The OpenAPI specification provides a standard way of describing REST APIs.
The OpenAPI Specification, originally known as the Swagger Specification, is a specification for machine-readable interface files for describing, producing, consuming, and visualizing RESTful web services.
OpenAPI descriptions are available in the documentation for each API. For example, this screen from the Create User Loan API:
In the near future, we plan to release additional support for OpenAPI, for example description of our object payloads in JSON schema format. Keep an eye out for blog posts and further updates.