Frontmatter parses, but isn't a mapping

What the rule is

The frontmatter parsed as valid YAML, but the result isn't a mapping (key: value pairs) at the top level — it parsed as a list, a single string, or an empty document instead.

`name` and `description` are read as top-level mapping keys. If the frontmatter's root isn't a mapping, there's nothing for a runtime to read those keys from, regardless of what content follows.

Where it comes from

• Agent Skills — SpecificationSpecification / 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

• Rewrite the frontmatter as `key: value` pairs, one per line, with `name` and `description` as top-level keys — not a list (`- item`) and not a single scalar value.

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.