\n: \n---\n lines. The block declares the file's identity (name, description) and scope (paths, loading, maintainer`) so the loader can target it correctly.

Frontmatter Block Present

Scoped rule files must begin with a --- YAML frontmatter delimiter as the very first line. The frontmatter block provides metadata like globs and description that controls how the agent discovers and loads the rule file.

Antipatterns

• Leading blank line before frontmatter like a newline then --- — the pattern \A--- requires the delimiter at byte position zero with no preceding whitespace or blank lines.
• Using a different delimiter like ~~~ or === — the check specifically matches --- followed by a newline.
• Frontmatter comment before the delimiter like <!-- metadata --> then --- — any content before the opening --- causes the pattern to fail.

Pass / Fail

Pass

---
globs: "src/**/*.py"
description: Python source conventions
---
# Source Conventions

Fail

---
globs: "src/**/*.py"
---
# Source Conventions
(leading blank line before ---)

Limitations

Checks for YAML frontmatter opening delimiter at the start of the file. Does not validate frontmatter content or schema compliance.