SmartBear has released a tool that it says is helpful for the testing that API providers might do during the API lifecycle. For example, it can reverse engineer an API definition by inspecting existing API calls. However, the experience is marred by several references to the Swagger specification.
In recent years, API specification/description formats, most notably OpenAPI, have become more common and have helped standardize the once-murky definition of an API. This standardization allowed the creation of a broad ecosystem of tooling for auto-generated testing and documentation.