A router that can't see the conversation can't classify "yes"
Classifying the word "yes"
A model router's job is to read a request and decide which model should answer it. Cheap questions go to a small model, hard ones to a large one, and the bill comes down. The whole arrangement rests on being able to tell the difference.
Then a user types "yes".
Or "continue". Or "do it". Nothing in those two or three characters says whether the work being approved is a spelling fix or a database migration. A router scoring the current message in isolation sees a very short string with no technical vocabulary, and does the obvious thing: cheapest model.
Which means if you route requests to save money, your cheapest tier is probably absorbing work it should never have seen — and your savings figure is partly fake. On 4 August LiteLLM published a benchmark that measures both halves of that: how wrong the routing gets, and what it costs to fix.