I use Spectral while designing APIs and during API design reviews. It is API an design UI that supports OpenAPI 2 and 3. It comes with a totally awesome Spectral (OpenAPI linter) integration. The UI does not cover all features of the OpenAPI formats, but it’s not a problem for most users: this is the most complete and the best tool of his kind. And I love being able to switch between UI and code views (this also helps to do what you can’t do with the UI).