd.com/u/davidbasilefilho)
- Updated: 2026-09-21
- Page: https://skillmd.com/skills/davidbasilefilho/tdd

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# Test-Driven Development

Read `CONTEXT.md` when present; match domain terms and respect relevant ADRs. See [tests.md](tests.md) and [mocking.md](mocking.md).

## Test Surface

Test behavior through public interfaces, never internals. Tests must read as specifications and survive refactors.

A seam is a public boundary where behavior is observed. Before testing, list proposed seams and confirm them with user. Write no test at an unconfirmed seam.

## Reject

- Implementation-coupled tests: private methods, internal mocks, side-channel verification.
- Tautologies: expected value recomputed like production code. Use independent literals, worked examples, or spec facts.
- Horizontal slicing: all tests before all code. Use vertical tracer bullets.

## Loop

1. Pick one confirmed seam and behavior.
2. Write one failing test; observe red.
3. Add only enough implementation for green. Avoid speculative behavior.
4. Repeat one vertical slice at a time.

Keep refactoring outside red-green cycles; perform it during review after tests pass.