Requirements
rizsotto/Bear
Write, modify, or review a requirement file under docs/requirements -- pick the single owning file, keep the text contract-only, name IDs so they need no explanation, and verify cross-references and…
Run or triage the mutation sweep in this repository. An agent skill from s3s-project/s3s.
Install Mutation Testing
$ npx skills add s3s-project/s3s --skill mutation-testing -a claude-codeProject install by default; add -g for ~/.claude/skills/.
$ gh skill install s3s-project/s3s 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/s3s-project/s3s.git skills-src && mkdir -p .claude/skills && cp -r skills-src/.agents/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/s3s-project/s3s/tree/main/.agents/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/s3s-project/s3s/tree/main/.agents/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 s3s-project/s3s --skill mutation-testing -a codexProject install goes to .agents/skills/; add -g for ~/.codex/skills/.
$ gh skill install s3s-project/s3s mutation-testing --agent codexProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/s3s-project/s3s.git skills-src && mkdir -p .agents/skills && cp -r skills-src/.agents/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/s3s-project/s3s/tree/main/.agents/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 s3s-project/s3s --skill mutation-testing -a cursorProject install goes to .agents/skills/; add -g for ~/.cursor/skills/.
$ gh skill install s3s-project/s3s mutation-testing --agent cursorProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/s3s-project/s3s.git skills-src && mkdir -p .cursor/skills && cp -r skills-src/.agents/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/s3s-project/s3s/tree/main/.agents/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/s3s-project/s3s.git --path .agents/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 s3s-project/s3s --skill mutation-testing -a gemini-cliProject install goes to .agents/skills/; add -g for ~/.gemini/skills/.
$ gh skill install s3s-project/s3s mutation-testing --agent gemini-cliProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/s3s-project/s3s.git skills-src && mkdir -p .gemini/skills && cp -r skills-src/.agents/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/s3s-project/s3s/tree/main/.agents/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 s3s-project/s3s 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 s3s-project/s3s --skill mutation-testing -a github-copilotProject install goes to .agents/skills/; add -g for ~/.copilot/skills/.
$ git clone --depth 1 https://github.com/s3s-project/s3s.git skills-src && mkdir -p .github/skills && cp -r skills-src/.agents/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/s3s-project/s3s/tree/main/.agents/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 s3s-project/s3s --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 s3s-project/s3s mutation-testing --agent opencodeProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/s3s-project/s3s.git skills-src && mkdir -p .opencode/skills && cp -r skills-src/.agents/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/s3s-project/s3s/tree/main/.agents/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
- 311
- Token cost
- ~917 tokens
- SKILL.md length
- 540 words
- Files
- 1
- Skills in repo
- 10
- Repo updated
- First seen
- Licence
- Apache-2.0
At a glance
Run or triage the mutation sweep in this repository. An agent skill from s3s-project/s3s.
- A survivor needs a decision
- SKILL.md covers Run a sweep locally, What runs on its own, Scope and The rotation, plus 2 more sections
- Calls cargo and just
- The sweep scope
What it does
Mutation Testing is an agent skill from s3s-project/s3s. Run or triage the mutation sweep in this repository. Use when a survivor needs a decision, when the sweep scope, budget or cursor is discussed, when the scheduled run fails, or when a new crate should join the mutation scope.
Its SKILL.md is about 920 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. The licence is Apache-2.0.
When your agent uses it
- A survivor needs a decision
- The sweep scope
- Cursor is discussed
- The scheduled run fails
Example prompts
- “/mutation-testing”
What it can do on your machine
Read from SKILL.md and the folder at commit 0fdcb86. 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:
cargojust
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.
Context cost
Mutation Testing loads about 917 tokens when it runs. Until then it costs about 61 tokens; SKILL.md has 540 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
The full file from s3s-project/s3s at commit 0fdcb86, republished under its Apache-2.0 licence (© s3s-project). 540 words, ~917 tokens.
Download SKILL.mdSave it as
.claude/skills/mutation-testing/SKILL.md (or your agent's skills folder).- name
- mutation-testing
- description
- Run or triage the mutation sweep in this repository. Use when a survivor needs a decision, when the sweep scope, budget or cursor is discussed, when the scheduled run fails, or when a new crate should join the mutation scope.
- license
- Apache-2.0
Mutation testing
The testing skill covers where the tests this sweep exercises live, and verification covers the red control that proves a new test can fail; this skill is about running the sweep and deciding what to do with a survivor.
Run a sweep locally
From the repository root, just mutants-run [BUDGET_MINUTES] [FILES] runs one budgeted sweep over the same rotation the scheduled run uses, and cargo run -p xtask -- mutants plan prints what that sweep would pick without mutating anything. One file is enough to iterate on: just mutants-run 6 crates/s3s-multipart/src/error.rs. Every unit prints the exact cargo mutants command it runs, including its --file and output directory, so that single line can be repeated by hand while a survivor is triaged, and cargo mutants --file <path> --list counts a file's mutants before spending time on it. Afterwards cargo run -p xtask -- mutants report --out mutants.out --check aggregates the outcome directories and fails on a survivor that is not accepted.
What runs on its own
.github/workflows/mutants.yml runs mutants run once a day with a fifteen-minute budget. It is scheduled only: pull requests never trigger it, and no local recipe drives it either. The sweep keeps a cursor as a CI artifact, and losing it only restarts the rotation.
Scope
The scope is the signing and parsing crates — s3s-multipart, s3s-chunked, s3s-sigv2, s3s-sigv4, s3s-rfc2047 — listed in SCOPE in xtask/src/mutants.rs. It cannot be examine_globs in .cargo/mutants.toml: that option makes cargo-mutants ignore --file, which the per-file rotation needs. Extending the scope is an edit to SCOPE and a decision about cost, not a side effect.
The rotation
Each run takes the files that are due: never swept first, then files whose content changed since their last sweep (a digest, never an mtime, because a checkout rewrites mtimes), then the oldest. A file is the unit; a file larger than the budget is started anyway and recorded as a partial sweep, so it makes progress instead of stalling the rotation.
Show full SKILL.md (215 more words)Show less
Triage a survivor
A survivor is a result, not a failure: cargo-mutants exit codes 0, 2 and 3 are all useful runs, and only 1, 4, 5, 6 and 70 mean the run itself broke. Every survivor gets one of two answers.
- A test gap: the mutant changes behaviour that nothing asserts. Write the test that fails for the right reason, then re-run that file.
- An equivalent mutant: the change cannot alter behaviour. Record it in the
ACCEPTEDtable inxtask/src/mutants.rswith a status and a reason a reviewer can check. The table is code so that it is reviewed together with the sweep that produced it.
Do not reach for exclude_globs or exclude_re to hide a survivor; those options are for code that cannot be mutated meaningfully, such as generated files. cargo run -p xtask -- mutants check validates the configuration and the accepted list without mutating anything.
Resources
A mutant can make a test allocate without limit: the sweep caps its own address space through rlimit where the platform has it, keeps cargo-mutants at two workers unless CARGO_MUTANTS_JOBS says otherwise, and bounds each run by the wall-clock budget. A run that has to be killed is a signal that the budget or the tests need attention, not that the cap should be raised quietly.
© s3s-project, Apache-2.0. 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 .agents/skills/mutation-testing of s3s-project/s3s.
Open the folder on GitHubat commit 0fdcb86
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 skills3s-project/s3s | 311 | — | ~917 | Automated safety check: Pass | Apache-2.0 | |
| Requirementsrizsotto/Bear | 6.5k | — | ~2k | Automated safety check: Pass | GPL-3.0 | |
| Crap Analysisardalis/RiverBooks | 135 | 2 repos | ~3.4k | Automated safety check: Pass | None | |
| Project Statusbactopia/bactopia | 522 | — | ~787 | Automated safety check: Pass | MIT | |
| Check Coverageldayton/Dippy | 243 | — | ~403 | Automated safety check: Pass | MIT | |
| Guarding Destructive Operationskajisho5/ffmpeg-skill | 1.9k | — | ~2.6k | Automated safety check: Pass | MIT |
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Project Status
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Check Coverage
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A skill your agent uses to add a third validation layer to the ATDD workflow — after acceptance tests verify WHAT and unit tests verify HOW, mutation testing verifies the tests actually catch bugs.
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Related
Categories
Questions about Mutation Testing
What does Mutation Testing do?
Run or triage the mutation sweep in this repository. An agent skill from s3s-project/s3s. Mutation Testing is an agent skill from s3s-project/s3s. Run or triage the mutation sweep in this repository.
When should I use Mutation Testing?
Mutation Testing fits situations like: A survivor needs a decision; the sweep scope; Cursor is discussed; the scheduled run fails.
How do I install Mutation Testing in Claude Code?
Run `npx skills add s3s-project/s3s --skill mutation-testing -a claude-code`. Or copy the skill folder (.agents/skills/mutation-testing in s3s-project/s3s) 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 s3s-project/s3s --skill mutation-testing -a codex`. Or copy the skill folder (.agents/skills/mutation-testing in s3s-project/s3s) 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 s3s-project/s3s --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 (cargo and just).
Does Mutation Testing 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 Mutation Testing 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 Mutation Testing use?
Mutation Testing is published under the Apache-2.0 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 917 tokens (SKILL.md is roughly 3.7k 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: Requirements (rizsotto/Bear, 6.5k stars), Crap Analysis (ardalis/RiverBooks, 135 stars), Project Status (bactopia/bactopia, 522 stars) and Check Coverage (ldayton/Dippy, 243 stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.
Who maintains Mutation Testing?
s3s-project (a GitHub organization) maintains it in s3s-project/s3s, which has 311 GitHub stars. The repository holds 10 skills in this directory. The repository was last updated on October 8, 2026.
Source: s3s-project/s3s on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.