No YAML frontmatter block found

What the rule is

A SKILL.md is a markdown file with YAML frontmatter, and the frontmatter is what tells an agent when the skill is relevant. Without a `---`-delimited block at the very top of the file, there's no frontmatter to read at all — not empty frontmatter, none.

This is different from a frontmatter block that's present but broken (SKM-F02–SKM-F04). Here, the opening delimiter line itself — a line containing only `---` — was never found, which usually means the file starts directly with prose, or the delimiter has stray characters on the same line.

Where it comes from

• Agent Skills — SpecificationSpecification / reference validator
• SKILL.md, the format hubSpecification / reference validator
• skills-ref validate (official reference validator, v0.1.5)Specification / reference validator

Strict spec conformance

Non-conformant. The official reference validator (skills-ref) rejects this — tested directly, not inferred from severity.

This is a separate question from the severity above and from which runtimes enforce it below — a rule can be a real, working extension in its home runtime while the reference validator still rejects the file outright, or vice versa. Both facts are reported, never one inferred from the other.

Which runtimes enforce it

How to fix it

• Add a line containing only `---` as the very first line of the file, followed by `name:` and `description:` fields, followed by a closing `---` line, before any body text.

# My Skill

This does a thing.

---
name: my-skill
description: Use when asked to do the thing.
---

# My Skill

Instructions.

Elsewhere

• SKILL.md, the formatThe full frontmatter field table this rule is drawn from.
• Agent Skills Not WorkingThe diagnostic reference this rule’s portability and failure evidence comes from.
• All rulesEvery check the validator runs, grouped by class.