# BRA103: What Is a Skill Book https://www.telosbrain.com/skills/BRA103?v=1 What a Skill Book is — a self-contained folder of categorised skills managed by skillbook.yml, portable between brains, with codes that let skills reference each other for progressive disclosure. Use when explaining skill books, placing a skill in a book, or deciding what belongs in skill content. ## Instructions A Skill Book is a named collection of skills organised into categories. Categories are the information architecture of a Skill Book — they define what kinds of skills belong in the book and how they are grouped. Each category covers a numeric range (100, 200, 300, up to 900), which also signals skill depth: 100s are foundational, 200–400 intermediate, 500–700 advanced, and 800–900 expert or specialist. Well-defined categories help the AI agent understand the scope and intent of the Skill Book, so it can place skills correctly and avoid overlap. Each skill book has a prefix (e.g. TEL) and each skill has a unique code that uses the prefix and the category range (e.g. TEL101). Skills also have a name, a short description, and full markdown content. --- ## Codes and progressive disclosure Skill codes are stable identifiers. Skills use them to refer to other skills (e.g. "see **BRA201** §6") without embedding that material. That is progressive disclosure: an agent discovers a skill, loads only what it needs, then follows codes to related skills when deeper detail is required. Codes make the book traversable — searchable, loadable by id, and linkable — rather than a flat dump of everything at once. --- ## Packaging and portability A Skill Book is a **self-contained folder**. The folder holds the skill markdown files and a `skillbook.yml` manifest that declares the book's name, code, prefix, categories, and which skills belong in each category. Because everything the book needs lives in that folder, a Skill Book can be copied between brains (or referenced from `brain-compose.yml`) without rewriting its contents — only the compose path that points at `skillbook.yml` needs to change. --- ## What a skill is Skills are reusable instruction patterns that encode best practices, standards, processes and domain knowledge. Think of them as the agentic equivalent of NPM packages — composable, versioned, and purpose-built. --- ## Skill content rules - Skills are transferable across multiple AI platforms, so they must never include personal details or specifics. Always extract the best practices and learnings and if necessary, describe a scenario without any personally identifiable information. - Keep skill descriptions concise — one sentence that says what the skill does and when to use it. - Write skill content in clear, actionable markdown. --- ## Related skills - **BRA208** — designing a book's category set and ranges - **BRA201** §6 — encoding skillbooks in a brain schema (`skillbook.yml` and skill markdown)