A draft OpenAPI definition of JSON:API is available at GitHub - shpoont/openapi-jsonapi-definition.
It is intended to provide a fully-documented, mockable OpenAPI example of JSON:API.
Since this is a draft, there is still work to be done. This includes proper schema definitions of new vs existing resources,
x-faker support, request and response bodies, OpenAPI links, additional examples, etc.
Your feedback, PRs and ideas would be greatly appreciated.