Mutation Testing Triage

**DO NOT** fall back to "manual verification" or "manual analysis"
as a substitute for running trailmark. Install it first. If installation
fails, report the error instead of switching to manual analysis.
- A **mutation testing framework** for the target language — if the framework
command fails (not found, not installed), install it using the instructions
in [references/mutation-frameworks.md](references/mutation-frameworks.md).
**DO NOT** fall back to "manual mutation analysis" or skip mutation testing.
Install the framework first. If installation fails, report the error
instead of switching to manual mutation analysis.
- **necessist** (optional, recommended) — if the target language is
supported (Go, Rust, Solidity/Foundry, TypeScript/Hardhat,
TypeScript/Vitest, Rust/Anchor), install with `cargo install necessist`.
See [references/mutation-frameworks.md](references/mutation-frameworks.md)
for details.
- An existing test suite that passes
- **macOS environment**: Run `ulimit -n 1024` before any `mull-runner`
invocation. macOS Tahoe (26+) sets unlimited file descriptors by
default, which crashes Mull's subprocess spawning. See
[references/mutation-frameworks.md](references/mutation-frameworks.md)
for details.

---

## Rationalizations to Reject

| Rationalization | Why It's Wrong | Required Action |
|-----------------|----------------|-----------------|
| "All survived mutants need tests" | Many are harmless or equivalent | Triage before writing tests |
| "Mutation testing is too noisy" | Noise means you're not triaging | Use graph data to filter |
| "Unit tests cover everything" | Complex data flows need fuzzing | Check entrypoint reachability |
| "Dead code mutants don't matter" | Dead code should be removed | Flag for cleanup |
| "Low complexity = low risk" | Boundary bugs hide in simple code | Check mutant location |
| "Tool isn't installed, I'll do it manually" | Manual analysis misses what tooling catches | Install the tool first |
| "Necessist isn't mutation testing, skip it" | Necessist finds what mutation testing misses: weak tests | Run both when the language supports it |

---

## Quick Start

```bash
# 1. Build the code graph
uv run trailmark analyze --language auto --summary {targetDir}

# 2. Run mutation testing (language-dependent)
# Python:
uv run mutmut run --paths-to-mutate {targetDir}/src
uv run mutmut results

# 2b. Run necessist (if language supported)
necessist

# 3. Analyze results with this skill's workflow (Phase 3)

Phase 1: Graph Build      → Parse codebase with trailmark
      ↓
Phase 2: Mutation Run     → Execute mutation testing framework
Phase 2b: Necessist Run   → Remove test statements (optional, parallel)
      ↓
Phase 3: Triage           → Classify findings using graph data
      ↓
Output: Categorized Report
  ├── Corroborated         (both tools flag same function — highest value)
  ├── False Positives      (harmless, skip)
  ├── Missing Tests        (write unit tests)
  └── Fuzzing Targets      (set up fuzz harnesses)

├─ Need to set up mutation testing for a language?
│  └─ Read: references/mutation-frameworks.md
│
├─ Need to set up necessist or find weak test statements?
│  └─ Read: references/mutation-frameworks.md (Necessist section)
│
├─ Need to understand the triage criteria in depth?
│  └─ Read: references/triage-methodology.md
│
├─ Need to understand how graph data informs triage?
│  └─ Read: references/graph-analysis.md
│
└─ Already have results + graph? Use Phase 3 below.

uv run trailmark analyze --language auto --summary {targetDir}

# Auto-detect framework
necessist

# Or target specific test files
necessist tests/test_parser.rs

# Export results
necessist --dump