# What Is the Purpose of the `validate_skill.py` Script? A Complete Guide

> Discover the purpose of validate_skill.py. This guide explains how this utility validates your SKILL.md file for AI lens compliance, ensuring it meets Agent Skills specification and host-specific rules.

- Repository: [Virgilio Junior/book-to-skill](https://github.com/virgiliojr94/book-to-skill)
- Tags: how-to-guide
- Published: 2026-09-01

---

**[`validate_skill.py`](https://github.com/virgiliojr94/book-to-skill/blob/main/validate_skill.py) is a linting and validation utility that checks a repository's [`SKILL.md`](https://github.com/virgiliojr94/book-to-skill/blob/main/SKILL.md) file for compliance with the formal Agent Skills specification and host-specific rules for AI lenses including Claude, GitHub Copilot CLI, Sourcegraph Amp, and Hermes Agent.**

The [`validate_skill.py`](https://github.com/virgiliojr94/book-to-skill/blob/main/validate_skill.py) script serves as the quality gate for the `virgiliojr94/book-to-skill` repository, ensuring that skill definitions remain portable, correctly structured, and compatible with multiple AI agent platforms. By validating YAML front-matter, enforcing field requirements, and checking tool declarations against known lens capabilities, it prevents malformed skills from entering the codebase.

## How [`validate_skill.py`](https://github.com/virgiliojr94/book-to-skill/blob/main/validate_skill.py) Validates Skill Definitions

The script operates as a standalone Python utility located at [`tools/validate_skill.py`](https://github.com/virgiliojr94/book-to-skill/blob/main/tools/validate_skill.py). It requires no external dependencies, making it ideal for continuous integration environments.

### Command-Line Interface and Basic Usage

The script accepts two primary arguments: the path to a skill file and the target lens for validation.

```bash

# Validate with default Claude lens

python3 tools/validate_skill.py

# Validate against GitHub Copilot CLI rules

python3 tools/validate_skill.py --lens copilot

# Validate a custom skill file with Amp lens

python3 tools/validate_skill.py path/to/CustomSkill.md --lens amp

```

The default lens is **Claude**, with `--lens` supporting: `claude`, `copilot`, `amp`, and `hermes`.

## Core Validation Architecture

Understanding the internal structure of [`validate_skill.py`](https://github.com/virgiliojr94/book-to-skill/blob/main/validate_skill.py) helps explain its comprehensive checking capabilities.

### Lens Definitions and Platform-Specific Rules

The script encodes platform requirements in the `LENSES` dictionary (lines 58-86). Each lens supplies:

- Recognized built-in tools
- Allowed front-matter keys
- Reserved words that cannot appear in skill names
- Severity levels for different violation types

This architecture enables **multi-platform validation** without maintaining separate scripts per target.

### Front-Matter Parsing and Field Extraction

The script implements specialized utilities for processing Markdown with YAML front-matter:

- `parse_frontmatter()` (lines 9-15) — isolates the YAML block from the body content
- `get_scalar()` and `get_list_items()` (lines 18-35) — extract scalar values and list items respectively

These functions enable robust handling of the [`SKILL.md`](https://github.com/virgiliojr94/book-to-skill/blob/main/SKILL.md) structure regardless of minor formatting variations.

### The `audit()` Function: Core Validation Logic

The `audit()` function orchestrates all validation checks. Here are the specific validations performed:

**Name Field Validation (lines 59-73)**
- Presence check (`name` is required)
- Length limits enforcement
- Pattern matching against allowed characters
- Reserved word restrictions (lens-specific prohibited terms)

**Description Validation (lines 74-85)**
- Presence requirement
- Hard length limits
- Optional soft-limit warnings for readability

**Allowed-Tools Analysis (lines 86-117)**
The most complex validation checks that declared tools match the lens's known capabilities:

- Validates each tool in `allowed-tools` against the lens's built-in set
- Detects Bash usage patterns when shell execution is declared
- Flags unknown or disallowed tool tokens
- Reports tools that may behave differently across lenses

**Additional Quality Checks (lines 118-125)**
- Unrecognized top-level front-matter keys
- Body line count warnings for oversized skills

### UTF-8 Output Handling

The script forces UTF-8 encoding on stdout/stderr (lines 35-42) to ensure check-mark glyphs (✓) and cross marks (✗) render correctly across all terminal environments.

## CI/CD Integration and Exit Behavior

[`validate_skill.py`](https://github.com/virgiliojr94/book-to-skill/blob/main/validate_skill.py) is designed as a **continuous integration gate**. It exits with non-zero status when ERROR conditions are detected, halting merges that would introduce incompatible skills.

### GitHub Actions Example

```yaml
- name: Validate skill across all lenses
  run: |
    python3 tools/validate_skill.py --lens ${{ matrix.lens }}
  env:
    matrix:
      lens: [claude, copilot, amp, hermes]

```

This matrix strategy ensures a single [`SKILL.md`](https://github.com/virgiliojr94/book-to-skill/blob/main/SKILL.md) complies with **all four supported platforms** before approval.

### Result Reporting Format

Validation results use consistent prefixes for machine parsing:

- `ERROR:` — blocking violations that cause non-zero exit
- `WARN:` — advisory issues that do not block
- Final status line with ✓ or ✗ emoji (lines 128-146)

## Skill Validation in the Repository Workflow

The script sits at a critical point in the `book-to-skill` architecture. According to [`AGENTS.md`](https://github.com/virgiliojr94/book-to-skill/blob/main/AGENTS.md) and [`CONTRIBUTING.md`](https://github.com/virgiliojr94/book-to-skill/blob/main/CONTRIBUTING.md), validation occurs after skill authoring and before any generation or extraction operations.

The validation step precedes:
- Content extraction from source books
- Skill generation for target platforms
- Repository contributions and pull requests

This ordering ensures that **invalid skills never propagate** to downstream processing stages.

## Summary

- **[`validate_skill.py`](https://github.com/virgiliojr94/book-to-skill/blob/main/validate_skill.py)** is a self-contained, dependency-free validation utility for [`SKILL.md`](https://github.com/virgiliojr94/book-to-skill/blob/main/SKILL.md) files
- Supports **four AI lenses**: Claude (default), GitHub Copilot CLI, Sourcegraph Amp, and Hermes Agent
- Validates **required fields**: `name`, `description`, and `allowed-tools` with lens-specific rules
- Enforces **quality constraints**: length limits, reserved words, unknown keys, and body size
- Designed for **CI integration**: non-zero exit on ERROR conditions enables automated blocking
- Located at [`tools/validate_skill.py`](https://github.com/virgiliojr94/book-to-skill/blob/main/tools/validate_skill.py) with no external dependencies beyond Python 3

## Frequently Asked Questions

### What file does [`validate_skill.py`](https://github.com/virgiliojr94/book-to-skill/blob/main/validate_skill.py) check by default?

By default, [`validate_skill.py`](https://github.com/virgiliojr94/book-to-skill/blob/main/validate_skill.py) validates [`SKILL.md`](https://github.com/virgiliojr94/book-to-skill/blob/main/SKILL.md) in the current working directory. You can specify an alternative path as the first positional argument, such as `python3 tools/validate_skill.py path/to/AnotherSkill.md`.

### Why does the script exit with non-zero status?

The non-zero exit code occurs only when **ERROR** severity violations are found. This design enables the script to function as a CI gate—failing builds before malformed skills merge into the repository. Warnings alone do not trigger failure.

### What happens if I declare a tool not supported by my target lens?

The `audit()` function's allowed-tools analysis (lines 86-117) flags unknown tool tokens as errors or warnings depending on lens configuration. For maximum portability, skills should restrict themselves to the intersection of tools supported across all target lenses.

### Can I run validation without installing dependencies?

Yes. [`validate_skill.py`](https://github.com/virgiliojr94/book-to-skill/blob/main/validate_skill.py) uses only Python 3 standard library modules. It requires no `pip install` step, making it safe to run in minimal CI containers and fresh clones of the `virgiliojr94/book-to-skill` repository.