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 ANcpLua/ancplua-claude-plugins --skill mutation-resistant-coveragegit clone --depth 1 https://github.com/ANcpLua/ancplua-claude-pluginsWrote 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/ancplua/ancplua-claude-plugins/mutation-resistant-coverage)<a href="https://agentmods.dev/skills/ancplua/ancplua-claude-plugins/mutation-resistant-coverage"><img src="https://agentmods.dev/badge/skills/ancplua/ancplua-claude-plugins/mutation-resistant-coverage/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/ancplua/ancplua-claude-plugins/mutation-resistant-coverage"><img src="https://agentmods.dev/badge/skills/ancplua/ancplua-claude-plugins/mutation-resistant-coverage.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 | $0.00071 | $0.01194 |
| Opus 5.5 | $0.00028 | $0.00478 |
| Sonnet 5.5 | $0.00014 | $0.00239 |
| Haiku 4.5 | $0.00007 | $0.00119 |
Grade A, and why
mutation-resistant-coverage 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 19d 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 — 124 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Mutation-Resistant Coverage
Core principle
Branches are not equal. A branch where success vs failure changes
observable behavior is worth a test. A branch where two formatting outputs
differ by a whitespace is not. You enumerate semantically distinct
outcomes, write one high-signal test per outcome, and coverage falls out as
a side effect.
You never write a test whose only purpose is to enter a branch.
When to invoke
- Pushing a file toward 100% branch coverage.
- A mutation-testing run (Stryker, Infection, Pitest, Mutmut) reports surviving mutants and you need to kill them.
- Opening a PR where the coverage gate demands more tests and you want to add them without producing cosmetics.
Semantic branch taxonomy
Classify every condition in the target:
| Class | Example | Test required? |
|---|---|---|
success vs failure |
try/catch, Result.Ok / Result.Err |
Yes, both sides |
empty vs non-empty |
items.length === 0, null collection |
Yes, both sides |
valid vs invalid |
schema validation, type narrowing | Yes, both sides |
authorized vs unauthorized |
role / permission check | Yes, both sides |
cached vs uncached |
memoization, TTL expiry | Yes, both sides |
primary vs fallback |
network → cache, default → override | Yes, both sides |
new vs existing |
create vs update, insert vs upsert | Yes, both sides |
defensive |
guard where caller already enforces the invariant | Consider deleting the branch |
cosmetic |
whitespace-only format variants | No; one test is enough |
Workflow
Step 1 — Enumerate
Read the target file. Produce a branch map:
| Location | Branch | Class | Needs test? | Notes |
|---|---|---|---|---|
file.ts:42 |
items.length === 0 |
empty/non-empty | yes | both sides |
file.ts:88 |
user.role === 'admin' |
auth/unauth | yes | both sides |
file.ts:104 |
if (!input) |
defensive | no | callers enforce; propose deletion |
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.
- 19d ago First seen · 124 lines · 71 tokens per session scan A 12e311921e97
mutation-resistant-coverage is a skill published in the GitHub repository ANcpLua/ancplua-claude-plugins (6 stars, last pushed 2d ago), licensed MIT. It adds 71 tokens to every session and 1,194 once invoked, about $0.0003 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 2026-10-01.
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