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 aj-geddes/useful-ai-prompts --skill mutation-testinggit clone --depth 1 https://github.com/aj-geddes/useful-ai-promptsWrote 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/aj-geddes/useful-ai-prompts/mutation-testing)<a href="https://agentmods.dev/skills/aj-geddes/useful-ai-prompts/mutation-testing"><img src="https://agentmods.dev/badge/skills/aj-geddes/useful-ai-prompts/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/aj-geddes/useful-ai-prompts/mutation-testing"><img src="https://agentmods.dev/badge/skills/aj-geddes/useful-ai-prompts/mutation-testing.svg" alt="Reviewed on agentmods" width="80" height="20"></a>- Socket pass
- Snyk pass
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 | $0.00040 | $0.00535 |
| Opus 5 | $0.00020 | $0.00267 |
| Sonnet 5 | $0.00008 | $0.00107 |
| Haiku 4.5 | $0.00004 | $0.00053 |
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 9d 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 — 80 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Mutation Testing
Table of Contents
Overview
Mutation testing assesses test suite quality by introducing small changes (mutations) to source code and verifying that tests fail. If tests don't catch a mutation, it indicates gaps in test coverage or test quality. This technique helps identify weak or ineffective tests.
When to Use
- Evaluating test suite effectiveness
- Finding untested code paths
- Improving test quality metrics
- Validating critical business logic is well-tested
- Identifying redundant or weak tests
- Measuring real test coverage beyond line coverage
- Ensuring tests actually verify behavior
Quick Start
Minimal working example:
# Install Stryker
npm install --save-dev @stryker-mutator/core @stryker-mutator/jest-runner
# Initialize configuration
npx stryker init
# Run mutation testing
npx stryker run
Reference Guides
Detailed implementations in the references/ directory:
| Guide | Contents |
|---|---|
| Stryker for JavaScript/TypeScript | Stryker for JavaScript/TypeScript |
| PITest for Java | PITest for Java |
| mutmut for Python | mutmut for Python |
| Mutation Testing Reports | Mutation Testing Reports |
Best Practices
✅ DO
- Target critical business logic for mutation testing
- Aim for 80%+ mutation score on important code
- Review survived mutants to improve tests
- Mark equivalent mutants to exclude them
- Use mutation testing in CI for critical modules
- Test boundary conditions thoroughly
- Verify actual behavior, not just code execution
❌ DON'T
- Expect 100% mutation score everywhere
- Run mutation testing on all code (too slow)
- Ignore equivalent mutants
- Test getters/setters with mutations
- Run mutations on generated code
- Skip mutation testing on complex logic
- Focus only on line coverage
What ships with it
6 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.
- 9d ago First seen · 80 lines · 40 tokens per session scan A 4e1242e2164c
mutation-testing is a skill published in the GitHub repository aj-geddes/useful-ai-prompts (338 stars, last pushed 6mo ago), licensed MIT. It adds 40 tokens to every session and 535 once invoked, about $0.0002 per session on Opus 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 2026-09-03.
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