At roughly twelve lines it is lean and assumes Claude's competence — no padding, no explaining what tests or edge cases are — and every line earns its place.

As an instruction-only skill it provides concrete, actionable guidance: named prioritize/skip criteria ("domain invariants, important edge cases, public interfaces, persisted data, security boundaries" vs "mechanical edits, implementation details, existing coverage") and an explicit decision rule. Per the rubric's instruction-skill note, absence of code is not penalized when guidance is this actionable.

The decision procedure is clearly sequenced — "identify the realistic failure it prevents and confirm that the maintenance cost is justified. If that case is weak, do not add one" — and per the simple-skill carve-out a clear, unambiguous single-purpose decision skill scores 3; no validation checkpoints are needed since this is not a destructive or batch operation.

Under 50 lines with no bundle files and no need for external references, it is well organized via a heading, intro, prioritization/skip bullets, and a closing rule, satisfying the simple-skill carve-out for progressive disclosure.

Lists multiple concrete actions within its domain — "Decide whether a change warrants new tests" and "choose proportionate verification" — rather than vague language. It uses imperative voice matching the good examples, with no first/second person, so no voice penalty applies.

It clearly answers both what ("Decide whether a change warrants new tests and choose proportionate verification") and when, with an explicit "Use before adding tests, when planning validation, or when reviewing..." trigger clause.

It includes some natural terms ("adding tests", "new tests") but leans formal ("planning validation", "reviewing whether a proposed test protects meaningful behavior") and misses common colloquial variations users would say like "write tests", "unit tests", "should I test this".

It carves a clear niche — judging test-worthiness and selecting proportionate verification — with specific triggers unlikely to fire for unrelated skills, distinguishing it from generic test-writing or code-review skills.