A practical guide to creating and reviewing Agent Skills, with advice on domain expertise, activation rules, progressive disclosure, and common mistakes.

Use it to create or improve skills, review their structure, reduce false activations, and test whether they respond to the right requests.

It helps authors make skills easier to discover, more precise, and more useful to agents and users.

skill-coach — Skill for Claude Code by curiositech

Install

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.

Any agent

npx skills add curiositech/some_claude_skills --skill skill-coach

Clone the repo

git clone --depth 1 https://github.com/curiositech/some_claude_skills

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.

README.md

[![agentmods](https://agentmods.dev/badge/skills/curiositech/some_claude_skills/skill-coach/github.svg)](https://agentmods.dev/skills/curiositech/some_claude_skills/skill-coach)

Your own site

<a href="https://agentmods.dev/skills/curiositech/some_claude_skills/skill-coach"><img src="https://agentmods.dev/badge/skills/curiositech/some_claude_skills/skill-coach/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.

Your own site · 80×15

<a href="https://agentmods.dev/skills/curiositech/some_claude_skills/skill-coach"><img src="https://agentmods.dev/badge/skills/curiositech/some_claude_skills/skill-coach.svg" alt="Reviewed on agentmods" width="80" height="20"></a>

Per session 74 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 2,541 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe. Third-party audits

NVIDIA SkillSpector warn 7 Sept 2026

SkillSpector: 1 finding, up to high

These are SkillSpector’s own severities. On a checked sample its high-severity flags on skills were ~96% false positives — a documented command, a public API, a “never do X” rule — so we show them as a caution to read, not a verdict. Why →

high Rogue Agent · line 218
Skill modifies its own code, configuration, or behavior at runtime. Self-modification enables an agent to escalate privileges, disable safety constraints, or install persistent backdoors.

Fix: Prevent the skill from modifying its own code, SKILL.md, or configuration files. Treat skill files as read-only at runtime.

How audits are shown

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.

Model	Per session	Once invoked
Fable 5.1	$0.00074	$0.02541
Opus 5	$0.00037	$0.01270
Sonnet 5	$0.00015	$0.00508
Haiku 4.5	$0.00007	$0.00254

Measured 8d ago against content hash 89d0a5866e71, method: parsed. Prices are Anthropic first-party input rates as of 2026-09-11, from the pricing page.

Security

Grade A, and why

skill-coach 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 8d ago.

The scan reads SKILL.md. This mod also ships 4 executable files (examples/good/clip-aware-embeddings/scripts/validate_clip_usage.py, scripts/check_self_contained.py, scripts/test_activation.py, …), listed below but not scanned — reading those needs a real analyzer, not pattern matching.

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/skills/skill-coach/SKILL.md · 308 lines

How it starts

The opening of the file, as written. The whole thing — 308 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Skill Coach: Creating Expert-Level Agent Skills

Encode real domain expertise, not just surface-level instructions. Focus on shibboleths - the deep knowledge that separates novices from experts.

When to Use This Skill

Use for:

Creating new Agent Skills from scratch
Reviewing/auditing existing skills
Improving skill activation rates
Adding domain expertise to skills
Debugging why skills don't activate

NOT for:

General Claude Code features (slash commands, MCPs)
Non-skill coding advice
Debugging runtime errors (use domain skills)

Quick Wins

Immediate improvements for existing skills:

Add NOT clause to description → Prevents false activation
Add 1-2 anti-patterns → Prevents common mistakes
Check line count (run validator) → Should be fewer than 500 lines
Remove dead files → Delete unreferenced scripts/references
Test activation → Questions that should/shouldn't trigger it

What Makes a Great Skill

Great skills are progressive disclosure machines that:

Activate precisely - Specific keywords + NOT clause
Encode shibboleths - Expert knowledge that separates novice from expert
Surface anti-patterns - "If you see X, that's wrong because Y, use Z"
Capture temporal knowledge - "Pre-2024: X. 2024+: Y"
Know their limits - "Use for A, B, C. NOT for D, E, F"
Provide decision trees - Not templates, but "If X then A, if Y then B"
Stay under 500 lines - Core in SKILL.md, deep dives in /references

Core Principles

Progressive Disclosure

Phase 1 (~100 tokens): Metadata - "Should I activate?"
Phase 2 (<5k tokens): SKILL.md - "How do I do this?"
Phase 3 (as needed): References - "Show me the details"

Critical: Keep SKILL.md under 500 lines. Split details into /references.

Description Formula

[What] [Use for] [Keywords] NOT for [Exclusions]

❌ Bad: "Helps with images"
⚠️ Better: "Image processing with CLIP"
✅ Good: "CLIP semantic search. Use for image-text matching.
   Activate on 'CLIP', 'embeddings'. NOT for counting, spatial reasoning."

Read the full file on GitHub · 308 lines

Files

What ships with it

17 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.

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.

8d ago First seen · 308 lines · 74 tokens per session scan A 89d0a5866e71

Subscribe to this mod's changes

skill-coach is a skill published in the GitHub repository curiositech/some_claude_skills (219 stars, last pushed 5d ago), licensed MIT. It adds 74 tokens to every session and 2,541 once invoked, about $0.0004 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 2026-09-03.

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