Polyglot Test Agent
boshi-xixixi/TraeSkill
Generates comprehensive, workable unit tests for any programming language using a multi-agent pipeline.
Validate test effectiveness with mutation testing using Stryker (TypeScript/JavaScript with Vitest or bun test via @hughescr/stryker-bun-runner) and mutmut (Python).
Install Mutation Testing
$ npx skills add secondsky/claude-skills --skill mutation-testing -a claude-codeProject install by default; add -g for ~/.claude/skills/.
$ gh skill install secondsky/claude-skills mutation-testing --agent claude-codeProject scope by default; add --scope user for a personal install. Needs GitHub CLI 2.90.0 or later (public preview).
$ git clone --depth 1 https://github.com/secondsky/claude-skills.git skills-src && mkdir -p .claude/skills && cp -r skills-src/plugins/mutation-testing/skills/mutation-testing .claude/skills/mutation-testing && rm -rf skills-srcUse ~/.claude/skills/ instead of .claude/skills for a personal install. The folder must contain SKILL.md.
Claude Code skills documentation · loads skills from .claude/skills/
Install the "mutation-testing" agent skill from https://github.com/secondsky/claude-skills/tree/main/plugins/mutation-testing/skills/mutation-testing into .claude/skills/mutation-testing/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "mutation-testing", then confirm the skill loads.Claude Code copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$skill-installer install https://github.com/secondsky/claude-skills/tree/main/plugins/mutation-testing/skills/mutation-testingType this inside Codex. $skill-installer <name> installs a curated skill from openai/skills. The installer writes to $CODEX_HOME/skills (default ~/.codex/skills). Restart Codex if the skill does not show up.
$ npx skills add secondsky/claude-skills --skill mutation-testing -a codexProject install goes to .agents/skills/; add -g for ~/.codex/skills/.
$ gh skill install secondsky/claude-skills mutation-testing --agent codexProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/secondsky/claude-skills.git skills-src && mkdir -p .agents/skills && cp -r skills-src/plugins/mutation-testing/skills/mutation-testing .agents/skills/mutation-testing && rm -rf skills-srcUse ~/.agents/skills/ instead of .agents/skills for a personal install.
Codex skills documentation · loads skills from .agents/skills/
Install the "mutation-testing" agent skill from https://github.com/secondsky/claude-skills/tree/main/plugins/mutation-testing/skills/mutation-testing into .agents/skills/mutation-testing/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "mutation-testing", then confirm the skill loads.Codex copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$ npx skills add secondsky/claude-skills --skill mutation-testing -a cursorProject install goes to .agents/skills/; add -g for ~/.cursor/skills/.
$ gh skill install secondsky/claude-skills mutation-testing --agent cursorProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/secondsky/claude-skills.git skills-src && mkdir -p .cursor/skills && cp -r skills-src/plugins/mutation-testing/skills/mutation-testing .cursor/skills/mutation-testing && rm -rf skills-srcUse ~/.cursor/skills/ instead of .cursor/skills for a personal install.
Cursor skills documentation · loads skills from .cursor/skills/, .agents/skills/, .claude/skills/, .codex/skills/
Install the "mutation-testing" agent skill from https://github.com/secondsky/claude-skills/tree/main/plugins/mutation-testing/skills/mutation-testing into .cursor/skills/mutation-testing/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "mutation-testing", then confirm the skill loads.Cursor copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$ gemini skills install https://github.com/secondsky/claude-skills.git --path plugins/mutation-testing/skills/mutation-testing--scope user (default) or --scope workspace; --path is the subfolder of the repo that holds the skill; --consent skips the security confirmation prompt.
$ npx skills add secondsky/claude-skills --skill mutation-testing -a gemini-cliProject install goes to .agents/skills/; add -g for ~/.gemini/skills/.
$ gh skill install secondsky/claude-skills mutation-testing --agent gemini-cliProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/secondsky/claude-skills.git skills-src && mkdir -p .gemini/skills && cp -r skills-src/plugins/mutation-testing/skills/mutation-testing .gemini/skills/mutation-testing && rm -rf skills-srcUse ~/.gemini/skills/ instead of .gemini/skills for a personal install, then run /skills reload.
Gemini CLI skills documentation · loads skills from .gemini/skills/, .agents/skills/
Install the "mutation-testing" agent skill from https://github.com/secondsky/claude-skills/tree/main/plugins/mutation-testing/skills/mutation-testing into .gemini/skills/mutation-testing/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "mutation-testing", then confirm the skill loads.Gemini CLI copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$ gh skill install secondsky/claude-skills mutation-testingInstalls for Copilot at project scope by default; add --scope user for a personal install. Preview a skill first with gh skill preview. Needs GitHub CLI 2.90.0 or later (public preview).
$ npx skills add secondsky/claude-skills --skill mutation-testing -a github-copilotProject install goes to .agents/skills/; add -g for ~/.copilot/skills/.
$ git clone --depth 1 https://github.com/secondsky/claude-skills.git skills-src && mkdir -p .github/skills && cp -r skills-src/plugins/mutation-testing/skills/mutation-testing .github/skills/mutation-testing && rm -rf skills-srcUse ~/.copilot/skills/ instead of .github/skills for a personal install. Commit .github/skills so cloud agent and code review can use it.
GitHub Copilot skills documentation · loads skills from .github/skills/, .claude/skills/, .agents/skills/
Install the "mutation-testing" agent skill from https://github.com/secondsky/claude-skills/tree/main/plugins/mutation-testing/skills/mutation-testing into .github/skills/mutation-testing/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "mutation-testing", then confirm the skill loads.GitHub Copilot copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$ npx skills add secondsky/claude-skills --skill mutation-testing -a opencodeOpenCode documents no install command of its own. Project install goes to .agents/skills/; add -g for ~/.config/opencode/skills/.
$ gh skill install secondsky/claude-skills mutation-testing --agent opencodeProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/secondsky/claude-skills.git skills-src && mkdir -p .opencode/skills && cp -r skills-src/plugins/mutation-testing/skills/mutation-testing .opencode/skills/mutation-testing && rm -rf skills-srcUse ~/.config/opencode/skills/ instead of .opencode/skills for a personal install.
OpenCode skills documentation · loads skills from .opencode/skills/, .claude/skills/, .agents/skills/
Install the "mutation-testing" agent skill from https://github.com/secondsky/claude-skills/tree/main/plugins/mutation-testing/skills/mutation-testing into .opencode/skills/mutation-testing/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "mutation-testing", then confirm the skill loads.OpenCode copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
Facts
- Skill name
mutation-testing- GitHub stars
- 227
- Token cost
- ~1.4k tokens
- SKILL.md length
- 230 words
- Files
- 1
- Skills in repo
- 169
- Repo updated
- First seen
- Licence
- MIT
At a glance
Validate test effectiveness with mutation testing using Stryker (TypeScript/JavaScript with Vitest or bun test via @hughescr/stryker-bun-runner) and mutmut (Python).
- Improve test quality
- SKILL.md covers Core Concept, TypeScript/JavaScript (Stryker), Python (mutmut) and Common Mutation Types, plus 5 more sections
- Calls bun, bunx and uv
- Tasks that involve Test coverage
What it does
Mutation Testing is an agent skill from secondsky/claude-skills. 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.
Its SKILL.md is about 1.4k tokens, which your agent loads only when the skill is triggered. It is a single SKILL.md file with no bundled scripts.
It sits in Testing & QA, covering Test coverage and Unit testing. It works with Vitest, JavaScript, Python and TypeScript. The repository describes itself as: Production-ready skills for Claude Code CLI - Cloudflare, React, Tailwind v4, and AI integrations. The licence is MIT.
When your agent uses it
- Improve test quality
- Tasks that involve Test coverage
- Tasks that involve Unit testing
Example prompts
- “/mutation-testing”
Requirements
- Python 3
- Node.js
- Pre-approved tools (allowed-tools): Bash, Read, Edit, Write, Grep, Glob, TodoWrite
What it can do on your machine
Read from SKILL.md and the folder at commit 8837836. It shows what the files ask for, not the result of running them.
Tool permissions
Pre-approves these tools, so the agent can use them without asking each time:
BashReadEditWriteGrepGlobTodoWrite
From allowed-tools in the SKILL.md frontmatter.
Runs code
Shell commands in SKILL.md call:
bunbunxuvnpm
From the folder's file list and the shell code blocks in SKILL.md.
Network
No URLs in SKILL.md. Its commands use bunx, uv and npm, which can reach the network depending on how they are called.
From URLs in SKILL.md, links to its own repository left out.
Credentials
Names no API keys, tokens, secrets or passwords.
From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.
Context cost
Mutation Testing loads about 1.4k tokens when it runs. Until then it costs about 65 tokens; SKILL.md has 230 words of instructions outside code blocks.
Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.
Safety
Auto-check: notes
The automated check noted patterns worth knowing about, such as sudo or a known installer.
- NotePre-approves every shell command (allowed-tools: Bash)SKILL.md
allowed-tools: Bash, Read, Edit, Write, Grep, Glob, TodoWrite
Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.
SKILL.md
The full file from secondsky/claude-skills at commit 8837836, republished under its MIT licence (© secondsky). 230 words, ~1,415 tokens.
Download SKILL.mdSave it as
.claude/skills/mutation-testing/SKILL.md (or your agent's skills folder).- name
- mutation-testing
- description
- 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.
- allowed-tools
- Bash, Read, Edit, Write, Grep, Glob, TodoWrite
- license
- MIT
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
# Using Bun
bun add -d @stryker-mutator/core @stryker-mutator/vitest-runner
# Using npm
npm install -D @stryker-mutator/core @stryker-mutator/vitest-runnerConfiguration
// 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)
@stryker-mutator/core ^9.6.1
Installation
bun add -D @hughescr/stryker-bun-runner @stryker-mutator/coreConfiguration
// stryker.conf.mjs
export default {
testRunner: 'bun',
coverageAnalysis: 'perTest',
mutate: ['src/**/*.ts', '!src/**/*.test.ts'],
thresholds: { high: 80, low: 60, break: 60 },
incremental: true,
bun: {
inspectorTimeout: 5000, // WebSocket Inspector connection timeout (ms)
// bunPath: '/path/to/bun', // only if custom Bun install
// timeout: 30000, // test timeout (ms)
// env: { DEBUG: 'true' },
// bunArgs: ['--bail'],
},
}Key Behaviors
- Sequential execution: Runs tests with
--concurrency=1for accurate per-test coverage. Slower than parallel but required for correct test-to-mutant correlation. - Concurrent test patching: Automatically patches
describe.concurrent(),test.concurrent(),it.concurrent()to run sequentially during mutation testing — no code changes needed. - Inspector Protocol: Uses Bun's WebSocket Inspector API to discover tests and correlate coverage.
Running Stryker
# Run mutation testing
bunx stryker run
# Incremental mode (only changed files)
bunx stryker run --incremental
# Specific files
bunx stryker run --mutate "src/utils/**/*.ts"
# Open HTML report
open reports/mutation/html/index.htmlExample: Weak Test
// Source code
function calculateDiscount(price: number, percentage: number): number {
return price - (price * percentage / 100)
}
// ❌ WEAK: Test passes even if we mutate calculation
test('applies discount', () => {
expect(calculateDiscount(100, 10)).toBeDefined() // Too weak!
})
// ✅ STRONG: Test catches mutation
test('applies discount correctly', () => {
expect(calculateDiscount(100, 10)).toBe(90)
expect(calculateDiscount(100, 20)).toBe(80)
expect(calculateDiscount(50, 10)).toBe(45)
})Python (mutmut)
Installation
uv add --dev mutmutRunning mutmut
# Run mutation testing
uv run mutmut run
# Show results
uv run mutmut results
# Show specific mutant
uv run mutmut show 1
# Generate HTML report
uv run mutmut html
open html/index.htmlCommon Mutation Types
// Arithmetic Operator
// Original: a + b → a - b, a * b, a / b
// Relational Operator
// Original: a > b → a >= b, a < b, a <= b
// Logical Operator
// Original: a && b → a || b
// Boolean Literal
// Original: true → falseMutation Score Targets
| Score | Quality | Action |
|---|---|---|
| 90%+ | Excellent | Maintain quality |
| 80-89% | Good | Small improvements |
| 70-79% | Acceptable | Focus on weak areas |
| < 60% | Poor | Major improvements needed |
Improving Weak Tests
Pattern: Insufficient Assertions
// Before: Mutation survives
test('calculates sum', () => {
expect(sum([1, 2, 3])).toBeGreaterThan(0) // Weak!
})
// After: Mutation killed
test('calculates sum correctly', () => {
expect(sum([1, 2, 3])).toBe(6)
expect(sum([0, 0, 0])).toBe(0)
expect(sum([])).toBe(0)
})Pattern: Boundary Conditions
// After: Tests boundaries
test('validates age boundaries', () => {
expect(isValidAge(18)).toBe(true) // Min valid
expect(isValidAge(17)).toBe(false) // Just below
expect(isValidAge(100)).toBe(true) // Max valid
expect(isValidAge(101)).toBe(false) // Just above
})Best Practices
- Start with core business logic modules
- Ensure 80%+ coverage before mutation testing
- Run incrementally (only changed files)
- Focus on important files first
- Don't expect 100% mutation score (equivalent mutants exist)
Workflow
# 1. Ensure good coverage first
bun test --coverage
# Target: 80%+ coverage
# 2. Run mutation testing
bunx stryker run
# 3. Check report
open reports/mutation/html/index.html
# 4. Fix survived mutants
# 5. Re-run incrementally
bunx stryker run --incremental
# or: npx stryker run --incrementalSee Also
vitest-testing- Unit testing frameworktest-quality-analysis- Detecting test smells
© secondsky, MIT. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file
Files
Just SKILL.md in plugins/mutation-testing/skills/mutation-testing of secondsky/claude-skills.
Open the folder on GitHubat commit 8837836
Compare with similar skills
Mutation Testing next to the 5 skills that share the most tags, products or categories with it. Stars are the repository's; “used in” counts other GitHub owners with a copy.
| Skill | Stars | Used in | Tokens | Auto-check | Licence | Repo updated |
|---|---|---|---|---|---|---|
| Mutation Testing this skillsecondsky/claude-skills | 227 | — | ~1.4k | Automated safety check: Notes | MIT | |
| Polyglot Test Agentboshi-xixixi/TraeSkill | 275 | — | ~1.7k | Automated safety check: Pass | MIT | |
| Test Writing WorkflowiOfficeAI/AionUi | 33k | 1 repos | ~1.2k | Automated safety check: Pass | Apache-2.0 | |
| Supercovsupercorp-ai/supercov | 150 | 1 repos | ~415 | Automated safety check: Pass | MIT | |
| Coverage Toolszkldi/Tachi | 237 | — | ~580 | Automated safety check: Pass | None | |
| Test Coveragescott-fryxell/brayness | 124 | — | ~1.6k | Automated safety check: Pass | MIT |
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iOfficeAI/AionUi
Sets the test-writing workflow for the repository: risk-first scenario lists, behavior-focused Vitest tests, a full run before each commit and a coverage target.
Supercov
supercorp-ai/supercov
Measures test coverage and code quality in a repository with the supercov CLI, and turns what it finds into small, focused tests or fixes.
Coverage Tools
zkldi/Tachi
Aggregates Vitest v8/Istanbul coverage across Tachi workspaces via tachi-coverage-tools (manifest, CLI, optional programmatic API).
Test Coverage
scott-fryxell/brayness
Write Vitest specs for Vue 3 JavaScript (Vite Plus, happy-dom, @vue/test-utils) and analyze V8 coverage + Fallow health to prioritize test-first refactors.
Supercov Security
supercorp-ai/supercov
Scans a repository's source for security vulnerabilities with the supercov CLI, pointing to the line of each finding and mapping it to CWE classes.
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Related
Works with
Categories
Questions about Mutation Testing
What does Mutation Testing do?
Validate test effectiveness with mutation testing using Stryker (TypeScript/JavaScript with Vitest or bun test via @hughescr/stryker-bun-runner) and mutmut (Python). Mutation Testing is an agent skill from secondsky/claude-skills. Validate test effectiveness with mutation testing using Stryker (TypeScript/JavaScript with Vitest or bun test via @hughescr/stryker-bun-runner) and mutmut (Python).
When should I use Mutation Testing?
Mutation Testing fits situations like: improve test quality; tasks that involve Test coverage; tasks that involve Unit testing.
How do I install Mutation Testing in Claude Code?
Run `npx skills add secondsky/claude-skills --skill mutation-testing -a claude-code`. Or copy the skill folder (plugins/mutation-testing/skills/mutation-testing in secondsky/claude-skills) into .claude/skills/mutation-testing in your project. Claude Code loads it when a task matches its description.
How do I install Mutation Testing in Codex?
Run `npx skills add secondsky/claude-skills --skill mutation-testing -a codex`. Or copy the skill folder (plugins/mutation-testing/skills/mutation-testing in secondsky/claude-skills) into .agents/skills/mutation-testing in your project. Codex loads it when a task matches its description.
Can I use Mutation Testing in Cursor, Gemini CLI or GitHub Copilot?
Cursor, Gemini CLI, GitHub Copilot and OpenCode also load SKILL.md folders. With the skills CLI, run `npx skills add secondsky/claude-skills --skill mutation-testing -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/mutation-testing, .gemini/skills/mutation-testing, .github/skills/mutation-testing and .opencode/skills/mutation-testing in your project.
What does Mutation Testing need to run?
Going by SKILL.md and its folder, Mutation Testing needs the command-line tools its instructions call (bun, bunx, uv and npm). Our summary lists: Python 3; Node.js. Its frontmatter pre-approves these tools: Bash, Read, Edit, Write, Grep, Glob, TodoWrite.
Does Mutation Testing access the network?
SKILL.md contains no URLs. Its commands use uv and npm, which can reach the network depending on how they are called. This is read from the text; nothing was executed.
Is Mutation Testing safe to install?
Our automated static check of SKILL.md found notes only (pre-approves every shell command (allowed-tools: bash)), nothing it rates as a warning. It is not a guarantee. Review the folder before installing.
What licence does Mutation Testing use?
Mutation Testing is published under the MIT licence (declared in SKILL.md). It allows redistribution, so the full SKILL.md is shown on this page.
How many tokens does Mutation Testing use?
About 1.4k tokens (SKILL.md is roughly 5.7k characters). Agents keep only the skill's name and description in context until a task matches; then they load SKILL.md in full.
What are the alternatives to Mutation Testing?
Skills that share tags, products or a category with Mutation Testing: Polyglot Test Agent (boshi-xixixi/TraeSkill, 275 stars), Test Writing Workflow (iOfficeAI/AionUi, 33k stars), Supercov (supercorp-ai/supercov, 150 stars) and Coverage Tools (zkldi/Tachi, 237 stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.
Who maintains Mutation Testing?
secondsky (a GitHub user) maintains it in secondsky/claude-skills, which has 227 GitHub stars. The repository holds 169 skills in this directory. The repository was last updated on September 28, 2026.
Source: secondsky/claude-skills on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.