I have used swagger a bit for JSON API development, but didn't find it added much over just keeping it simple and reacting to client needs.

I typically split my externally facing JSON API out from the main application (which uses HTML/intercooler) because the use cases are typically so different: the JSON API needs to be general and abstract whereas the web application UI has a lot of fiddly little specific data needs that are better implemented on the back end, where you have full access to the domain model and a proper query language.