Effective agent skills

Agent Skills: A Complete Guide

When to Use

• Use when creating, editing, reviewing, or debugging an agent SKILL.md file.
• Use when you need quality guidance for triggers, examples, limitations, and safety notes.

A consolidated reference on what agent skills are, why they exist, how they work, and how to write effective ones.

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1. What agent skills are

An Agent Skill is a folder containing a SKILL.md file (YAML frontmatter + markdown instructions), plus optional subfolders for scripts, references, and assets that the agent loads on demand.

my-skill/
├── SKILL.md          # Required: metadata + instructions
├── scripts/          # Optional: executable code (CLIs, validators, helpers)
├── references/       # Optional: detailed docs loaded only when needed
└── assets/           # Optional: templates, fonts, static files

Skills are an open standard (agentskills.io), originally created by Anthropic and adopted by OpenAI Codex, Cursor, Gemini CLI, Microsoft Agent Framework, Google ADK, and 40+ other agent products. A skill written once works across all compatible agents.

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2. Why this abstraction exists

Base LLMs are generalists. Real work requires procedural knowledge, organizational context, and repeatable workflows. Every prior alternative had a failure mode:

Approach	Problem
Stuff it into the system prompt	Always loaded → context bloat at scale
Re-paste instructions each session	No version control, no consistency
Fine-tuning	Slow, expensive, opaque, vendor-locked
MCP servers alone	Give the agent tools but no workflows for using them

Skills solve four problems at once:

• Context efficiency — instructions load only when relevant
• Repeatability — multi-step procedures become auditable workflows
• Composability — multiple skills combine at runtime per task
• Portability — same files work across vendors and surfaces

Mental model: skills are to LLMs what man pages, runbooks, and team handbooks are to engineers — reference material loaded into working memory only when the task demands it.

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3. How they work — progressive disclosure

The architectural core. Three-stage loading:

Level 1 — Discovery (~100 tokens per skill, always in context): Only name + description from frontmatter are injected into the system prompt at startup. Agent knows the skill exists and when it applies. You can install dozens of skills with negligible overhead.

Level 2 — Activation (<5,000 tokens, loaded on match): When the user's request matches a skill's description, the agent reads the full SKILL.md body into context.

Level 3 — Execution (unbounded, on demand): The agent reads referenced files (references/foo.md) or runs scripts (scripts/validate.py) only as needed. Scripts can execute without their source being loaded into context at all.

This is why bundled content has no practical limit. Files don't consume tokens until accessed.

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4. SKILL.md anatomy

---
name: skill-name
description: What this skill does AND when to use it. Include trigger phrases the user will say.
---

# Skill Name

## Quick start
[Minimal working example]

## Workflow
[Step-by-step procedure with checklists]

## Output format
[What the user/agent should expect back]

## Advanced
[Link to references/ for rarely-needed detail]

Frontmatter constraints:

• name is lowercase, hyphens only, 1–64 chars, exactly matches the parent folder name
• Avoid < and > in frontmatter (they can inject into the system prompt)
• Invalid YAML silently prevents loading

Optional standard fields:

• disable-model-invocation: true — stops the agent from auto-loading the skill based on the conversation; it can only be triggered manually (e.g. /skill-name). Now a standard Agent Skills spec field, so it works across spec-compliant clients (Claude Code, Copilot, etc.), not just Claude. Caveat: it prevents auto-invocation, but some clients (Claude Code, open bug) still inject the description into context, so it doesn't always save the discovery-level tokens. Use for manual-only utilities you don't want firing automatically.

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5. Two design philosophies

Skills tend to fall into one of two patterns. Both are valid; they solve different problems.

Pattern A — Capability primitives (tool wrappers)

The skill is a thin wrapper over a deterministic CLI or script. Logic lives in code. SKILL.md teaches the agent how to invoke it.

• Adds: new capabilities (search, email, browser, API access)
• Reliability via: shell tools, not prompts
• Typical length: 30–80 lines, mostly command examples
• Use when: the bottleneck is "the agent can't do X"

Pattern B — Process primitives (cognitive disciplines)

The skill encodes a methodology the agent should follow. Pure prompt engineering — no scripts needed.

• Adds: structured workflows (TDD, code review, design alignment, debugging loops)
• Reliability via: exp