# Skill Authoring Best Practices

Source: [platform.claude.com/docs/en/agents-and-tools/agent-skills/best-practices](https://platform.claude.com/docs/en/agents-and-tools/agent-skills/best-practices)

## Core Principles

### Concise is Key

The context window is a public good. Your Skill shares the context window with everything else Claude needs to know.

**Default assumption**: Claude is already very smart. Only add context Claude doesn't already have.

Challenge each piece of information:
- "Does Claude really need this explanation?"
- "Can I assume Claude knows this?"
- "Does this paragraph justify its token cost?"

**Good example (concise, ~50 tokens):**
```markdown
## 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 example (too verbose, ~150 tokens):**
```markdown
## Extract PDF text

PDF (Portable Document Format) files are a common file format that contains
text, images, and other content. To extract text from a PDF, you'll need to
use a library. There are many libraries available...
```

### Set Appropriate Degrees of Freedom

Match specificity to task fragility and variability.

**High freedom** (multiple valid approaches):
```markdown
## Code review process

1. Analyze the code structure and organization
2. Check for potential bugs or edge cases
3. Suggest improvements for readability
4. Verify adherence to project conventions
```

**Medium freedom** (preferred pattern with variation):
```markdown
## Generate report

Use this template and customize as needed:

```python
def generate_report(data, format="markdown"):
    # Process data
    # Generate output in specified format
```
```

**Low freedom** (fragile, exact sequence required):
```markdown
## Database migration

Run exactly this script:

```bash
python scripts/migrate.py --verify --backup
```

Do not modify the command or add flags.
```

### Test With All Models

Skills act as additions to models. Test with Haiku, Sonnet, and Opus.

- **Haiku**: Does the Skill provide enough guidance?
- **Sonnet**: Is the Skill clear and efficient?
- **Opus**: Does the Skill avoid over-explaining?

## Naming Conventions

Use **gerund form** (verb + -ing) for Skill names:

**Good:**
- `processing-pdfs`
- `analyzing-spreadsheets`
- `managing-databases`
- `testing-code`
- `writing-documentation`

**Acceptable alternatives:**
- Noun phrases: `pdf-processing`, `spreadsheet-analysis`
- Action-oriented: `process-pdfs`, `analyze-spreadsheets`

**Avoid:**
- Vague: `helper`, `utils`, `tools`
- Generic: `documents`, `data`, `files`
- Reserved: `anthropic-*`, `claude-*`

## Writing Effective Descriptions

**Always write in third person.** The description is injected into the system prompt.

**Be specific and include key terms:**

```yaml
# PDF Processing skill
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.

# Excel Analysis skill
description: Analyze Excel spreadsheets, create pivot tables, generate charts. Use when analyzing Excel files, spreadsheets, tabular data, or .xlsx files.

# Git Commit Helper skill
description: Generate descriptive commit messages by analyzing git diffs. Use when the user asks for help writing commit messages or reviewing staged changes.
```

**Avoid vague descriptions:**
```yaml
description: Helps with documents  # Too vague!
description: Processes data       # Too generic!
description: Does stuff with files # Useless!
```

## Progressive Disclosure Patterns

### Pattern 1: High-level guide with references

```markdown
---
name: pdf-processing
description: Extracts text and tables from PDF files, fills forms, merges documents.
---

# PDF Processing

## Quick start

```python
import pdfplumber
with pdfplumber.open("file.pdf") as pdf:
    text = pdf.pages[0].extract_text()
```

## Advanced features

**Form filling**: See [FORMS.md](FORMS.md)
**API reference**: See [REFERENCE.md](REFERENCE.md)
**Examples**: See [EXAMPLES.md](EXAMPLES.md)
```

### Pattern 2: Domain-specific organization

```
bigquery-skill/
├── SKILL.md (overview and navigation)
└── reference/
    ├── finance.md (revenue, billing)
    ├── sales.md (opportunities, pipeline)
    ├── product.md (API usage, features)
    └── marketing.md (campaigns, attribution)
```

### Pattern 3: Conditional details

```markdown
# DOCX Processing

## Creating documents

Use docx-js for new documents. See [DOCX-JS.md](DOCX-JS.md).

## Editing documents

For simple edits, modify the XML directly.

**For tracked changes**: See [REDLINING.md](REDLINING.md)
**For OOXML details**: See [OOXML.md](OOXML.md)
```

## Keep References One Level Deep

Claude may partially read files when they're referenced from other referenced files.

**Bad (too deep):**
```markdown
# SKILL.md
See [advanced.md](advanced.md)...

# advanced.md
See [details.md](details.md)...

# details.md
Here's the actual information...
```

**Good (one level deep):**
```markdown
# SKILL.md

**Basic usage**: [in SKILL.md]
**Advanced features**: See [advanced.md](advanced.md)
**API reference**: See [reference.md](reference.md)
**Examples**: See [examples.md](examples.md)
```

## Workflows and Feedback Loops

### Workflow with Checklist

```markdown
## Research synthesis workflow

Copy this checklist:

```
- [ ] Step 1: Read all source documents
- [ ] Step 2: Identify key themes
- [ ] Step 3: Cross-reference claims
- [ ] Step 4: Create structured summary
- [ ] Step 5: Verify citations
```

**Step 1: Read all source documents**

Review each document in `sources/`. Note main arguments.
...
```

### Feedback Loop Pattern

```markdown
## Document editing process

1. Make your edits to `word/document.xml`
2. **Validate immediately**: `python scripts/validate.py unpacked_dir/`
3. If validation fails:
   - Review the error message
   - Fix the issues
   - Run validation again
4. **Only proceed when validation passes**
5. Rebuild: `python scripts/pack.py unpacked_dir/ output.docx`
```

## Common Patterns

### Template Pattern

```markdown
## Report structure

Use this template:

```markdown
# [Analysis Title]

## Executive summary
[One-paragraph overview]

## Key findings
- Finding 1 with supporting data
- Finding 2 with supporting data

## Recommendations
1. Specific actionable recommendation
2. Specific actionable recommendation
```
```

### Examples Pattern

```markdown
## Commit message format

**Example 1:**
Input: Added user authentication with JWT tokens
Output:
```
feat(auth): implement JWT-based authentication

Add login endpoint and token validation middleware
```

**Example 2:**
Input: Fixed bug where dates displayed incorrectly
Output:
```
fix(reports): correct date formatting in timezone conversion
```
```

### Conditional Workflow Pattern

```markdown
## Document modification

1. Determine the modification type:

   **Creating new content?** → Follow "Creation workflow"
   **Editing existing?** → Follow "Editing workflow"

2. Creation workflow:
   - Use docx-js library
   - Build document from scratch

3. Editing workflow:
   - Unpack existing document
   - Modify XML directly
   - Validate after each change
```

## Content Guidelines

### Avoid Time-Sensitive Information

**Bad:**
```markdown
If you're doing this before August 2025, use the old API.
```

**Good:**
```markdown
## Current method

Use the v2 API endpoint: `api.example.com/v2/messages`

## Old patterns

<details>
<summary>Legacy v1 API (deprecated 2025-08)</summary>
The v1 API used: `api.example.com/v1/messages`
</details>
```

### Use Consistent Terminology

**Good - Consistent:**
- Always "API endpoint"
- Always "field"
- Always "extract"

**Bad - Inconsistent:**
- Mix "API endpoint", "URL", "API route", "path"
- Mix "field", "box", "element", "control"

## Anti-Patterns to Avoid

### Windows-Style Paths

- **Good**: `scripts/helper.py`, `reference/guide.md`
- **Avoid**: `scripts\helper.py`, `reference\guide.md`

### Too Many Options

**Bad:**
```markdown
You can use pypdf, or pdfplumber, or PyMuPDF, or pdf2image, or...
```

**Good:**
```markdown
Use pdfplumber for text extraction:
```python
import pdfplumber
```

For scanned PDFs requiring OCR, use pdf2image with pytesseract instead.
```

## Checklist for Effective Skills

### Core Quality
- [ ] Description is specific and includes key terms
- [ ] Description includes both what and when
- [ ] SKILL.md body under 500 lines
- [ ] Additional details in separate files
- [ ] No time-sensitive information
- [ ] Consistent terminology
- [ ] Examples are concrete
- [ ] References one level deep
- [ ] Progressive disclosure used appropriately
- [ ] Workflows have clear steps

### Code and Scripts
- [ ] Scripts handle errors explicitly
- [ ] No "voodoo constants" (all values justified)
- [ ] Required packages listed
- [ ] Scripts have clear documentation
- [ ] No Windows-style paths
- [ ] Validation steps for critical operations
- [ ] Feedback loops for quality-critical tasks

### Testing
- [ ] At least three test scenarios
- [ ] Tested with Haiku, Sonnet, and Opus
- [ ] Tested with real usage scenarios
- [ ] Team feedback incorporated