JSON Schema is a vocabulary for describing and constraining JSON-compatible data. The data being assessed is the instance; the document containing its constraints is the schema.

A schema can describe expected types, required properties, constants, arrays, nested objects, and relationships between constraints. Keywords such as $schema, $id, and $ref identify a schema dialect and connect reusable schema resources.

md-utils uses JSON Schema to assess structured frontmatter. JSON Schema covers the data shape, while mdtype and project rules add Markdown-aware concerns such as headings, sections, file selection, and logical paths.