A model router should optimise for boring, not cheap

Every multi model product ends up building a router, and the first version always optimises for cost. Cheapest model that can plausibly handle the request. It works, and then it doesn't, in a way that's hard to see. The answer comes back confident and slightly wrong, nobody complains because nothing looked broken, and they quietly stop coming back.

What I actually want from a router is consistency. Same question, same shape of answer, tomorrow as well as today. Someone who has learned what your product is bad at can work around it. Someone who gets a different quality of answer every time can't learn anything, and that's worse than being reliably mediocre.

So the rule I've landed on is that a route only changes when I can explain why to the person using it, and cost isn't a reason I can say out loud.

The open question for me is detecting the plausible but wrong case without a human reading outputs. Everything I've tried is either a second model grading the first, which shares the blind spot, or a rule that only fires on mistakes nobody was going to make. If you're routing across models and have solved this, I'd like to hear how.