Facility is best with new APIs. Design a Facility Service Definition (FSD) for your API and then leverage various tools and libraries to implement, document, and consume your API. The Facility Editor is a great way to experiment with API ideas and preview the client and server code that would be generated.
Facility can be used for existing APIs by creating and using an FSD that reflects that API. Note that an existing API may use features that aren’t supported by Facility, especially since a limited feature set is one of the design goals of Facility.