Conciseness

The body is dense and Stryker-specific — config tables, commands, and anti-patterns all earn their place — but includes minor over-explanation: the Overview paragraph explains what mutation testing is ("Mutation testing plants small bugs (mutants) in production code and re-runs the tests") and the 'When to use' section restates the frontmatter description. Matches the score-4 anchor ('efficient; minor instances of over-explanation that could be trimmed'); not score 5 because a few sentences could be cut without loss.

Actionability

Fully executable throughout: "npm install --save-dev @stryker-mutator/core @stryker-mutator/jest-runner", a complete stryker.conf.json example, "npx stryker run --dryRunOnly", "npx stryker run --mutate src/cart.ts", and a copy-paste-ready GitHub Actions workflow. Matches the score-5 anchor ('copy-paste ready code or commands; specific examples cover the common cases').

Workflow Clarity

Six clearly sequenced steps with explicit validation checkpoints: Step 3 mandates a cheap dry run first ("a failing dry run means the runner or config is wrong"), a single-file trial run, then the full run; Step 4 closes the loop ("Add that test, re-run the file with --mutate src/cart.ts, and confirm the mutant is now Killed"), and the 'Judgement rules' section gives error-recovery guidance. Matches the score-5 anchor ('explicit validation steps; feedback loops for error recovery').

Progressive Disclosure

Well-organized single-file skill with clear section headers and clearly signaled one-level-deep external links ([introduction][si], [configuration][cfg], [incremental][inc]), but no bundle files exist while the body runs ~175 lines — the ~30-line CI workflow YAML and the config-field reference table are candidates for a references/ file. Matches the score-4 anchor ('good structure; most content is appropriately placed; minor organization gaps'); not score 5 since content that could be split out is inlined.

Specificity

Quotes: "Sets up StrykerJS: the Jest, Vitest, Mocha or Karma runner, stryker.conf.json, thresholds and incremental runs on pull requests" — multiple specific concrete actions (runner setup, config file, thresholds, incremental PR runs) with comprehensive coverage of the skill's scope. This matches the score-5 anchor ('lists multiple specific concrete actions; comprehensive coverage') and exceeds the score-4 anchor, which expects minor gaps in coverage.

Completeness

Explicitly answers 'when' with a concrete trigger clause ("Use when a JavaScript or TypeScript suite has high coverage but bugs still slip through, and the team wants mutation testing to find weak tests") and 'what' with concrete capabilities ("Sets up StrykerJS: ... thresholds and incremental runs on pull requests"). Matches the score-5 anchor exactly; score 4 would require the 'when' to be less explicit or specific.

Trigger Term Quality

Quotes: "JavaScript or TypeScript suite", "high coverage but bugs still slip through", "mutation testing", "weak tests", "pull requests" — good, natural keyword coverage a user would actually say. Not score 5 because a few natural synonyms are missing (e.g., 'mutation score', 'test quality', 'tests don't catch bugs'); clearly above score 3, which expects missing common variations.

Distinctiveness Conflict Risk

Clear niche: StrykerJS mutation testing for JavaScript/TypeScript, distinct from generic testing or coverage skills and from sibling mutation-testing skills for other ecosystems (.NET, Java, Python). Trigger conditions ('high coverage but bugs still slip through') are unlikely to fire for the wrong skill, matching the score-5 anchor's 'clear niche with distinct triggers; minimal conflict risk'.