test-gap-analysis

Use pseudo-mutation analysis to ask whether any test would fail if a line of production code were changed. The Skill identifies mutation points, reasons about covering tests, then applies each candidate mutation and re-runs the tests to confirm genuine gaps.

Line and branch coverage measure which code executed during tests, not whether assertions verify behavior. Mutation testing measures whether tests detect code changes, making it the gold standard for test effectiveness.

Yes, the mutation catalog is language-neutral and maps to language-specific idioms like None vs nil, raise vs panic, and Rust's ? operator. A language extension file guides test file discovery and framework-specific assertion patterns.

Use an actual mutation framework like Stryker, mutmut, PIT, go-mutesting, or cargo-mutants when you want exhaustive automated mutation runs. This Skill suits fast, reasoning-driven analysis and targeted gap closure without tool setup.

Static reasoning alone can mislabel mutations as survived when tests actually catch them. The Skill prevents this by applying each candidate mutation, re-running the covering tests, and reverting, labeling findings unverified only when the suite cannot run.