author here:

I'm not really sure why you'd say that OpenAPI isn't a JSON Schema document: there are published JSON Schema files on the official OpenAPI website. See for example:

One using the draft-04 of JSON schema: https://spec.openapis.org/oas/3.0/schema/2024-10-18.html One using the 2020-12 version of JSON schema: https://spec.openapis.org/oas/3.2/schema/2025-09-17.html