LLM routing usually starts as if statements. One service picks the cheap model for ticket summaries. Another hardcodes the strong model because it needs tool calling. A third has a comment reading “never send this to the cloud” directly above a fallback that sends it to the cloud when the local server times out. Changing a model, a price or a provider means editing all three.

The router worth building is a small one: ordered rules over properties you can compute before the request leaves, written like an nginx config in a single file, with no database and no UI. Input size, whether tools or images are present, a data classification tag, a cost ceiling for the single request, which upstreams are healthy. One binary on a cheap VPS reads the file, accepts the OpenAI request shape and picks where each request goes. The rule that justifies the exercise is that privacy routing fails closed.