mutation-testing
EXPERIMENTAL — mutation testing with muter to measure whether tests actually assert anything: mutants that survive reveal assertion-free coverage. Advisory report only, never a merge gate. Use on engine/logic targets when coverage numbers look good but bugs still slip through.
npx skills add rshankras/claude-code-apple-skills --skill mutation-testing --agent claude-code
Same command for any agent — swap --agent for codex, cursor, copilot.
Weekly change comes from our own snapshots, not the repository page — it measures attention, not adoption.
# Mutation Testing (Experimental) **Status: EXPERIMENTAL — advisory report only. Never wire this as a merge gate.** Coverage proves a line *ran* under test; mutation testing proves a test would *notice if the line were wrong*. Muter mutates the source (flips `>` to `<`, `&&` to `||`, deletes statements), reruns the suite per mutant, and reports the mutants that **survived** — code that is covered but effectively unasserted. It is the truest answer to "did the agent write real tests or coverage theater," and also the slowest and most fragile tool in the gauntlet, which is why it stays advisory. ## When This Skill Activates Use this skill when: - Coverage is healthy but bugs still slip through covered code - Auditing agent-written test suites for assertion-free tests - A periodic (nightly/weekly) quality audit of engine/logic targets Do NOT use as a per-commit or merge-blocking check — a full mutation run multiplies the suite's runtime by the mutant count. ## Setup 1. Install muter and confirm it runs against the current Xcode before investing further: ```bash brew install muter-mutation-testing/formulae/muter muter --version ``` 2. Copy `templates/muter.conf.yml` to the repo root as
- When This Skill Activates
- Setup
- Scoping: Engines Only
- Reading the Report
- Caveats (Why This Stays Advisory)
- Deterministic Fallback: Manual Mutation Spot-Checks
- Common Pitfalls
- References
brew install muter-mutation-testing/formulae/muter muter --version muter run # writes an HTML/console report of surviving mutants
What does the mutation-testing skill do?
EXPERIMENTAL — mutation testing with muter to measure whether tests actually assert anything: mutants that survive reveal assertion-free coverage. Advisory report only, never a merge gate. Use on engine/logic targets when coverage numbers look good but bugs still slip through.
How do I install it?
Run `npx skills add rshankras/claude-code-apple-skills --skill mutation-testing --agent claude-code` — it drops the skill into your project so the agent can pick it up. Swap the --agent value for codex, cursor or copilot if you use one of those.
Where does this skill come from?
From rshankras/claude-code-apple-skills, a repository with 589 stars. We read it straight from the repository tree rather than a submitted listing, so what you see here is what is actually published.
Is a popular skill a good skill?
Not necessarily. Stars measure attention, not adoption — a repository can trend for a week and be abandoned. That is why we show the weekly change from our own snapshots next to the total, instead of a single flattering number.