In other words, you're excluding candidates based on their willingness to expend a lot of their time rather than some of your own.

The screening call goes both ways, ya know. I use that half hour to judge if I want to work at the company, as much as you want to find out if I might possibly fit in. If it looks good, then sure, I might give you an hour for an assessment project.

But I'd never do this FIRST.