Getting it into your agent
It runs from inside its repository, so the clone comes first — what it calls does not travel with the file alone.
git clone --depth 1 https://github.com/andr-ca/agentharnessnpx agentmods add rules/andr-ca/agentharness/mutation-testingWrote 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/rules/andr-ca/agentharness/mutation-testing)<a href="https://agentmods.dev/rules/andr-ca/agentharness/mutation-testing"><img src="https://agentmods.dev/badge/rules/andr-ca/agentharness/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/rules/andr-ca/agentharness/mutation-testing"><img src="https://agentmods.dev/badge/rules/andr-ca/agentharness/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 | $0.00055 | $0.00748 |
| Opus 5.5 | $0.00022 | $0.00299 |
| Sonnet 5.5 | $0.00011 | $0.00150 |
| Haiku 4.5 | $0.00006 | $0.00075 |
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 1mo 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 — 88 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Mutation Testing
Mutation testing verifies whether your tests detect bugs, not just
whether they execute code. A "mutant" is a small deliberate change
(flip > to >=, delete a return) applied to the source. If your
tests pass with the mutant, they would not have caught that bug.
Deeper reference: patterns/mutation-testing/MUTATION_TESTING.md
(full operator list, equivalent mutants, cost management strategies).
When to use it
- After reaching line/branch coverage targets but before shipping critical business logic (payment, auth, data validation).
- When a production bug slips through a test suite that claims high coverage.
- As a periodic quality audit (weekly/monthly) rather than a per-commit gate.
When to skip: Prototype and Internal tiers; generated code; pure data classes with no conditional logic; large legacy codebases where the cost of triaging hundreds of mutants outweighs the benefit.
Mutation score
$$\text{mutation score} = \frac{\text{killed}}{\text{total} - \text{equivalent}} \times 100%$$
| Score | Verdict |
|---|---|
| ≥ 80% | Strong; surviving mutants warrant individual inspection |
| 60–79% | Adequate; identify surviving categories and add targeted tests |
| Below 60% | Test suite quality is insufficient; line/branch percentages alone are misleading |
Python — mutmut
pip install mutmut
mutmut run --paths-to-mutate src/billing.py
mutmut results # list surviving mutants
mutmut show 15 # show the diff for mutant #15
TypeScript / JavaScript — Stryker
npm install --save-dev @stryker-mutator/core @stryker-mutator/jest-runner
npx stryker init
npx stryker run
stryker.config.js:
module.exports = {
testRunner: 'jest',
coverageAnalysis: 'perTest', // only test mutants actually covered
mutate: ['src/billing/**/*.ts'],
thresholds: { high: 80, low: 60, break: 50 }, // break = CI gate
};
Go — gremlins
go install github.com/go-gremlins/gremlins/cmd/gremlins@latest
gremlins unleash ./internal/billing/...
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.
- 1mo ago First seen · 88 lines · 55 tokens per session scan A 104fa1d078c0
mutation-testing is a cursor rule published in the GitHub repository andr-ca/agentharness (1 stars, last pushed 5d ago), licensed MIT. It adds 55 tokens to every session and 748 once invoked, about $0.0002 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 2026-09-03.
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