I think Toptal’s model is as ideal as it gets. Engineers are vetted and interviewed once, companies can hire without interviewing, if it doesn’t work out in the first week Toptal covers the cost. This incentivizes Toptal to do effective evaluation of engineers, engineers don’t have to repeat interviews over and over, and companies save time and money.

I don’t see how that model could be any better really. Referrals are also a good model, but the talent pool is limited.