lfe-mutation-verify

LFE Mutation Verify — Prompt-Based Test Quality Reasoning

Position in Pipeline

• Phase: 3 (Inspector sub-skill)
• Persona: Inspector (read-only; no src/ writes)
• Trigger: Invoked by /lfe-inspector Sub-Skill Dispatch when lfe-mutation-verify: true in .docs/quality/inspector-config.md
• Output: .plans/checks/mutation_findings.md — aggregated by Inspector into critique.md

Mission

Reason about test quality by asking: "If we made this change to the production code, would the existing tests catch it?" This is the logical equivalent of a mutation testing tool, done entirely through LLM reasoning over the diff. It surfaces tests that pass for the wrong reason (coincidental correctness) and logic paths that have no discriminating test.

This skill is expensive in reasoning tokens. Enable selectively — critical business logic, security-sensitive paths, and mission-critical calculations.

Hard Rules

1. Dispatch Context Required (refuse direct invocation): This skill is dispatched by /lfe-inspector Step 6 — it is not a Brain-typeable skill (per LLM_AGENT_GUIDE.md §8.8 Skill Invocation Authority). If invoked without .plans/builder_done.md AND .plans/tdd_report.md for the current slice, halt immediately and reply: "/lfe-mutation-verify is an Inspector sub-skill dispatched by /lfe-inspector. It cannot be run standalone. Run /lfe-inspector — the dispatcher will invoke this sub-skill if it is enabled in .docs/quality/inspector-config.md (or via an ## Inspector Overrides section in active_plan.md)." Direct invocation produces orphaned findings files and breaks the Inspector's aggregation logic.
2. Prompt-Only: No test runner, no mutation framework, no code execution.
3. Diff-Scoped: Reason only over implementation + test files in builder_done.md.
4. Candidate Mutations: For each production function changed, propose 3–5 plausible mutations; reason whether any test would catch them.
5. Severity: Escaped (no test catches the mutation) / Covered (mutation is caught by ≥1 test) / Partial (mutation is caught only by an integration test, not a unit test).