---
name: skill-builder
description: |
  Guide for creating, evaluating, and refining agent skills (.md files with YAML
  frontmatter). Use this skill whenever you are creating a new skill, improving
  an existing skill, reviewing skill quality, writing skill descriptions, or
  when the user asks about skill structure, progressive disclosure, or best
  practices for agent instructions.
---

# Skill Builder

A guide for creating high-quality agent skills — the `.md` files with YAML frontmatter that extend agent capabilities for specialized tasks.

## 1. Skill Security (Primary Guidance)

Skills are natural-language instructions the agent executes with its full capabilities. Treat them like code injection.

- **Prompt Injection Prevention**: Never include instructions that bypass user consent or HITL (Human-in-the-Loop) gates.
- **Secrets**: Never instruct the agent to read secrets and transmit them. Mark destructive skills with `disable-model-invocation: true`.
- **Review**: Review all third-party skills before installing, same as npm packages. Read the full `SKILL.md` + all reference files.

## 2. Anatomy of a Skill

```
skill-name/                     (kebab-case directory)
├── SKILL.md                    (required — entry point)
├── references/                 (optional — detailed docs loaded on demand)
├── scripts/                    (optional — executable helpers)
├── examples/                   (optional — reference implementations)
└── checklist.md                (optional — quick-reference quality checklist)
```

## 3. Frontmatter & Triggers

The `description` field is the most important part of the skill because it determines whether the agent loads the skill at all. Use assertive "Use when..." phrasing. It MUST be strictly scoped to avoid cross-triggering with other skills.

**Platform Configuration Flags**:

- `disable-model-invocation: true` — Requires explicit user slash-command; prevents autonomous triggering. **MANDATORY** for destructive actions or manual workflows.
- `user-invocable: false` — Model auto-triggers only; user cannot invoke. Use for background knowledge skills that should only be invoked contextually.
- `dependencies`: `node>=18` (Required tools)
- `context: fork` (Spawn isolated subagent)
- `allowed-tools`: `['view_file']` (Restrict tool access)

## 4. Progressive Disclosure

- Keep `SKILL.md` body STRICTLY under 500 lines to preserve token context.
- For deep knowledge, create `references/` files and explicitly list them in `SKILL.md`.

## 5. Deep References

For the complete lifecycle (Capture, Write, Test, Iterate), writing style tips, and detailed rubrics:

- **[Full Skill Builder Tutorial & Rubric](references/tutorial.md)**
- **[Quality Checklist](checklist.md)**