What is mutation-testing?

A testing method that deliberately inserts one small bug at a time into production code and checks whether the test suite detects it. Each undetected bug points to a missing or weak test.

Use it to assess test-suite gaps, focus on meaningful logic, and optionally add tests for bugs the suite misses.

It shows whether tests actually protect important behavior, rather than only measuring how much code they execute.

mutation-testing — Skill for Claude Code by hugobowne

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

Any agent

npx skills add hugobowne/show-us-your-agent-skills --skill mutation-testing

Clone the repo

git clone --depth 1 https://github.com/hugobowne/show-us-your-agent-skills

Made for: Claude Code.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

README.md

[![agentmods](https://agentmods.dev/badge/skills/hugobowne/show-us-your-agent-skills/mutation-testing/github.svg)](https://agentmods.dev/skills/hugobowne/show-us-your-agent-skills/mutation-testing)

Your own site

<a href="https://agentmods.dev/skills/hugobowne/show-us-your-agent-skills/mutation-testing"><img src="https://agentmods.dev/badge/skills/hugobowne/show-us-your-agent-skills/mutation-testing/github.svg" alt="Measured on agentmods" height="20"></a>

Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.

Your own site · 80×15

<a href="https://agentmods.dev/skills/hugobowne/show-us-your-agent-skills/mutation-testing"><img src="https://agentmods.dev/badge/skills/hugobowne/show-us-your-agent-skills/mutation-testing.svg" alt="Reviewed on agentmods" width="80" height="20"></a>

Per session 42 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 2,465 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe. Third-party audits

NVIDIA SkillSpector pass 7 Sept 2026

How audits are shown

Origin original No closer match found in the catalogue.

Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

Model	Per session	Once invoked
Fable 5.1	$0.00042	$0.02465
Opus 5	$0.00021	$0.01233
Sonnet 5	$0.00008	$0.00493
Haiku 4.5	$0.00004	$0.00247

Measured 11d ago against content hash 30444930254c, method: parsed. Prices are Anthropic first-party input rates as of 2026-09-10, from the pricing page.

Security

Grade A, and why

mutation-testing scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 11d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

skills/mutation-testing/SKILL.md · 267 lines

How it starts

The opening of the file, as written. The whole thing — 267 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Project: honnibal/claude-skills. "Claude skills I'm experimenting with. Please review carefully before use."

License: MIT License. Full license text is in LICENSE alongside this file.

Snapshot: Frozen copy of mutation-testing.md.txt as of 2026-05-22. The maintained version lives upstream and may have evolved since this snapshot.

Mutation Testing

You are now in mutation testing mode. Your job is to assess the strength of the project's test suite by introducing deliberate bugs (mutations) into the source code, one at a time, and checking whether any test fails. A mutation that the tests don't catch reveals a gap in coverage.

Scope

$ARGUMENTS

If the user names specific files or directories, restrict mutations to those.
If no argument is given, look at the project's source layout and use AskUserQuestion to agree on a starting scope. Don't try to mutate everything at once — pick a module or package and work through it.
Test files themselves are out of scope for mutation. Only mutate production code.
Prioritise code that has meaningful logic (branching, arithmetic, state changes) over boilerplate, config, or trivial accessors.

Pre-flight

Before mutating anything:

Clean working tree. Run git status on the files in scope. If there are uncommitted changes to any file you plan to mutate, stop and ask the user to commit or stash first. You need a clean baseline so every mutation can be reverted cleanly with git checkout -- <file>.
Find the test runner. Look for pytest.ini, pyproject.toml [tool.pytest], setup.cfg [tool:pytest], tox.ini, a Makefile test target, or a tests/ directory. Ask the user if you can't determine how to run tests. Confirm that the test suite passes on the unmodified code before starting — if it doesn't, stop and tell the user.
Map the code. Read the files in scope. Build a mental model of each module's structure and data flow so you can choose mutations that are meaningful rather than trivially dead.

Read the full file on GitHub · 267 lines

Files

What ships with it

3 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

11d ago First seen · 267 lines · 42 tokens per session scan A 30444930254c

Subscribe to this mod's changes

mutation-testing is a skill published in the GitHub repository hugobowne/show-us-your-agent-skills (67 stars, last pushed 29d ago), licensed MIT. It adds 42 tokens to every session and 2,465 once invoked, about $0.0002 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.

Testing

research-engineer

An uncompromising Academic Research Engineer. Operates with absolute scientific rigor, objective criticism, and zero flair. Focuses on theoretical correctness, formal verification, and optimal implementation across any required technology.

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Compare extracts from two Tika builds over a corpus to detect regressions in content, encoding, exceptions, and embedded-document handling. Use for "compare before/after extracts", "eval this change against the corpus".

apache/tika · 50 tokens

neuron-evaluation-engineer

Create and run AI evaluations with datasets, assertions, and output drivers in Neuron AI. Use this skill whenever the user mentions evaluation, testing AI systems, creating evaluators, dataset-driven testing, assertion-based validation, or wants to measure AI system performance. Also trigger for tasks involving…

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mutation-testing