Frontmatter block is not closed

What the rule is

An opening `---` delimiter was found, but no second `---` line appears anywhere after it. Every implementation of the format needs both delimiters to know where the YAML ends and the markdown body begins — without the closing line, the parser has no way to tell.

This usually happens when a description field's multi-line value is indented incorrectly and swallows the closing delimiter, or when the closing `---` was accidentally deleted during an edit.

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

• Add a line containing only `---` immediately after the last frontmatter field and before the markdown body.

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.