From file to useful result
How to ask with this skill installed
Installing the skill gives your AI a method. Your request still has to provide the case-specific facts, constraints and expected output.
Describe the decision or deliverable, not just the topic.
Add source material, audience, limits and known facts.
Set format, quality criteria and checks.
When to use it
- After mutation testing reveals survived mutants that need triage
- Identifying where unit tests would have the highest impact
- Finding functions that need fuzz harnesses instead of unit tests
- Prioritizing test improvements using data flow context
- Filtering out harmless mutants from actionable ones
- Finding unnecessary test statements that indicate weak assertions (necessist)
When not to use it
- Codebase has no existing test suite (write tests first)
- Pure documentation or configuration changes
- Single-file scripts with trivial logic
What you need first
- trailmark installed — if
uv run trailmarkfails, run:
uv pip install trailmark
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
- necessist (optional, recommended) — if the target language is
- An existing test suite that passes
- macOS environment: Run
ulimit -n 1024before anymull-runner
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.
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.
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.
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