Ralph Tdd

---
name: ralph-tdd
description: "Test-driven implementation for a Ralph iteration. Use when implementing backend or library code: one failing test, the minimum code to make it pass, repeat per vertical slice, tests at public seams."
---

# Test-Driven Development

> Adapted from [mattpocock/skills](https://github.com/mattpocock/skills) `skills/engineering/tdd` (MIT, upstream commit `3216582`) for an unattended Ralph iteration: there is no user to confirm seams with, and refactoring belongs to Ralph's reviewer stage. `tests.md` and `mocking.md` are upstream's, unchanged.

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 it 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 seams you named first.** This is an unattended run: there is no user to ask. Before writing any test, write down the seams under test (the public interface, and which of its boundaries the task exercises) and keep to them. List those seams in the commit body so the reviewer stage and the next iteration can see the choice. You can't test everything, so naming the seams up front is how testing effort lands on the critical paths and complex logic instead of every edge case.

## 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 Ralph's reviewer stage, not the red → green implementation cycle.

## Documentation is part of done

When a change affects architecture, interfaces, or invariants, update the relevant docs before finishing. Delegate the docs pass to a sub-agent.

The primary implementer must review the sub-agent's documentation diff against the implemented behavior before declaring completion. The delegate edits docs only: the implementer owns the red → green cycle, and Ralph's reviewer stage retains refactoring and defect-review ownership.