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.
Run mutant, read mutation reports, fix alive mutations, and verify coverage.
Install Mutant
$ npx skills add mbj/mutant --skill mutant -a claude-codeProject install by default; add -g for ~/.claude/skills/.
$ gh skill install mbj/mutant mutant --agent claude-codeProject scope by default; add --scope user for a personal install. Needs GitHub CLI 2.90.0 or later (public preview).
Claude Code skills documentation · loads skills from .claude/skills/
Install the "mutant" agent skill from https://github.com/mbj/mutant/tree/main into .claude/skills/mutant/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "mutant", 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.
$ npx skills add mbj/mutant --skill mutant -a codexProject install goes to .agents/skills/; add -g for ~/.codex/skills/.
$ gh skill install mbj/mutant mutant --agent codexProject scope by default (.agents/skills/); add --scope user for a personal install.
Codex skills documentation · loads skills from .agents/skills/
Install the "mutant" agent skill from https://github.com/mbj/mutant/tree/main into .agents/skills/mutant/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "mutant", 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 mbj/mutant --skill mutant -a cursorProject install goes to .agents/skills/; add -g for ~/.cursor/skills/.
$ gh skill install mbj/mutant mutant --agent cursorProject scope by default (.agents/skills/); add --scope user for a personal install.
Cursor skills documentation · loads skills from .cursor/skills/, .agents/skills/, .claude/skills/, .codex/skills/
Install the "mutant" agent skill from https://github.com/mbj/mutant/tree/main into .cursor/skills/mutant/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "mutant", 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.
$ npx skills add mbj/mutant --skill mutant -a gemini-cliProject install goes to .agents/skills/; add -g for ~/.gemini/skills/.
$ gh skill install mbj/mutant mutant --agent gemini-cliProject scope by default (.agents/skills/); add --scope user for a personal install.
Gemini CLI skills documentation · loads skills from .gemini/skills/, .agents/skills/
Install the "mutant" agent skill from https://github.com/mbj/mutant/tree/main into .gemini/skills/mutant/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "mutant", 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 mbj/mutant mutantInstalls 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 mbj/mutant --skill mutant -a github-copilotProject install goes to .agents/skills/; add -g for ~/.copilot/skills/.
GitHub Copilot skills documentation · loads skills from .github/skills/, .claude/skills/, .agents/skills/
Install the "mutant" agent skill from https://github.com/mbj/mutant/tree/main into .github/skills/mutant/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "mutant", 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 mbj/mutant --skill mutant -a opencodeOpenCode documents no install command of its own. Project install goes to .agents/skills/; add -g for ~/.config/opencode/skills/.
$ gh skill install mbj/mutant mutant --agent opencodeProject scope by default (.agents/skills/); add --scope user for a personal install.
OpenCode skills documentation · loads skills from .opencode/skills/, .claude/skills/, .agents/skills/
Install the "mutant" agent skill from https://github.com/mbj/mutant/tree/main into .opencode/skills/mutant/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "mutant", 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
mutant- GitHub stars
- 2.2k
- Token cost
- ~2k tokens
- SKILL.md length
- 1,115 words
- Files
- 716
- Skills in repo
- 1
- Repo updated
- First seen
- Licence
- Proprietary
At a glance
Run mutant, read mutation reports, fix alive mutations, and verify coverage.
- Works in 2 steps: Mutation results (alive count, coverage… → Action for each alive mutation.
- Running mutation testing
- SKILL.md covers When to Activate, When Not to Use, Inputs and Outputs (Fixed Order), plus 5 more sections
- Calls bundle
What it does
Mutant is an agent skill from mbj/mutant. Run mutant, read mutation reports, fix alive mutations, and verify coverage. Use when running mutation testing or responding to alive mutations.
Its SKILL.md is about 2k tokens, which your agent loads only when the skill is triggered. The skill folder holds 718 other files (for example `.github/dependabot.yml`, `.github/workflows/ci.yml` and `CONTRIBUTING.md`). Compatibility notes: Unified agent skills CLI
It sits in Testing & QA, covering Test coverage. It works with Ruby. The repository describes itself as: Mutation testing for Ruby. AI writes your code. AI writes your tests. But who tests the tests?
When your agent uses it
- Running mutation testing
- Responding to alive mutations
Example prompts
- “/mutant”
Requirements
- Compatibility (from SKILL.md): Unified agent skills CLI
Workflow steps
2 steps, taken from the first numbered list in SKILL.md.
- Mutation results (alive count, coverage percentage).
- Action for each alive mutation.
What it can do on your machine
Read from SKILL.md and the folder at commit d20fb1f. It shows what the files ask for, not the result of running them.
Tool permissions
Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.
From allowed-tools in the SKILL.md frontmatter.
Runs code
Shell commands in SKILL.md call:
bundle
From the folder's file list and the shell code blocks in SKILL.md.
Network
No URLs in SKILL.md.
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.
Compatibility
Unified agent skills CLI
From compatibility in the SKILL.md frontmatter.
Context cost
Mutant loads about 2k tokens when it runs. Until then it costs about 38 tokens; SKILL.md has 1,115 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 passed
The automated check found no risky patterns in SKILL.md.
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
Its licence (Proprietary) doesn't allow us to republish the file, so here is its outline and opening line. It has 1,115 words (~1,982 tokens).
- name
- mutant
- compatibility
- Unified agent skills CLI
- metadata.author
- dkubb
- metadata.version
- 2026-03-v22
- triggers
- mutation testing, mutant, alive mutation, mutation coverage
Files
SKILL.md and 715 other files in the repository root of mbj/mutant.
- SKILL.md
- .github/dependabot.yml
- .github/workflows/ci.yml
- .gitignore
- CONTRIBUTING.md
- Cargo.lock
- Cargo.toml
- Changelog.md
- LICENSE
- README.md
- docs/ast-pattern.md
- docs/commercial.md
- docs/concurrency.md
- docs/configuration.md
- docs/debugging.md
- docs/hooks.md
- docs/in-the-wild.md
- docs/incremental.md
- … and 698 more
Open the folder on GitHubat commit d20fb1f
Compare with similar skills
Mutant 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 |
|---|---|---|---|---|---|---|
| Mutant this skillmbj/mutant | 2.2k | — | ~2k | Automated safety check: Pass | Proprietary | |
| Supercovsupercorp-ai/supercov | 150 | 1 repos | ~415 | Automated safety check: Pass | MIT | |
| Supercov Securitysupercorp-ai/supercov | 150 | 1 repos | ~236 | Automated safety check: Pass | MIT | |
| Testing Patternsdilolabs/nosia | 213 | 1 repos | ~1.4k | Automated safety check: Pass | MIT | |
| Test Writing WorkflowiOfficeAI/AionUi | 33k | 1 repos | ~1.2k | Automated safety check: Pass | Apache-2.0 | |
| Evaluate PR Testsdotnet/maui | 23k | — | ~2.9k | Automated safety check: Pass | MIT |
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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.
Testing Patterns
dilolabs/nosia
Writes Minitest tests with fixtures following 37signals conventions.
Test Writing Workflow
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.
- Official
Evaluate PR Tests
dotnet/maui
Reviews the tests added in a pull request for fix coverage, quality, edge cases and test type, and recommends lighter test types where they would do.
Integration Test
tracewayapp/traceway
Run a live-instance verification of traceway-cli that goes beyond the Go smoke suite — exercises real-data detail endpoints, TTY-default rendering, adaptive metric-name discovery, and emits a…
Related
Works with
Categories
Questions about Mutant
What does Mutant do?
Run mutant, read mutation reports, fix alive mutations, and verify coverage. Mutant is an agent skill from mbj/mutant. Run mutant, read mutation reports, fix alive mutations, and verify coverage.
When should I use Mutant?
Mutant fits situations like: running mutation testing; responding to alive mutations.
How do I install Mutant in Claude Code?
Run `npx skills add mbj/mutant --skill mutant -a claude-code`. Or copy the skill folder (the mbj/mutant repository) into .claude/skills/mutant in your project. Claude Code loads it when a task matches its description.
How do I install Mutant in Codex?
Run `npx skills add mbj/mutant --skill mutant -a codex`. Or copy the skill folder (the mbj/mutant repository) into .agents/skills/mutant in your project. Codex loads it when a task matches its description.
Can I use Mutant 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 mbj/mutant --skill mutant -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/mutant, .gemini/skills/mutant, .github/skills/mutant and .opencode/skills/mutant in your project.
What does Mutant need to run?
Going by SKILL.md and its folder, Mutant needs the command-line tools its instructions call (bundle). Compatibility (from SKILL.md): Unified agent skills CLI.
Does Mutant access the network?
SKILL.md contains no URLs. Any network use would come from the scripts or tools the agent runs. This is read from the text; nothing was executed.
Is Mutant safe to install?
Our automated static check of SKILL.md found no risky patterns, such as piping downloads into a shell, reading credential files or hidden Unicode. It is not a guarantee. Review the folder before installing.
What licence does Mutant use?
Mutant carries a proprietary licence (from the LICENSE file in the skill folder). It is published on GitHub, but it is not open source: read the licence file before using or sharing it.
How many tokens does Mutant use?
About 2k tokens (SKILL.md is roughly 7.9k 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 Mutant?
Skills that share tags, products or a category with Mutant: Supercov (supercorp-ai/supercov, 150 stars), Supercov Security (supercorp-ai/supercov, 150 stars), Testing Patterns (dilolabs/nosia, 213 stars) and Test Writing Workflow (iOfficeAI/AionUi, 33k stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.
Who maintains Mutant?
mbj (a GitHub user) maintains it in mbj/mutant, which has 2,207 GitHub stars. The repository was last updated on October 2, 2026.
Source: mbj/mutant on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.