Parses GraphQL schema language — object, interface, union, enum, input and scalar types, schema and directive definitions, extend blocks, field arguments with defaults, list and non-null wrappers, directives and descriptions — and checks it: unknown or duplicate types, interfaces not fully implemented, invalid union members, input/argument types that are not input types, undefined directives and missing root types. Output is normalized SDL, an introspection-style JSON document (__schema) or the list of relationships between types (field, implements, union). Everything runs locally.