mutation-testing SKILL.md skillA
mutation-testing from secondsky/claude-skills is an agent skill: a SKILL.md folder an AI agent loads when a task calls for it.
mdr add secondsky/claude-skills/mutation-testing@git:20260803.875f1a8Pins git:20260803.875f1a8 in mdr.lock. Pin the fingerprint instead.
- Audit grade
- A, 17 of 17 checks passed
- Kind
- SKILL.md skill
- Repository
- secondsky/claude-skills
- Path
plugins/mutation-testing/skills/mutation-testing/SKILL.md- Versions recorded
- 4
- Latest version
- git:20260803.875f1a8, committed Aug 3, 2026
- First version recorded
- Dec 24, 2025
- Size
- 5,672 bytes
- Repository stars
- 243
- License
- MIT
Validate test effectiveness with mutation testing using Stryker (TypeScript/JavaScript with Vitest or bun test via @hughescr/stryker-bun-runner) and mutmut (Python). Find weak tests that pass despite code mutations. Use to improve test quality.
Collected from public GitHub. Every version is saved exactly as it was and never changes. Install it at an exact version with the mdr tool, and every file is checked before it reaches your agent. The main file is checked against the SHA-256 fingerprint recorded here, and the others against the fingerprints GitHub keeps for its source commit. The automatic checks below grade the latest version, and the same checks always give the same file the same grade.
What the file says
# Mutation Testing
Expert knowledge for mutation testing - validating that your tests actually catch bugs by introducing deliberate code mutations.
## Core Concept
- **Mutants**: Small code changes introduced automatically
- **Killed**: Test fails with mutation (good - test caught the bug)
- **Survived**: Test passes with mutation (bad - weak test)
- **Score**: Percentage of mutants killed (aim for 80%+)
## TypeScript/JavaScript (Stryker)
### Vitest Runner
#### Installation
```bash
# Using Bun
bun add -d @stryker-mutator/core @stryker-mutator/vitest-runner
# Using npm
npm install -D @stryker-mutator/core @stryker-mutator/vitest-runner
```
#### Configuration
```typescript
// stryker.config.mjs
export default {
packageManager: 'bun',
reporters: ['html', 'clear-text', 'progress'],
testRunner: 'vitest',
coverageAnalysis: 'perTest',
mutate: ['src/**/*.ts', '!src/**/*.test.ts'],
thresholds: { high: 80, low: 60, break: 60 },
incremental: true,
}
```
### Bun Native Runner (`bun test`)
Use when projects use `bun test` directly (not Vitest).
#### Requirements
- Bun **>= 1.3.7** (needs TestReporter WebSocket events from Bun PR #25986)
…Read the whole file at its exact version.
How to install
mdr add secondsky/claude-skills/mutation-testing@sha256:ef325228876bba65The line at the top of this page pins the label git:20260803.875f1a8, which is what most people want. This one pins the sha256 fingerprint of the contents instead, so it survives a label being moved. Either way the exact version is written to mdr.lock, and mdr install downloads and checks those contents again, so it installs them exactly or stops with an error.
[](https://markdownregistry.com/a/art_5xxlauvwzmx7mlja)
3 badge image fetches in 30 days. What the badge shows.
Versions
The registry has recorded 4 versions of this file, between Dec 24, 2025 and Aug 3, 2026, about one every 74 days.
| version | committed | commit | size | audit | |
|---|---|---|---|---|---|
| git:20260803.875f1a8 latest | 2026-08-03 | 875f1a8 | 5,672 B | A | view · diff |
| git:20260303.7e67f1a | 2026-03-03 | 7e67f1a | 5,672 B | A | view · diff |
| git:20260303.3b60bbf | 2026-03-03 | 3b60bbf | 5,659 B | A | view · diff |
| git:20251224.ba827ce | 2025-12-24 | ba827ce | 4,270 B | A | view |
Audit of the latest version
- pass: Frontmatter block present
- pass: Frontmatter declares a name
- pass: Frontmatter declares a description
- pass: Size between 200 bytes and 200 KB (5672 bytes)
- pass: No zero-width or bidi control characters
- pass: No instruction hidden inside an HTML comment
- pass: No link to an exfiltration or paste host
- pass: No credential-shaped string
- pass: No instruction to send local credentials anywhere
- pass: No text hidden with inline styles
- pass: No prompt-injection phrasing
- pass: No curl or wget piped into a shell
- pass: No recursive delete of root, home or parent
- pass: No instruction to read or print local credentials
- pass: No base64 blob over 200 characters
- pass: No link to a raw IP address
- pass: No script tag
Source
secondsky/claude-skills · 243 stars · license MIT · pushed 2026-10-31 · branch main
API
GET https://markdownregistry.com/api/v1/artifacts/art_5xxlauvwzmx7mlja GET https://markdownregistry.com/api/v1/resolve?ref=secondsky/claude-skills/mutation-testing GET https://markdownregistry.com/api/v1/blob/ef325228876bba659511a7435a9286b8632eaef4241e8133e624e386a5ab4609
Your AI assistant does the legwork. You hear about the deals worth your word. Hand yours the standing instructions at modelranch.com and it joins the network that reads files like this one.
Guides: the SKILL.md format and its required fields · what to check before you install a skill · how to pin a skill to an exact version · where to find agent skills · keeping a team on the same skills · which agent file does what.
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