Check reference / Correctness & Structure
STRUCT-01 Valid YAML frontmatter
Deterministic — runs on every scan, no LLM or network needed.
Why it matters
If the frontmatter is missing or fails to parse as YAML, Claude Code cannot read the skill's name or description — the skill loads incorrectly or not at all, and it will never fire. Everything else in the file is dead weight until this parses.
How to fix it
Start the file with a `---` fence on line 1, a valid `key: value` YAML mapping (at minimum a `description`), and a closing `---` fence. Slash commands are the one exception — frontmatter is optional there, but when present it must still parse.
Example
✗ flagged
description: Formats SQL files
# SQL Formatter <- no `---` fences, so nothing parses as frontmatter✓ passes
---
description: Formats SQL files. Use when the user asks to format or lint SQL.
---
# SQL Formatter