What problem does it solve? Code coverage shows which lines your tests execute but not whether they would catch real bugs. This Skill verifies test effectiveness by introducing small mutations into production code and checking whether your test suite kills them, exposing weak assertions, missing boundary tests, and untested edge cases before a PR ships. ## Core Features & Use Cases - Stryker Setup and Configuration: Detects existing Stryker config, recommends exact pinned versions, and generates a starting configuration matched to the project's test runner (Vitest, Jest, Mocha). - Branch-Diff Mutation Runs: Provides a diff-scoped workflow that mutates only files changed against the review base, with clean-working-tree enforcement and monorepo-aware scoping. - Survivor Triage and TDD Killing: Categorizes mutants as killed, survived, no-coverage, or equivalent, then drives a focused loop of writing behavior tests that kill worthwhile survivors. - Use Case: After finishing a feature branch, run the diff-scoped mutation check as a PR-readiness gate, discover that a boundary condition mutant survived, add a boundary test, and re-run the focused check before opening the PR. ## Quick Start Ask the assistant to set up Stryker and run mutation testing on the current branch diff, then strengthen any tests that let mutants survive.
