Writing Skills

Scope: covers SKILL.md authoring. For agent writing, see [[writing-agents]]. For plugin architecture, see [[writing-plugins]].

1. The Description is Everything

The description field determines when Claude loads this skill. It is not a summary -- it is a trigger mechanism.

Bad (score 55):

description: "Helpful skill for React development"

Good (score 95):

description: "Use when building React components, debugging re-renders, optimizing performance with useMemo/useCallback, or fixing hook dependency arrays"

Description Checklist

Criterion	Test
3+ trigger phrases	Count distinct action phrases separated by commas
Action-oriented	Starts with "Use when..." or "How to..."
Includes tool/framework name	The technology name appears explicitly
Matches real queries	Would a user's actual question contain any of your trigger words?

Trigger Phrase Construction

Map real user queries to trigger phrases:

User says	Trigger phrase to include
"My component re-renders too much"	"debugging re-renders"
"How do I memoize this?"	"optimizing performance with useMemo/useCallback"
"My useEffect runs in a loop"	"fixing hook dependency arrays"
"I need a new component for..."	"building React components"

Rule: if you can't list 3 real user queries that match your description, rewrite it.

2. Body Structure

Section Order

1. Scope note (if related skills exist) -- tells Claude when to use THIS skill vs another
2. Most commonly needed patterns -- what users ask about 80% of the time
3. Decision matrices -- when to use A vs B (tables)
4. Worked examples -- before/after with scores
5. Common mistakes -- anti-patterns to avoid
6. References -- links to deep dives

Heading Rules

• H1 (#): skill title only (one per file)
• H2 (##): major sections (numbered: ## 1. Section Name)
• H3 (###): subsections within a major section
• Never skip heading levels (no H2 followed directly by H4)

Code Examples

Every code example must show the problem, then the solution:

### Bad (breaks on concurrent requests)
` ` `python
global_state = {}  # shared mutable state
` ` `

### Good (request-scoped)
` ` `python
def handler(request):
    state = {}  # local to this request
` ` `

Rules for code examples:

• Runnable, not pseudocode
• Contextual -- show enough surrounding code to understand placement
• Annotated -- comment the critical line, not every line

3. Progressive Disclosure

Keep SKILL.md under 500 lines. Use the file system for depth:

skills/my-domain/my-skill/
  SKILL.md          # core patterns (< 500 lines)
  references/       # deep dives, edge cases, full API docs
    advanced.md
    api-reference.md
  examples/         # working code samples
    basic-setup.ts
    advanced-config.ts
  scripts/          # utility scripts
    validate.sh

When to Extract to references/

Content type	Keep in SKILL.md?	Extract to references/?
Top 5 patterns everyone needs	Yes	No
Full API reference (50+ entries)	No	Yes
Edge cases (< 5% of users)	No	Yes
Configuration matrix (20+ options)	No	Yes
Quick decision table (< 10 rows)	Yes	No

4. Worked Example: Improving a Skill

Before (score 55/100)

---
name: docker-helper
description: "Information about Docker"
version: 0.1.0
---

# Docker Helper

Docker is a containerization platform. Here are some useful commands.

## Commands
- `docker build` - builds an image
- `docker run` - runs a container
- `docker ps` - lists containers

## Dockerfile
A Dockerfile contains instructions for building an image.

## Docker Compose
Docker Compose is for multi-container applications.

Problems:

• Description is a label, not a trigger mechanism (0 trigger phrases)
• Body teaches theory Claude already knows from training
• No code examples showing problem/solution
• No decision matrices
• No scope note

After (score 92/100)

---
name: docker-helper
description: "Use when writing Dockerfiles, debugging container networking, optimizing image size with multi-stage builds, or configuring Docker Compose services. Covers build cache, volume mounts, health checks, and compose profiles."
version: 0.1.0
---

# Docker Helper

> Scope: covers Docker CLI, Dockerfiles, and Compose. For Kubernetes deployment, see [[k8s-deploy]].

## 1. Image Size Optimization

### Before (1.2 GB)
` ` `dockerfile
FROM node:20
COPY . .
RUN npm install
RUN npm run build
CMD ["node", "dist/index.js"]
` ` `

### After (148 MB -- 88% smaller)
` ` `dockerfile
FROM node:20-slim AS build
WORKDIR /app
COPY package*.json ./
RUN npm ci --only=production
COPY . .
RUN npm run build

FROM node:20-slim
WORKDIR /app
COPY --from=build /app/dist ./dist
COPY --from=build /app/node_modules ./node_modules
CMD ["node", "dist/index.js"]
` ` `

Key changes: multi-stage build, slim base, copy only production artifacts.

## 2. Base Image Selection

| Use case | Base image | Size |
|----------|-----------|------|
| Node.js production | node:20-slim | 180 MB |
| Node.js minimal | node:20-alpine | 130 MB |
| Python production | python:3.12-slim | 150 MB |
| Static files only | nginx:alpine | 40 MB |
| From scratch (Go/Rust) | scratch | < 20 MB |

Changes made:

1. Description: 0 trigger phrases -> 8 trigger phrases (+37 points)
2. Scope note added (+5 points)
3. Replaced theory with problem/solution examples (+25 points)
4. Added decision matrix (+10 points)
5. Removed content Claude knows from training (-15 lines, +5 points for conciseness)

5. Common Mistakes

Mistake	Why it hurts	Fix
Description is a feature list	Claude can't match user queries to the skill	Rewrite as trigger phrases starting with "Use when..."
Body teaches theory	Wastes tokens on content Claude already knows	Show patterns and decisions, not definitions
Over 500 lines	Bloats context window, penalized by linters	Extract to references/, keep core patterns only
No scope note	Claude doesn't know when to use THIS skill vs a related one	Add scope note referencing related skills
Pseudocode examples	Not actionable, can't be copy-pasted	Use runnable code with enough context
Every section has equal weight	Buries the most useful content	Lead with the 80% patterns, push edge cases to references/

6. Quality Checklist

Before shipping a skill, verify:

• Description has 3+ specific trigger phrases
• Description starts with "Use when..." or "How to..."
• Scope note present (if related skills exist)
• H2 sections numbered, H3 subsections under them
• Code examples show problem then solution
• Decision tables for A-vs-B choices
• Total lines < 500
• No content Claude already knows from training
• At least one worked example with before/after