skills-best-practices

What this skill does

# Skills Best Practices

Comprehensive reference for building Agent Skills that follow Anthropic's official guidelines. Skills are folders containing instructions, scripts, and resources that teach Claude how to handle specific tasks. They follow the [Agent Skills open standard](https://agentskills.io).

## Quick Start

A minimal skill is a directory with a `SKILL.md` file:

```
my-skill/
├── SKILL.md          # Required - instructions with YAML frontmatter
├── references/       # Optional - detailed docs loaded on demand
├── scripts/          # Optional - executable code
└── assets/           # Optional - templates, fonts, icons
```

Minimal `SKILL.md`:

```yaml
---
name: my-skill-name
description: What it does. Use when [specific triggers].
---

# My Skill Name

[Instructions here]
```

Only `name` and `description` are required in frontmatter.

## Core Design Principles

### Progressive Disclosure (Most Important)

Skills load information in three levels to minimize token usage:

| Level | When Loaded | Token Cost | Content |
|-------|------------|------------|---------|
| **1: Metadata** | Always (startup) | ~100 tokens | `name` + `description` from frontmatter |
| **2: Instructions** | When skill triggers | <5k tokens (recommended) | SKILL.md body |
| **3: Resources** | As needed | Effectively unlimited | Bundled files, scripts |

Keep SKILL.md under **500 lines**. Move detailed docs to separate files and reference them:

```markdown
## Advanced features
- **Form filling**: See [FORMS.md](FORMS.md)
- **API reference**: See [reference.md](reference.md)
```

Claude reads referenced files only when the task requires them.

### Composability

Skills work alongside other skills. Don't assume yours is the only one loaded.

### Portability

Skills work across Claude.ai, Claude Code, API, and Agent SDK without modification (if dependencies are available).

## Writing the Description (Critical)

The description is the **single most important field** - it determines when your skill activates. Claude uses it to decide relevance from potentially 100+ available skills.

### Rules

- Write in **third person** ("Processes files..." not "I help you process files...")
- Include **WHAT** it does + **WHEN** to use it
- Max 1024 characters, no XML angle brackets
- Be slightly "pushy" - Claude tends to **undertrigger** rather than overtrigger
- Include specific trigger phrases users would naturally say

### Good vs Bad

```yaml
# GOOD - specific, actionable, includes triggers
description: Extract text and tables from PDF files, fill forms, merge
  documents. Use when working with PDF files or when the user mentions
  PDFs, forms, or document extraction.

# BAD - too vague
description: Helps with documents.

# BAD - missing triggers
description: Creates sophisticated multi-page documentation systems.
```

More examples in [references/description-guide.md](references/description-guide.md).

## Frontmatter Reference

### Required Fields

| Field | Rules |
|-------|-------|
| `name` | Kebab-case, max 64 chars, lowercase + numbers + hyphens only. No "claude" or "anthropic" |
| `description` | Non-empty, max 1024 chars, no XML tags. WHAT + WHEN |

The agentskills.io standard and the Claude API require both fields. Claude Code is more lenient: `name` falls back to the directory name, and `description` falls back to the first markdown paragraph. Write both anyway for portability.

### Optional Fields (Claude Code)

| Field | Purpose |
|-------|---------|
| `argument-hint` | Autocomplete hint, e.g. `[issue-number]` |
| `when_to_use` | Extra trigger context, appended to `description` in the skill listing |
| `arguments` | Named positional arguments for `$name` substitution (space-separated string or list) |
| `disable-model-invocation` | `true` = only user can invoke (for deploy, commit) |
| `user-invocable` | `false` = hidden from `/` menu (background knowledge) |
| `allowed-tools` | Pre-approves tools (no permission prompt); space-separated, e.g. `Read Grep Glob` |
| `model` | Override model for this skill; accepts `inherit`. Lasts the current turn only |
| `effort` | Override effort level: `low`, `medium`, `high`, `xhigh`, `max` |
| `context` | `fork` = run in isolated subagent |
| `agent` | Subagent type when `context: fork` (e.g. `Explore`, `Plan`) |
| `hooks` | Hooks scoped to this skill's lifecycle |
| `paths` | Glob patterns limiting when skill activates |

> **Publishing caveat:** every field above except `allowed-tools` is Claude Code-specific. They work in Claude Code at runtime, but the **official `agentskills validate` spec validator rejects them** - it allows only `name`, `description`, `license`, `compatibility`, `metadata`, `allowed-tools`, with no relax flag. If your repo or CI runs that validator (most ClawHub-publishing repos do), a skill using these fields fails validation unless you strip them from the copy you validate/publish. The ClawHub registry itself tends to tolerate extra top-level fields on publish, but the reference validator in your pipeline will not. See [Validate Against the Spec](#validate-against-the-spec).

### Naming Conventions

The `name` (and its folder) must: be 1-64 chars; use only lowercase letters, numbers, and hyphens; not start or end with a hyphen; not contain consecutive hyphens (`--`); and match the parent directory name. Anthropic surfaces also reject the reserved words `claude` and `anthropic`.

Prefer **gerund form** for clarity:

- `processing-pdfs`, `analyzing-spreadsheets`, `managing-databases`
- Also acceptable: `pdf-processing`, `process-pdfs`
- Avoid: `helper`, `utils`, `tools`, `documents`

## Structuring Instructions

### Be Concise

Claude is smart. Only add context it doesn't already have:

```markdown
# GOOD (~50 tokens)
## Extract PDF text
Use pdfplumber for text extraction:
```python
import pdfplumber
with pdfplumber.open("file.pdf") as pdf:
    text = pdf.pages[0].extract_text()
```

# BAD (~150 tokens)
## Extract PDF text
PDF files are a common file format containing text and images.
To extract text, you need a library. There are many available...
```

### Avoid Too Many Options

Don't present multiple approaches unless necessary. Give one default with an escape hatch:

```markdown
# BAD: "Use pypdf, or pdfplumber, or PyMuPDF, or pdf2image..."
# GOOD: "Use pdfplumber for text extraction. For scanned PDFs needing
#        OCR, use pdf2image with pytesseract instead."
```

### Set Degrees of Freedom

- **High freedom** (text guidelines): Multiple approaches valid, context-dependent
- **Medium freedom** (pseudocode/templates): Preferred pattern exists, some variation OK
- **Low freedom** (exact scripts): Operations are fragile, consistency critical

### Recommended SKILL.md Structure

```markdown
# Skill Name

## Quick start
[Minimal working example]

## Workflow Decision Tree
[Route to the right approach based on task type]

## Detailed Instructions
[Step-by-step for each workflow]

## Examples
[Concrete input/output pairs]

## Troubleshooting
[Common errors and fixes]
```

### Reference Files

Keep references **one level deep** from SKILL.md. "Depth" means the reference *chain* (a file linking to a file linking to a file), not filesystem nesting - a `references/` subdirectory is fine. In a chain, Claude may preview files with partial reads (`head`) and miss content.

```markdown
# BAD: Too deep
SKILL.md -> advanced.md -> details.md -> actual info

# GOOD: One level
SKILL.md -> advanced.md (contains the info directly)
SKILL.md -> reference.md (contains the info directly)
```

For reference files >100 lines, include a **table of contents** at the top. Watch file *size* too: a single reference of many hundreds of lines defeats progressive disclosure even at one level deep, because Claude loads the whole file for any subtopic. Split large references by subtopic so each task pulls only what it needs.

## Patterns

### Sequential Workflow

```markdown
## Step 1: Analyze input
Run: `python scripts/analyze.py input