# Tdd

> Test-driven development. Use when the user wants to build features or fix bugs test-first, mentions "red-green-refactor", or wants integration tests.

- Skill: `ravihammond/tdd` (Agent Skill, multi-file: 3 files)
- Install (CLI): `npx skillmds@latest add ravihammond/tdd`
- Raw SKILL.md: https://api.skillmd.com/api/skills/ravihammond/tdd/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Integrations & APIs
- License: MIT
- Author: ravihammond (https://skillmd.com/u/ravihammond)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/ravihammond/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 `CONTEXT.md` (if it exists) so test names and interface vocabulary match the project's domain language, and respect 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 a real production contract: a public interface that existing callers use or that naturally belongs in the production design. Tests observe behavior through that boundary without reaching inside. A test-only entry point is not a seam.

**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 test first and confirm it fails for the intended reason; only then write 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.
- **Test-blind green.** GREEN is complete only when the behavior exists through production paths. Inspect every modified production-runtime file and remove test-only branches, parameters, exports, environment checks, modes, fixtures, mocks, and wiring before declaring GREEN.
- **Refactoring is not part of the loop.** It belongs to the review stage (see the `code-review` skill), not the red → green implementation cycle.