Conciseness

The ~25-line body is lean and assumes Claude's competence: no explanation of what TDD is or why it matters, no library tutorials, no padding. Phrases like "Write one test for one behavior" and "Add nothing the test does not need" earn their tokens; the one-line opening summary is orientation, not over-explanation, so the level-5 anchor fits better than level 4.

Actionability

The guidance is directive and concrete — "Run the project's test command", "The test must fail because the behavior is missing, not because of a typo or a missing import", "If it passes, it tests nothing new: change the test" — with clear failure criteria and decision points. It sits at level 4 rather than 5 because there are no concrete examples or commands (e.g., a sample failing test or how to find the project's test command), a minor gap even for an instruction-only skill.

Workflow Clarity

The five-step cycle is clearly sequenced with explicit validation checkpoints at every phase ("Watch it fail", "Watch it pass. Run the full suite", "Run the suite again" after refactor) and genuine feedback loops for error recovery ("If it passes... change the test"; code written before its test "is deleted and written again from the test"). This matches the level-5 anchor including its feedback-loop criterion.

Progressive Disclosure

The body keeps only the cycle and rules inline, and defers test-writing detail to a single one-level-deep reference that is real (references/writing-good-tests.md) and clearly signaled with a conditional load cue ("When writing or changing a test, read..."). Structure and navigation match the level-5 anchor; nothing is inlined that belongs in a separate file.

Specificity

The description lists four concrete, sequential actions covering the full cycle — "write one failing test, run it and watch it fail, write the minimal code to pass, then refactor with the tests green" — with no vague filler. Coverage of the TDD workflow is comprehensive, matching the level-5 anchor rather than the level-4 anchor ('minor gaps in coverage'), since nothing in the cycle is omitted.

Completeness

Both questions are answered explicitly: the 'what' is "Implements a feature or bug fix test-first: write one failing test... then refactor with the tests green" and the 'when' is an explicit "Use when a change alters code behavior: a new feature, a bug fix, or a behavior change" clause with concrete trigger phrases. This matches the level-5 anchor exactly; level 4 would require the 'when' to be less explicit or specific.

Trigger Term Quality

Natural trigger phrases are present: "a new feature", "a bug fix", "a behavior change", "test-first" — terms users would genuinely say when this skill applies. It falls at level 4 rather than 5 because a few natural synonyms are missing (e.g., "TDD", "test-driven", "write the tests first"), and it is above level 3 since common variations of the core triggers are covered.

Distinctiveness Conflict Risk

The skill carves a clear niche — the test-first implementation cycle — with distinct process triggers, so it is mostly distinguishable from other skills. It is not level 5 because the 'when' clause ("a change alters code behavior") is broad enough to overlap with general implementation or testing skills, but the overlap is minor and the methodology is unambiguous, placing it above level 3.