Mutation Tester

What Mutation Tester is

Mutation Tester is a subagent published in the jeremylongshore/tons-of-skills-marketplace repository on GitHub, which has about 2.8k stars. The repository describes itself as: “Model-agnostic agent-skills platform with a harness-free canonical layer, verified adapters, and the ccpi package manager. Explore at tonsofskills.com.”

A subagent is a specialist assistant that Claude can hand part of a task to. It is a markdown file whose frontmatter sets a name, a description that tells Claude when to delegate, and optionally the tools and model it may use; the body becomes the subagent's own system prompt.

Because a subagent works in its own context, it keeps the main conversation focused: Claude can send a narrow job, such as a review or a specialised analysis, to Mutation Tester and get back a compact result.

How to install Mutation Tester

New to extending Cowork? Our plugins guide and Customize guide explain how skills, plugins, and connectors fit together.

Inside the source file

An excerpt from plugins/testing/mutation-test-runner/agents/mutation-tester.md, shared under the repository's MIT license. Read the full file on GitHub.

// Original code
function isPositive(num) {
  return num > 0;
}

// Mutation 1: Change > to >=
function isPositive(num) {
  return num >= 0;  // Should fail test for isPositive(0)
}

// Mutation 2: Change > to <
function isPositive(num) {
  return num < 0;  // Should fail all tests
}

Mutation Testing Report
=======================
Total Mutants: 150
Killed: 142 (94.7%) 
Survived: 8 (5.3%) 
Timeout: 0
No Coverage: 0

Mutation Score: 94.7%

Survived Mutants:
  src/utils/validator.js:23
    - Replaced > with >= 
    - Suggests missing boundary test

  src/api/users.js:45
    - Replaced && with ||
    - Suggests missing logic test
…

Before you install

• Read the whole file first. Skills, commands, and subagents are instructions Claude will follow, so make sure they match what you want.
• Check which tools, scripts, or MCP servers it uses. Local servers and scripts run with your permissions.
• Try it in a test project or a copy of your files before pointing it at real work.
• Pin the version you tested, and review changes before updating.
• Watch for instructions that fetch web content or run shell commands; those are where prompt injection risks start. See our prompt injection guide.

FAQ