# Tdd

> Test-driven development — the red/green/refactor loop, what makes a test worth keeping (behavior through public interfaces, not implementation details), where tests go (seams agreed up front), and the anti-patterns that make tests lie (implementation-coupled, tautological, horizontal-sliced). Use when the user wants to build features or fix bugs test-first, mentions "red-green-refactor", or wants integration tests written well.

- Skill: `ever-just/tdd` (Agent Skill, multi-file: 3 files)
- Install (CLI): `npx skillmds@latest add ever-just/tdd`
- Raw SKILL.md: https://api.skillmd.com/api/skills/ever-just/tdd/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Integrations & APIs
- Author: ever-just (https://skillmd.com/u/ever-just)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/ever-just/tdd

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# Test-Driven Development

TDD is the red → green loop. This skill is the reference that makes that loop produce tests worth keeping: what a good test is, where tests go, the anti-patterns, and the rules of the loop. Every section applies on every cycle — consult them before and during the loop, not after.

When exploring the codebase, read a project glossary/CONTEXT doc if one exists so test names and interface vocabulary match the project's domain language, and respect any architecture decision records (ADRs) in the area you're touching.

## What a good test is

Tests verify behavior through public interfaces, not implementation details. Code can change entirely; tests shouldn't. A good test reads like a specification — "user can checkout with valid cart" tells you exactly what capability exists — and survives refactors because it doesn't care about internal structure.

See [tests.md](tests.md) for examples and [mocking.md](mocking.md) for mocking guidelines.

## 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 — 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?"

## Anti-patterns

- **Implementation-coupled** — mocks internal collaborators, tests private methods, or verifies through a side channel (querying the database instead of using the interface). The tell: the test breaks when you refactor but behavior hasn't changed.
- **Tautological** — the assertion recomputes the expected value the way the code does (`expect(add(a, b)).toBe(a + b)`, a snapshot derived by hand the same way, a constant asserted equal to itself), so it passes by construction and can never disagree with the code. Expected values must come from an independent source of truth — a known-good literal, a worked example, the spec.
- **Horizontal slicing** — writing all tests first, then all implementation. Bulk tests verify _imagined_ behavior: you test the _shape_ of things rather than user-facing behavior, the tests go insensitive to real changes, and you commit to test structure before understanding the implementation. Work in **vertical slices** instead — one test → one implementation → repeat, each test a **tracer bullet** that responds to what the last cycle taught you.

## Rules of the loop

- **Red before green.** Write the failing test first, then only enough code to pass it. Don't anticipate future tests or add speculative features.
- **One slice at a time.** One seam, one test, one minimal implementation per cycle.
- **Refactoring is not part of the loop.** It belongs to a separate review stage, not the red → green implementation cycle.

## Combining with other skills

- `diagnosing-bugs` — Phase 5's regression test follows this skill's seam discipline: test at the real call site, not a shallow stand-in.
- `resolving-merge-conflicts` — step 4 of that skill runs the project's tests; a merge that breaks a TDD-style seam test is a real regression, not a flaky test to silence.

## Attribution

Adapted from [mattpocock/skills](https://github.com/mattpocock/skills) (`skills/engineering/tdd`), MIT licensed, © Matt Pocock.