There should be UI before that point, because a UI will help expose the need for any changes needed to the API.
Note the API is functional now. I added a comment to your issue in github that discusses the problem you were encountering and what you need to do to overcome it, btw.
As far as the above comment, not sure if that was your issue or someone else's (a lot of aliases, so it gets confusing).
But in any event, you should be able to proceed with a functional prototype now, and it would be great to have one as soon as possible.
So does this mean you're hired to create a UI for the new wallet website? That was what I was a bit uncertain of.