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 agentmods add skills/me2resh/apexyard/mutation-testnpx skills add me2resh/apexyard --skill mutation-testgit clone --depth 1 https://github.com/me2resh/apexyardWrote 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/me2resh/apexyard/mutation-test)<a href="https://agentmods.dev/skills/me2resh/apexyard/mutation-test"><img src="https://agentmods.dev/badge/skills/me2resh/apexyard/mutation-test.svg" alt="Measured on agentmods" 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 | $0.00040 | $0.03940 |
| Opus 5 | $0.00020 | $0.01970 |
| Sonnet 5 | $0.00008 | $0.00788 |
| Haiku 4.5 | $0.00004 | $0.00394 |
Grade A, and why
mutation-test 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 4d 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 — 351 lines — stays where its author put it; the contents beside it link to each section on GitHub.
/mutation-test — Behaviour-Quality Sensor
Runs mutation testing against a project to measure whether the test suite constrains behaviour, not just executes lines. Coverage % answers "did the test run this line?"; mutation testing answers "if I broke this line, would the test catch it?".
Pairs with /launch-check (fans out to this skill at milestone boundaries) and complements the existing > 80% coverage gate from .claude/rules/workflow-gates.md. Not run per-PR — see § "When to use this" for the cadence rationale.
Runtime requirements
| Dependency | Used for | Without it |
|---|---|---|
bash ≥ 4 |
The skill itself | Required |
git |
Project detection + path resolution | Required |
stryker (@stryker-mutator/core) |
TS / JS mutation runner | Skill exits 3 with the install one-liner if TS/JS detected |
mut.py (MutPy) |
Python mutation runner | Same shape |
go-mutesting |
Go mutation runner | Same shape |
mutant (mutant-rspec / mutant-minitest) |
Ruby mutation runner | Same shape |
jq |
Stryker JSON report parsing | Required when Stryker is the chosen runner |
Same disclosure shape as /pdf and /process — disclosed up front, surfaced when invoked, never silently fails.
Path resolution
source "$(git rev-parse --show-toplevel)/.claude/hooks/_lib-read-config.sh"
source "$(git rev-parse --show-toplevel)/.claude/hooks/_lib-portfolio-paths.sh"
projects_dir=$(portfolio_projects_dir)
workspace_dir=$(portfolio_workspace_dir)
Defaults to single-fork mode. Split-portfolio v2 adopters resolve to the sibling private repo transparently. Don't hardcode projects/ or workspace/ literals.
Usage
/mutation-test # run against cwd (must be inside workspace/<name>/)
/mutation-test workspace/example-app # run against an explicit project path
/mutation-test --language=python # force the Python runner (skip detection)
/mutation-test --runner=stryker # force a specific runner regardless of language
/mutation-test --threshold=70 # one-off threshold override (doesn't write to config)
/mutation-test --check-only # report which runners are installed, do nothing else
What ships with it
2 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.
- 4d ago First seen · 351 lines · 40 tokens per session scan A 0076b3257448
mutation-test is a skill published in the GitHub repository me2resh/apexyard (498 stars, last pushed today), licensed MIT. It adds 40 tokens to every session and 3,940 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 2026-08-30.
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