LLM multi-model routing: the invariants that hold when you serve traffic from more than one provider
LLM multi-model routing spreads traffic across two or more model providers so a single-vendor outage, price change, or policy shift does not stop production. The pattern is simple in principle and complicated in practice because different providers have different token formats, streaming semantics, tool-call schemas, and safety-refusal patterns. This piece walks through the six invariants that hold regardless of provider (identity resolution, classification, policy, audit, idempotency, and response normalization) and the three variances that do not (token accounting, streaming chunking, and tool-call format).