Conciseness

The body is ~19 lean lines that assume Claude already knows what TDD is and adds only project-specific rules ("No production code without a failing test first", "Code written before its test is deleted and written again from the test"). Every token earns its place — no concept explanations, no padding — matching the score-5 anchor exactly.

Actionability

For an instruction-only process skill, the guidance is fully directive: the four imperative steps of the cycle, concrete rules covering the common cases (new feature, bug fix via "a test that reproduces the bug", hard-to-write test → "simplify the interface"), and an explicit done-checklist. Per the rubric's scoring note, absence of code is not penalized when the guidance is actionable, and nothing here is vague.

Workflow Clarity

The multi-step cycle is sequenced explicitly ("Write the test first. Watch it fail. Write the minimal code to pass. Refactor with the tests green.") with built-in validation checkpoints — watching the test fail is the checkpoint, and "Before you say it is done" requires that "Each new behavior has a test that you saw fail first" and "The full suite passes on the final code". This matches the score-5 anchor (explicit validation steps plus a checklist); the workflow is not destructive or batch, so no cap applies.

Progressive Disclosure

The body is well under 50 lines with test-quality detail correctly split into references/writing-good-tests.md (verified to exist, 8KB), referenced exactly once, one level deep, with a clear load condition ("When writing or changing a test, read…") that the reference file itself echoes ("Load this reference when: writing or changing tests…"). This matches the score-5 anchor for well-signaled one-level-deep references in a compact skill.

Specificity

The description enumerates the full TDD cycle as concrete actions — "write one failing test, run it and watch it fail, write the minimal code to pass, then refactor with the tests green" — comprehensive coverage with no vague filler. This matches the score-5 anchor (multiple specific concrete actions, comprehensive) rather than 4, which expects minor gaps in coverage.

Completeness

It explicitly answers both questions: "what" ("Implements a feature or bug fix test-first: write one failing test, run it and watch it fail, write the minimal code to pass, then refactor") and "when" ("Use when a change alters code behavior: a new feature, a bug fix, or a behavior change") with concrete trigger phrases. This is a textbook match for the score-5 anchor; score 4 would require the 'when' clause to be less explicit.

Trigger Term Quality

Natural phrases like "a new feature, a bug fix, or a behavior change" are terms users would actually say, but common trigger synonyms are absent: "TDD", "test-driven", "unit tests", "write tests first" never appear. Good coverage with a few natural terms missing matches the score-4 anchor; it is not score 5 (comprehensive synonyms/extensions) but clearly above the midpoint of score 3 ("some relevant keywords but missing common variations").

Distinctiveness Conflict Risk

"Test-first" methodology is a clear niche distinct from general coding or testing skills, and the trigger clause scopes it to behavior-altering changes. Minor overlap risk remains with a general testing or bug-fixing skill since "a bug fix" is a broadly shared trigger — matching "mostly distinct; minor overlap risk" (4) rather than "minimal conflict risk" (5).