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tdd: add tautological-test anti-pattern

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Tests whose assertion is recomputed the way the code computes it pass by
construction and give zero confidence. Add it as a peer of the existing
implementation-coupling anti-pattern: a Philosophy principle, a per-cycle
checklist gate, and a BAD/GOOD example pair in tests.md. Includes a patch
changeset.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
This commit is contained in:
Matt Pocock 2026-06-29 21:22:50 +01:00
parent a116824938
commit 43ea0884b0
3 changed files with 24 additions and 0 deletions
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3
skills/engineering/tdd/SKILL.md
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@ -13,6 +13,8 @@ description: Test-driven development. Use when the user wants to build features
**Bad tests** are coupled to implementation. They mock internal collaborators, test private methods, or verify through external means (like querying a database directly instead of using the interface). The warning sign: your test breaks when you refactor, but behavior hasn't changed. If you rename an internal function and tests fail, those tests were testing implementation, not behavior.
**Tautological tests** restate the implementation inside the assertion, so they pass by construction and give zero confidence. When the expected value is computed the way the code computes it — `expect(add(a, b)).toBe(a + b)`, snapshotting a figure you derived by hand the same way the code does, asserting a constant equals itself — the test can never disagree with the code: break the code wrong and the assertion breaks wrong with it. The expected value must come from an independent source of truth — a known-good literal, a worked example, the spec.
See [tests.md](tests.md) for examples and [mocking.md](mocking.md) for mocking guidelines.
## Anti-Pattern: Horizontal Slices
@ -103,6 +105,7 @@ After all tests pass, look for [refactor candidates](refactoring.md):
[ ] Test describes behavior, not implementation
[ ] Test uses public interface only
[ ] Test would survive internal refactor
[ ] Expected values are independent literals, not recomputed from the code
[ ] Code is minimal for this test
[ ] No speculative features added
```