Agent-Skills.md

Agent Skills: Mutation Testing

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UncategorizedID: jagreehal/jagreehal-claude-skills/mutation-testing

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jagreehal/jagreehal-claude-skills
jagreehalLicense: MIT
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pnpm dlx add-skill https://github.com/jagreehal/jagreehal-claude-skills/tree/HEAD/skills/mutation-testing

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skills/mutation-testing/SKILL.md

Skill Metadata

Name
mutation-testing
Description
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Mutation Testing

Critical rules

  • Coverage = execution. Mutation = detection. Prefer Stryker over manual mutation for JS/TS.
  • Run at the PR-readiness / end-of-phase gate — never inside each RED-GREEN increment.
  • Diff-scoped runs for branches; full runs for setup, CI, or baseline work.
  • Mutate production source only — never tests, fixtures, snapshots, generated, .d.ts, build output.
  • Every survivor: kill with a behavior test, classify equivalent, or ask the user.
  • New tests must fail against the mutant for the right reason before restoring production code.
  • Assert exact observable behavior — not toBeDefined / weak inequalities.
  • Before operators, survivors, or WRONG/CORRECT examples, read the matching resource below.

Workflow

  1. Inspect — Find package manager, runner, Stryker config; git diff <base>...HEAD --name-only. Monorepo: smallest affected package first.
  2. Setup if missing — npm init stryker@latest; Vitest plugin; coverageAnalysis: perTest; mutate src/**/*.ts excluding tests. Add scripts: mutation, mutation:incremental, mutation:diff. Before editing config details, read references/setup.md.
  3. Run — Prefer mutation:diff for branch feedback. Full mutation for first setup / shared infra / CI gates.
  4. Triage — Killed: ok. Survived: strengthen the assertion or change the input (equal original/mutant results for the current input means no assertion can kill it), unless equivalent. No Coverage: add behavior test. High-value code (money, permissions, safety): fix immediately. Subtle/unspecified: ask user with concrete choices.
  5. Kill survivors (TDD) — Keep mutant → write smallest failing behavior test → restore code → verify pass → re-run scoped mutate then diff. Assert returns, persisted state, events — not implementation details.
  6. CI — Start report-only; add failing thresholds after a baseline. Persist HTML/clear-text artifacts. Incremental locally; force full periodically.

During RED (cheap): use mutator heuristics mentally — see references/mutator-rules.md. Manual fallback only when Stryker can't target the code.

Resources

  • references/mutator-rules.md — operators, heuristics, equivalent mutants, manual procedure. Read when planning tests or triaging survivors.
  • references/setup.md — Stryker config, scripts, diff scoping. Read when adding or tuning the harness.
  • references/examples.md — weak coverage vs boundary-killing tests. Read when strengthening survivors.

Validation

  • [ ] Harness is Stryker (or documented manual fallback)
  • [ ] Run timed at PR gate, not per TDD increment
  • [ ] Diff-scoped for the branch; mutate production only
  • [ ] Survivors killed, marked equivalent, or escalated to user
  • [ ] Killer tests failed against the mutant first; assert exact behavior

Constraints

  • Not for authoring first tests (writing-tests) or load/chaos (performance-testing).
  • Adjacent: tdd-workflow (cycles before this gate), testing-strategy (pyramid placement), verification-before-completion (include mutation report in evidence).