JSON Schema [1] is actually a mature standard now, with decent tooling support, mostly through OpenAPI (formerly Swagger), which extends it with support for endpoints.
It's much simpler to use than XML Schemas, and arguably results in cleaner data models, since it doesn't have anything analogous to XML namespaces that allow for arbitrary mixing of schemas.
It's much simpler to use than XML Schemas, and arguably results in cleaner data models, since it doesn't have anything analogous to XML namespaces that allow for arbitrary mixing of schemas.
[1] https://json-schema.org/