This looks nice and clean compared to some of the other tools in this vein that I've used. That said, I've fairly recently decided that Jupyter notebooks are a

This looks nice and clean compared to some of the other tools in this vein that I've used. That said, I've fairly recently decided that Jupyter notebooks are a better mousetrap than GUI-based tools for this kind of thing. Python's a pretty ergonomic language for this kind of thing, and having a full programming language to work with makes some testing scenarios more convenient. Plus, you can get some nice spinoffs out of the deal. Once you get everything how you like it, you might be only an nbconvert away from having a decent suite of automated acceptance tests. Or sprinkle some markdown cells in between your tests, and now you've got executable, hackable documentation for your API. Check that into GitHub, and you've also got reasonably pretty documentation with detailed code samples that's easy to view in the repository browser.