I'm one of the founders at Devskiller. I don't want to promoted here, just wanted to help you with your question.
The problem that we found on the market is that there are a lot of companies that want to help you to source candidates, but have problems with checking their technical knowledge. Most of them just look through their CVs and call them to ask some questions about their previous projects. To vet candidates in the way you would like them too they would need to have a SME in each technology that they would check their skills and that would be very costly.
Devskiller was born out of frustration. we had actually the same problems as you mentioned because what was available on the market just verified algorithmic knowledge. We wasted a lot of time vetting people if they are real programming problem solvers in certain technologies. IMHO we achieved our goal, but off course, you would have to try it yourself to see if that is the thing that you are looking for.
If you have any questions just let me know.