What is mutation-testing?

A testing method that changes code in small ways, called mutants, to check whether tests notice the changes. This skill sets up and runs Stryker mutation testing and uses surviving mutants to improve tests.

Use it as a final review check after implementation or refactoring, either across the project or on the current review changes.

A test suite can pass while missing important behavior. Mutation testing shows which code changes the tests fail to catch.

mutation-testing — Skill for Claude Code by citypaul/.dotfiles

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 citypaul/.dotfiles --skill mutation-testing

Clone the repo

git clone --depth 1 https://github.com/citypaul/.dotfiles

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/citypaul/.dotfiles/mutation-testing/github.svg)](https://agentmods.dev/skills/citypaul/.dotfiles/mutation-testing)

Your own site

<a href="https://agentmods.dev/skills/citypaul/.dotfiles/mutation-testing"><img src="https://agentmods.dev/badge/skills/citypaul/.dotfiles/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/citypaul/.dotfiles/mutation-testing"><img src="https://agentmods.dev/badge/skills/citypaul/.dotfiles/mutation-testing.svg" alt="Reviewed on agentmods" width="80" height="20"></a>

Per session 97 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 3,524 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.

Origin unknown 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	$0.00097	$0.03524
Opus 5.5	$0.00039	$0.01410
Sonnet 5.5	$0.00019	$0.00705
Haiku 4.5	$0.00010	$0.00352

Measured 2mo ago against content hash 95e393a706f8, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, 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 2mo 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.

claude/.claude/skills/mutation-testing/SKILL.md · 246 lines

The source is not reproduced here

A licence we could not identify

The repository carries a LICENSE file, but it is custom or dual enough that GitHub cannot name it and neither can this catalogue. Unknown terms are not permission, so the body is not copied here. Read the licence at the source and decide for yourself.

Read it on GitHub

Files

What ships with it

1 file 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.

resources/mutator-rules.md 7.5 KB

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.

2mo ago Changed · +8 lines 95e393a706f8
2mo ago First seen · 238 lines · 97 tokens per session scan A 4ff95fa328da

Subscribe to this mod's changes

mutation-testing is a skill published in the GitHub repository citypaul/.dotfiles (757 stars, last pushed 4d ago), with no licence file. It adds 97 tokens to every session and 3,524 once invoked, about $0.0004 per session on Opus 5.5. 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

mutation-testing next to 6 skills from other repositories
Mod	Type	Stars	Last push	Licence	Scan	Tokens	Works with
mutation-testingcitypaul/.dotfiles	Skill	757	4d ago	none	A	97	Claude Code
imagegenopenai/codex	Skill	134k	today	Apache-2.0	A	113	Codex
local-ai-agentsmicrosoft/ai-agents-for-beginners	Skill	79k	yesterday	MIT	A	200	Claude Code, Codex
playwright-climicrosoft/playwright	Skill	100k	yesterday	Apache-2.0	A	19	Claude Code
browser-automationopenclaw/openclaw	Skill	394k	today	MIT	A	36	Claude Code
systematic-debuggingobra/superpowers	Skill	314k	4d ago	MIT	A	21	Claude Code
next-cache-components-adoptionvercel/next.js	Skill	143k	today	MIT	A	95	Claude Code