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.
git clone --depth 1 https://github.com/olgasafonova/SkillCheck-FreeWrote 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/olgasafonova/skillcheck-free/skill-check)<a href="https://agentmods.dev/rules/olgasafonova/skillcheck-free/skill-check"><img src="https://agentmods.dev/badge/rules/olgasafonova/skillcheck-free/skill-check/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/olgasafonova/skillcheck-free/skill-check"><img src="https://agentmods.dev/badge/rules/olgasafonova/skillcheck-free/skill-check.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 | $0.00026 | $0.05785 |
| Opus 5.5 | $0.00010 | $0.02314 |
| Sonnet 5.5 | $0.00005 | $0.01157 |
| Haiku 4.5 | $0.00003 | $0.00579 |
Grade A, and why
skill-check 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 20d 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 — 602 lines — stays where its author put it; the contents beside it link to each section on GitHub.
SkillCheck (Free)
Check skills against Anthropic guidelines and the agentskills specification. This file contains Free tier validation rules.
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When to Use
Activate when user:
- Says "check skill", "skillcheck", or "validate SKILL.md"
- Wants to verify skills meet Anthropic best practices
- Asks to find issues in skill definitions
Prerequisites
- Any AI assistant with file Read capability (Claude Code, Cursor, Windsurf, Codex CLI)
- Works on any platform (Unix/macOS/Windows)
- No special tools required (Read-only)
How to Check a Skill
- Locate: Find target SKILL.md file(s)
- Read: Load the content
- Validate: Apply each rule section below
- Report: List issues found with severity and fixes
Free Tier Validation Rules
Apply these checks in order. Each subsection defines patterns to match and issues to flag.
1. Frontmatter Structure
Every SKILL.md must start with YAML frontmatter between --- markers.
Required Fields
| Field | Required | Rules |
|---|---|---|
name |
Yes | Lowercase, hyphens only, 1-64 chars |
description |
Yes | WHAT + WHEN pattern, 1-1024 chars |
Optional Fields (Spec)
Fields defined in the agentskills.io specification:
| Field | Purpose |
|---|---|
license |
License name or reference to bundled license file |
allowed-tools |
Tools the skill can use (space-separated or YAML list) |
compatibility |
Platform compatibility info (max 500 chars) |
metadata |
Additional key-value pairs |
Claude Code Extensions
Recognized by Claude Code (per the Claude Code skills docs) but not part of the agentskills.io spec. Other agents may ignore these fields, and claude.ai uploads and the Skills API reject them (see 1.19).
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.
- 20d ago Changed · +41 lines 248c531e5b27
- 1mo ago Changed · +12 lines 5a0fc7ca4b97
- 2mo ago First seen · 549 lines · 26 tokens per session scan A b47939ef30ce
skill-check is a cursor rule published in the GitHub repository olgasafonova/SkillCheck-Free (42 stars, last pushed 19d ago), licensed MIT. It adds 26 tokens to every session and 5,785 once invoked, about $0.0001 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 2026-08-30.
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