It depends on the extent to which "validation" is recomputing exactly the same information. If you have a game that runs every single computation server-side it

It depends on the extent to which "validation" is recomputing exactly the same information. If you have a game that runs every single computation server-side it's not a sanity check, it's just a game running on a server. What we have set up is more akin to a game that runs both server side and on the client, with some added latency on both sides because every update has to be cross checked between the two. You don't see a lot of games like this, because it would be absurd. To be fair this is a slight oversimplification. I've lived in the US and dealt with the IRS, and while most of the information must be completed server-side there are some things (like deductions) that would indeed need an API. I think your suggestion, allowing multiple clients to use that API, is a good one. But there's no reason to force the client to recompute everything the server already has.