Skill Authoring Guide

Size Budget (ADR-001)

SKILL.md files must stay under 100 lines. If content exceeds this, split into reference files in a references/ subdirectory.

• Reference files use disable-model-invocation: true in frontmatter — they are data, not behavioral prompts
• SKILL.md is always loaded when the skill is active; reference files are loaded on demand
• Existing skills exceeding the limit should be refactored when touched, not proactively

Frontmatter

Required fields (enforced by tests/validate-frontmatter.test.ts):

Optional fields:

Description Format

• Max 1024 characters
• Third person, not imperative (the description talks about the skill, not to the user)
• Must include "Use when [trigger condition]" — this guides the auto-triggering system
• Describe what the skill enforces or enables, not just what it is

Good: Use when implementing any feature or bugfix, before writing implementation code — enforces red-green-refactor cycle with configurable strictness

Bad: TDD skill for testing (no trigger, no enforcement description)

File Structure

skills/<name>/
  SKILL.md              — Main file (always loaded, under 100 lines)
  references/           — Optional, lazy-loaded sub-topic files
    example.md          — Data tables, examples, checklists

• Cross-references use root-relative paths: skills/git-practices/SKILL.md
• Never use .claude/skills/... or relative ../ paths

Progressive Disclosure

SKILL.md contains behavioral rules — the instructions the AI must follow when the skill is active. Keep it focused: conditions, rules, verification steps.

Reference files contain supporting data — comparison tables, example lists, rationalization catalogs, detailed checklists. They are read only when the specific sub-topic is relevant.

This split is how you stay under 100 lines without losing depth. See skills/test-driven-development/ for the model: SKILL.md at 98 lines, with references/test-quality.md and references/rationalizations.md loaded on demand.

Scripts

Scripts are for deterministic operations only: file manipulation, formatting, validation, template generation. Never use scripts for AI judgment calls.

• Skill-specific scripts go in the skill directory
• Shared scripts go in scripts/ at the repo root

Review Checklist

Before merging a new or modified skill, verify:

• SKILL.md is under 100 lines
• Frontmatter has name and description
• Description is under 1024 chars, third person, includes "Use when [trigger]"
• Cross-references use root-relative paths
• Reference files (if any) have disable-model-invocation: true in frontmatter
• Content split: behavioral rules in SKILL.md, data/examples in reference files
• npm test passes (frontmatter validation, file reference validation)