Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx skills add nikeyes/stepwise-dev --skill mutation-testinggit clone --depth 1 https://github.com/nikeyes/stepwise-devWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/nikeyes/stepwise-dev/mutation-testing)<a href="https://agentmods.dev/skills/nikeyes/stepwise-dev/mutation-testing"><img src="https://agentmods.dev/badge/skills/nikeyes/stepwise-dev/mutation-testing/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/nikeyes/stepwise-dev/mutation-testing"><img src="https://agentmods.dev/badge/skills/nikeyes/stepwise-dev/mutation-testing.svg" alt="Reviewed on agentmods" width="80" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00097 | $0.01491 |
| Opus 5.5 | $0.00039 | $0.00596 |
| Sonnet 5.5 | $0.00019 | $0.00298 |
| Haiku 4.5 | $0.00010 | $0.00149 |
Grade A, and why
mutation-testing scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 8d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 92 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Mutation Testing
Answers one question: "If I introduced a bug here, would a test fail?" Coverage tells you a line was executed; mutation testing tells you a change to that line would be detected.
Attribution: Adapted from eferro's skill-factory (Apache-2.0), itself based on Paul Hammond's mutation-testing skill.
Usage
/mutation-testing <file> # whole module
/mutation-testing # branch diff: git diff main...HEAD plus uncommitted changes
/mutation-testing --changed <files...> # only changed hunks: git diff HEAD -- <files>; untracked files whole
/mutation-testing --tool [...] # force the project's mutation tool (see Modes)
Only production code is mutated. Skip test files, generated code and pure configuration.
Modes
- Mental (default): you generate mutants by reading the code and reason about whether the tests would fail. Fast, works in any language, leaves the working tree untouched except for the empirical checks below.
- Tool: run the project's mutation tool restricted to the scope. Use it when
--toolis passed or the project already configures one (stryker.config.*/stryker.conf.*,[tool.mutmut]inpyproject.toml/setup.cfg, the pitest plugin inpom.xml/build.gradle*). See the tool reference for the scoped command.
Never install a mutation tool or add its configuration. If --tool is passed and no tool is set up, say so and continue in mental mode.
Workflow
- Resolve scope. List the production files and, for diff modes, the changed line ranges.
- Check the baseline. Run the tests that cover the scope. If they fail, stop and report: mutation results are meaningless on a red suite.
- Generate mutants. For each function in scope, apply the operators in operators.md plus the language reference: python.md, javascript.md (JS/TS), java.md, kotlin.md. Favor the high-yield operators listed there; skip mutants that would not compile. In tool mode, run the tool instead: mutmut.md, stryker.md, pitest.md.
- Evaluate each mutant. Find the tests that exercise the line and trace a concrete input: would an assertion produce a different result? Count a mutant as killed only when you can name the test and the input that kills it. When unsure, check empirically (see Applying Mutants).
- Classify each survivor:
What ships with it
8 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 8d ago First seen · 92 lines · 97 tokens per session scan A a78d379c8bc0
mutation-testing is a skill published in the GitHub repository nikeyes/stepwise-dev (24 stars, last pushed 6d ago), licensed Apache-2.0. It adds 97 tokens to every session and 1,491 once invoked, about $0.0004 per session on Opus 5.5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-10-11.
Other skills, from other repositories
engram-testing-coverage
TDD and coverage standards for Engram. Trigger: When implementing behavior changes in any package.
conductor-implement
Execute tasks from a track's implementation plan following TDD workflow.
principle-test-behavior-not-implementation
Apply when you write, change, or keep a test. Assert what the code's users can observe, so that the test fails when that behavior breaks and keeps passing when only the internals change.
iterative-development
TDD iteration loops using Claude Code Stop hooks - runs tests after each response, feeds failures back automatically.
python
Python development with ruff, mypy, pytest - TDD and type safety.
tdd
This skill should be used when the user wants to implement features or fix bugs using test-driven development. Enforces the RED-GREEN-REFACTOR cycle with vertical slicing, context isolation between test writing and implementation, human checkpoints, and auto-test feedback loops. Uses multi-agent orchestration with the…