# Good Tests

> Use whenever designing, writing, modifying, or reviewing tests; choosing what or where to test; using TDD, BDD, ATDD, or red-green-refactor; creating or reviewing User Journey Tests; or making decisions about mocks and other test doubles. Keeps tests focused on observable behavior through public seams so they survive refactors. Do not use when only running existing tests or reporting their results.

- Skill: `choegyumin/good-tests` (Agent Skill, multi-file: 6 files)
- Install (CLI): `npx skillmds@latest add choegyumin/good-tests`
- Raw SKILL.md: https://api.skillmd.com/api/skills/choegyumin/good-tests/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: choegyumin (https://skillmd.com/u/choegyumin)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/choegyumin/good-tests

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# Good Tests

This skill is the reference for creating tests worth keeping: what a good test is, where tests go, the anti-patterns, and the rules of the loop. It does not require a loop or cycle such as TDD. When the work has a loop, apply these instructions before and during it, not after.

## Required terminology

This skill uses the terms **User Journey Test** and **Focused Test**. User Journey Tests commonly appear as acceptance or E2E tests, but the terms are not synonymous. Before reading the following sections or references, or before designing, writing, modifying, or reviewing tests, read [terms.md](./terms.md) and understand the definitions precisely.

## Seams: where tests go

A **seam** is the public boundary you test at: the interface where you observe behavior without reaching inside. Tests live at seams, never against internals.

**Test only at pre-agreed seams.** Before writing any test, write down the seams under test and confirm them with the user. No test is written at an unconfirmed seam. You can't test everything, so agreeing the seams up front is how testing effort lands on the critical paths and complex logic instead of every edge case.

Ask: "What's the public interface, and which seams should we test?"

When the shape of that interface is itself in question, address the interface design before writing tests. Use the existing code and ADRs to identify the public interface and candidate seams, explain the trade-offs of each candidate, and agree on them with the user.

## TDD loop

When following TDD (Test-Driven Development), use these principles. BDD (Behavior-Driven Development), ATDD (Acceptance Test-Driven Development), and related test-first methods follow the same principles.

- **Red before green.** Write and run the failing test first. Confirm that it fails for the expected reason because the intended behavior does not exist yet. If it already passes, check whether the behavior already exists and whether the chosen seam, inputs, preconditions, and expected result are correct.
- **One slice at a time.** One seam, one test, one minimal implementation per cycle. Move to the next slice only after the test passes. Do not anticipate future tests or add speculative features.
- **Green is not always followed immediately by refactoring.** Finish the implementation through repeated red-green cycles, then refactor during the review stage. Read red-green-refactor as red-green-red-...-green-refactor, not mechanically as refactoring after every green.

## Reference router

Route by the decision the work requires, not by the testing term the user happens to use. Read only the documents whose conditions apply. If several conditions overlap, read all applicable documents.

| Decision or task | Document to read |
| --- | --- |
| Design, write, modify, or review tests. | [tests.md](./tests.md) |
| Change a user journey, scenario, or story, or write, modify, or review a User Journey Test. | Also read [user-journey-tests.md](./user-journey-tests.md) |
| Choose, use, modify, or review a test double, or design an interface for one. | Also read [test-doubles.md](./test-doubles.md) |

When several conditions overlap, read only the required documents in this order: `tests.md` → `user-journey-tests.md` → `test-doubles.md`.