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skill-check: Instructions file for Claude Code

CLAUDE.md

skill-check CLAUDE.md is an instructions file for Claude Code from mctar/skill-check. It costs 881 tokens per session, scanned A, original, MIT.

Repository instructions for Skill Check, a security scanner for Claude skills, which are prompt extensions stored in SKILL.md files. They describe its seven static checks, scanner structure, tests, threat model, and known limits.

In plain words
What is it for?
Use them when developing or testing the scanner, updating threat rules, reviewing detection behavior, or working with malicious and benign test skills.
Why use it?
They help an agent understand how the scanner detects suspicious patterns and how to make safe, tested changes.

Instructions file for Claude Code

Written for Claude Code: the file is CLAUDE.md. Also seen: mentions CLAUDE.md; mentions Claude Code.

This is mctar/skill-check's own configuration. It tells Claude Code how to work on skill-check itself, so it is not a mod to install elsewhere. Copy it as a starting point and replace the rules that are about this project. Everything skill-check configures →

not rated 2repo 8mo ago A scan 881 tokens original MIT
View source ↗ mctar/skill-check
Reuse

Borrowing it

Nothing to install: this file belongs to mctar/skill-check. Take a copy, put it at the same path in your own repository, and replace the rules that are about this project with yours.

Copy the file
curl -O https://raw.githubusercontent.com/mctar/skill-check/main/CLAUDE.md
Clone the repo
git clone --depth 1 https://github.com/mctar/skill-check

Made for: Claude Code.

Wrote 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.

agentmods badge for skill-check CLAUDE.md

README.md
[![agentmods](https://agentmods.dev/badge/instructions/mctar/skill-check/claude-md/github.svg)](https://agentmods.dev/instructions/mctar/skill-check/claude-md)
Your own site
<a href="https://agentmods.dev/instructions/mctar/skill-check/claude-md"><img src="https://agentmods.dev/badge/instructions/mctar/skill-check/claude-md/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.

agentmods 80×15 button for skill-check CLAUDE.md

Your own site · 80×15
<a href="https://agentmods.dev/instructions/mctar/skill-check/claude-md"><img src="https://agentmods.dev/badge/instructions/mctar/skill-check/claude-md.svg" alt="Reviewed on agentmods" width="80" height="20"></a>
Per session 881 This file is loaded in full into every session.
When invoked 881 The same file — it is already loaded in full.
Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe.
Origin original No closer match found in the catalogue.
Token cost

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.

ModelPer sessionOnce invoked
Fable 5.1 $0.00881 $0.00881
Opus 5.5 $0.00352 $0.00352
Sonnet 5.5 $0.00176 $0.00176
Haiku 4.5 $0.00088 $0.00088

Measured 1mo ago against content hash 57009b308e55, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, from the pricing page.

Security

Grade A, and why

skill-check CLAUDE.md 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.

CLAUDE.md · 91 lines

How it starts

The opening of the file, as written. The whole thing — 91 lines — stays where its author put it; the contents beside it link to each section on GitHub.

CLAUDE.md

This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.

Project Overview

Skill Check is a security analysis tool for Claude skills (SKILL.md-based prompt extensions). It runs seven static-analysis checks against a skill's files to detect known threat patterns. Ships as two artifacts with identical detection logic: a standalone Python script (zero dependencies, Python 3.8+) and a Claude skill wrapper.

Repository: gervilabs/skill-check Authors: Gervi Labs (Thordur Arnason & Lena Thorsmæhlum)

Architecture

scanner/skill_check.py      # Single-file standalone scanner (zero external deps)
rules/threat_rules.json     # Detection rules (human-readable JSON, separate from code)
tests/malicious/             # 7 red-team test skills (one per check)
tests/benign/                # 3 false-positive verification skills
tests/test_skill_check.py   # Automated test runner
SKILL.md                    # Claude skill wrapper
CAPABILITY.md               # Honest limitations document
docs/threat_model.md        # Detailed threat model

Key design decision: one Python file + one JSON rules file. Rules are separate so they can be audited without reading code. This separation is a trust signal.

The Seven Checks

Each check is independent (pass/fail on its own). Ordered by detection confidence:

  1. Arcana (Remote Code Execution) — highest confidence
  2. Perception (Exfiltration)
  3. Sleight of Hand (Credential Harvesting)
  4. Survival (Filesystem Overreach)
  5. Investigation (Obfuscation)
  6. History (Dependency Risk)
  7. Insight (Behavioral Manipulation) — lowest confidence

Names reference tabletop RPG ability checks; each is always paired with a plain-language label.

Scanner Core Classes

  • Finding — single detected issue (file, line, rule_id, check, severity, matched content)
  • CheckResult — result of one check (name, pass/fail, list of Findings)
  • ScanResult — all seven CheckResults + summary
  • RuleEngine — loads rules from threat_rules.json, applies by check
  • SkillScanner — orchestrates the seven checks against a skill directory

Read the full file on GitHub · 91 lines

Changes

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.

  1. 1mo ago First seen · 91 lines · 881 tokens per session scan A 57009b308e55

Subscribe to this mod's changes

skill-check CLAUDE.md is an instructions file published in the GitHub repository mctar/skill-check (2 stars, last pushed 8mo ago), licensed MIT. It adds 881 tokens to every session, about $0.0035 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 2026-08-31.

Security
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