“Before I build a router I spend the cheaper levers: cache the prefix, then sweep effort on one model. A router has to beat that whole curve on cost per completed task, not just one model’s default.
Then I route in two stages. Hard rules first, as yes or no: does the prompt fit, do the tools work on that tier, may this data go there. Then a cheap classifier — Haiku 5.5 at low effort with a structured output — that labels the request, and escalates when it’s unsure, because a wrong ‘easy’ costs a bad answer and a wrong ‘hard’ costs a few tenths of a cent.
The decision rule is two prices. At 800 tokens in and 250 out, a Haiku 5.5 call is about $0.0002 and a Sonnet 5.5 call about $0.004. If a wrong answer costs me 80 cents, the small tier pays only while its extra wrong rate stays under about half a percent. I measure that from a weekly judged sample, not from the router’s confidence.
On a 60/25/15 mix that’s about $2,400 per million requests against $4,100 all on Sonnet — roughly 40% off — and I keep each conversation on one tier, because caches and thinking blocks don’t move between models.”