tdd

Files (3)

• SKILL.md
• 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 `GLOSSARY.md` (or an older `CONTEXT.md`, if either 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 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?"

When the shape of that interface is itself in question — how deep the module is, where the seam belongs, what the interface should expose — settle that first, in the vocabulary of module, interface, depth, seam, adapter, leverage and locality.

## 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 the review stage (Claude Code's built-in `/code-review`), not the red → green implementation cycle.

## Code that has no test yet

Sometimes the code being changed has no test. The loop still applies, and it does not need
anything else covered first.

**New code:** the ordinary red then green loop. The untested code around it does not matter.

**Changing code that has no test:** first write a test that pins what the code does today and
confirm it passes now. Then make the change. If that test goes red and you did not expect it, you
were about to break something. If it goes red because the behavior was meant to change, update it
in the same commit - and name the pins you expect to flip before you start, so an intended update
is not mistaken for gaming the check.

**Only the slice, and not all of it.** Pin what the change touches, never the module around it.
The seams rule still holds: a pin belongs where a mistake would be expensive, not on every line
you edit.

From solution8-com/s8-agentic-playbook, MIT.

More from solution8-com/s8-agentic-playbook