What problem does it solve? Line and branch coverage only prove code executed during tests, not that tests would catch real bugs. This Skill applies mutation testing to reveal which behaviors are genuinely verified by introducing small code changes and checking whether tests fail. ## Core Features & Use Cases - Mutant Analysis: Explains the standard mutation operators (boundary, negation, arithmetic, constant, return value, statement removal) and what each surviving mutant proves about test gaps. - Score Interpretation: Guides reading mutation scores, handling equivalent mutants, and setting CI break thresholds that ratchet upward instead of chasing 100%. - Cost-Scoped Execution: Provides strategies for diff-scoped incremental runs in PRs and nightly full sweeps, since mutation testing cost scales with mutants times covering tests. - Use Case: A pricing function shows 100% coverage but mutants survive on the discount boundary and membership check; the Skill walks through triaging each survivor into missing assertion, missing input, equivalent, or dead code and writing the parametrized tests that kill them. ## Quick Start Ask the assistant to apply mutation testing to a specific module and triage the surviving mutants into actionable test improvements.
