tdd

Files (4)

• SKILL.md
• openai.yaml · agents/
• mocking.md
• tests.md

SKILL.md (first lines)

# 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 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 agreed seams.** Before writing any test, write down the seams under test. 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.

- **Interactive:** confirm them with the user before the first test. Ask: "What's the public interface, and which seams should we test?"
- **Unattended** (a loop or dispatched agent with no user to ask): take the seams from the story's acceptance criteria, and state them in the pull request body so a reviewer can disagree.

When the shape of that interface is itself in question (how deep the module is, where the seam belongs, what the interface should expose), call the Skill tool with "codebase-design" for the vocabulary. It is the shared source of the module, interface, depth, seam, adapter, leverage and locality terms, and it is a reference to consult, not a session to run.

## Which layer

Prove each behavior at the cheapest layer that can actually observe it: unit, then integration, then end-to-end. Cost is a reason to move a test down only when the lower test proves the same thing.

- **Moving a test down needs a named replacement.** Before deleting or demoting an end-to-end test, cite the lower-layer test that fails when the behavior breaks. A plausible-sounding lead is not that test: verify it by breaking the behavior and watching that named test go red. Leads are often wrong.
- **Keep the end-to-end test when it is the only proof.** These behaviors only exist in a real browser or a running system:
  - touch, pointer and drag hit-testing;
  - focus order;
  - hydration timing;
  - layout (computed sizes, overflow, overlap);
  - canvas and map rendering;
  - flows that cross services.

  Keep them even when it grows the suite: a gate that is smaller because it lost its only proof of a behavior is a coverage regression, not a saving.
- **Write a new end-to-end test only for a journey (a multi-page user flow crossing services) or one of those behaviors. Field validation and single-component states are not journeys.** Anything a lower layer can prove goes there.

## 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.
- **Run the narrowest test that can fail.** One file or one filter for the slice, not the suite. Where and how much more to run is the repo's call (its `AGENTS.md`, its CI); follow it.
- **Refactor after green, never while red.** Keep the refactor in its own step, with the tests green before and after.

From delarooster/skills, MIT.

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