# What Is the Role of SKILL.md in book-to-skill? Generator Spec Explained

> Understand SKILL.md's role in book-to-skill. This generator spec defines workflows, modes, and output structures for converting book content into Agent Skills.

- Repository: [Virgilio Junior/book-to-skill](https://github.com/virgiliojr94/book-to-skill)
- Tags: internals
- Published: 2026-08-27

---

**[`SKILL.md`](https://github.com/virgiliojr94/book-to-skill/blob/main/SKILL.md) serves as the generator specification that drives the agent-driven second half of the *book-to-skill* pipeline, defining workflows, operating modes, token budgets, and output file structures for converting extracted book content into fully-featured Agent Skills.**

The *book-to-skill* repository (`virgiliojr94/book-to-skill`) implements a two-stage architecture: a **deterministic Python extractor** ([`scripts/extract.py`](https://github.com/virgiliojr94/book-to-skill/blob/main/scripts/extract.py) → `book_to_skill/`) prepares clean text and metadata, then an **agent-driven generator** follows the instructions encoded in [`SKILL.md`](https://github.com/virgiliojr94/book-to-skill/blob/main/SKILL.md) to transform that raw extraction into a complete, loadable skill. This separation keeps extraction logic stable while allowing generation behavior to evolve through a spec-driven, reviewable document.

## Core Responsibilities Defined in SKILL.md

[`SKILL.md`](https://github.com/virgiliojr94/book-to-skill/blob/main/SKILL.md) acts as the **single source of truth** for the generator. According to the source code in `virgiliojr94/book-to-skill`, it specifies six critical areas:

### 1. Workflow Definition

The document enumerates a **ten-step process** that the generator executes sequentially. Starting from an out-of-scope check and input validation, it proceeds through content analysis, user interaction phases, file generation, and concludes with security scanning and cleanup. Step 9.5 invokes [`tools/scan_generated_skill.py`](https://github.com/virgiliojr94/book-to-skill/blob/main/tools/scan_generated_skill.py) for advisory security checks, while Step 10 handles final cleanup.

### 2. Mode Selection

[`SKILL.md`](https://github.com/virgiliojr94/book-to-skill/blob/main/SKILL.md) defines **four operating modes** with clear triggers:

- **Full Conversion** — complete book-to-skill transformation
- **Analyze-Only** — structure extraction without generation
- **Generate-from-Prior-Analysis** — skips extraction, uses cached analysis
- **Update/Fold-in** — incremental updates to existing skills

This mode system lets users re-run the pipeline at different stages without redundant processing.

### 3. User Interaction Protocol

Steps 0–4 establish a **REPL-style probing sequence** where the generator queries the user for:

- Content type classification
- Target purpose for the skill
- Desired depth level per chapter

These inputs shape how much detail each chapter summary receives, directly influencing the final skill's granularity.

### 4. Token Budget Control

Step 7 implements a **`BOOK_TYPE × DEPTH` matrix** that calculates per-chapter token allocations. This budget system ensures generated skills stay within cost limits while maintaining quality. The matrix scales allocation based on book complexity and user-selected depth preference.

### 5. File Generation Structure

[`SKILL.md`](https://github.com/virgiliojr94/book-to-skill/blob/main/SKILL.md) prescribes exact output formats:

- Individual **chapter files** with standardized naming ([`ch01-intro.md`](https://github.com/virgiliojr94/book-to-skill/blob/main/ch01-intro.md), etc.)
- **Glossary** for domain terminology
- **Patterns** document for recurring techniques
- **Cheatsheet** for quick reference
- **Master [`SKILL.md`](https://github.com/virgiliojr94/book-to-skill/blob/main/SKILL.md)** with front-matter, indexes, and core framework section (capped at ≤ 4,000 tokens)

### 6. Security and Validation

The specification mandates two quality gates before completion:

- **Step 9.5**: Security scan via [`tools/scan_generated_skill.py`](https://github.com/virgiliojr94/book-to-skill/blob/main/tools/scan_generated_skill.py)
- **Step 10**: Cleanup routine ensuring generated skills are safe to load in downstream agents

This validation chain prevents malformed or unsafe skills from entering the user's skill directory.

## How SKILL.md Fits the Architecture

The repository's design, documented in [`docs/architecture.md`](https://github.com/virgiliojr94/book-to-skill/blob/main/docs/architecture.md), maintains strict separation between stages:

| Stage | Component | Driven By | Output |
|-------|-----------|-----------|--------|
| Extraction | [`scripts/extract.py`](https://github.com/virgiliojr94/book-to-skill/blob/main/scripts/extract.py), `book_to_skill/` package | Deterministic Python code | Clean text, metadata JSON |
| Generation | Agent runtime | [`SKILL.md`](https://github.com/virgiliojr94/book-to-skill/blob/main/SKILL.md) spec | Complete Agent Skill directory |

This split enables **hot-swappable generation logic** — updating [`SKILL.md`](https://github.com/virgiliojr94/book-to-skill/blob/main/SKILL.md) immediately changes behavior without modifying the extractor's core parsing and sanitization code in `book_to_skill/`.

## Running the Pipeline

To execute the extractor and trigger the generator that reads [`SKILL.md`](https://github.com/virgiliojr94/book-to-skill/blob/main/SKILL.md):

```bash

# Locate the extractor shim across possible host paths

SCRIPT_PATH=$(for f in \
  "$HOME/.copilot/skills/book-to-skill/scripts/extract.py" \
  "$HOME/.agents/skills/book-to-skill/scripts/extract.py" \
  "$HOME/.claude/skills/book-to-skill/scripts/extract.py" \
  ".github/skills/book-to-skill/scripts/extract.py"; do
    [ -f "$f" ] && echo "$f" && break
done)

# Run extraction; generator follows SKILL.md internally

python3 "$SCRIPT_PATH" path/to/book.pdf

```

## Generated Skill Structure

After successful execution, the output directory follows the layout defined in [`SKILL.md`](https://github.com/virgiliojr94/book-to-skill/blob/main/SKILL.md):

```bash
$SKILLS_HOME/my-skill/
├── SKILL.md          # Core frameworks & indexes (≤ 4k tokens)

├── chapters/
│   ├── ch01-intro.md
│   └── ...
├── glossary.md
├── patterns.md
└── cheatsheet.md

```

## Key Source Files

- **[`SKILL.md`](https://github.com/virgiliojr94/book-to-skill/blob/main/SKILL.md)** — Generator specification [source](https://github.com/virgiliojr94/book-to-skill/blob/master/SKILL.md)
- **[`docs/architecture.md`](https://github.com/virgiliojr94/book-to-skill/blob/main/docs/architecture.md)** — Extractor/generator split diagram [source](https://github.com/virgiliojr94/book-to-skill/blob/master/docs/architecture.md)
- **[`scripts/extract.py`](https://github.com/virgiliojr94/book-to-skill/blob/main/scripts/extract.py)** — Extraction shim [source](https://github.com/virgiliojr94/book-to-skill/blob/master/scripts/extract.py)
- **[`tools/scan_generated_skill.py`](https://github.com/virgiliojr94/book-to-skill/blob/main/tools/scan_generated_skill.py)** — Security scanner invoked at Step 9.5 [source](https://github.com/virgiliojr94/book-to-skill/blob/master/tools/scan_generated_skill.py)
- **`book_to_skill/`** — Core extractor package [source](https://github.com/virgiliojr94/book-to-skill/tree/master/book_to_skill)

## Summary

- **[`SKILL.md`](https://github.com/virgiliojr94/book-to-skill/blob/main/SKILL.md) is the generator spec**, not the extractor — it governs how agent-driven generation behaves after deterministic extraction completes.
- It defines **ten sequential steps** from validation through security scanning, with four selectable operating modes for different use cases.
- **Token budgeting** via the `BOOK_TYPE × DEPTH` matrix controls costs while preserving quality.
- The specification mandates **specific output structures**: chapter files, glossary, patterns, cheatsheet, and a master [`SKILL.md`](https://github.com/virgiliojr94/book-to-skill/blob/main/SKILL.md) capped at 4,000 tokens.
- **Security validation** through [`tools/scan_generated_skill.py`](https://github.com/virgiliojr94/book-to-skill/blob/main/tools/scan_generated_skill.py) is hardcoded at Step 9.5.
- This architecture enables **spec-driven evolution** — modify [`SKILL.md`](https://github.com/virgiliojr94/book-to-skill/blob/main/SKILL.md) to change generation without touching `book_to_skill/` extraction code.

## Frequently Asked Questions

### What happens if I modify SKILL.md?

Changes take effect immediately on the next generator run. The extractor phase remains unaffected since it operates from hardcoded Python logic in `book_to_skill/`. This design lets you experiment with generation strategies, adjust token budgets, or add new operating modes without risking extraction stability.

### Can I run the generator without the extraction step?

Yes. [`SKILL.md`](https://github.com/virgiliojr94/book-to-skill/blob/main/SKILL.md) defines the **Generate-from-Prior-Analysis** mode specifically for this workflow. If you've cached analysis output from a previous run, the generator skips the extractor entirely and proceeds directly to skill construction using stored metadata.

### How does book-to-skill prevent overspending on API tokens?

Step 7's **`BOOK_TYPE × DEPTH` matrix** calculates per-chapter budgets before any generation begins. The generator requests user confirmation at Step 2.5 (Pre-flight cost estimate) when projections exceed thresholds, and the hard ceiling on master [`SKILL.md`](https://github.com/virgiliojr94/book-to-skill/blob/main/SKILL.md) length (≤ 4,000 tokens) caps final output size regardless of input complexity.

### Where does the final generated skill live?

Output location depends on your configured `SKILLS_HOME` environment variable. The generator creates a subdirectory matching your skill name, populating it with the structured file layout defined in [`SKILL.md`](https://github.com/virgiliojr94/book-to-skill/blob/main/SKILL.md) Steps 6–9. Standard paths include `$HOME/.copilot/skills/`, `$HOME/.agents/skills/`, or `$HOME/.claude/skills/` depending on your agent runtime.