I build production AI systems where an LLM runs only where natural-language judgment is unavoidable. Everything that can be deterministic is, and the boundary between the two is tested.
A deterministic spine does the work that can be specified. The model is one stage in the middle: it reads intent and proposes, then plain code enforces the parts that carry consequences, before anything is said back.
I am building a career-mode cricket game for mobile. A deterministic match engine resolves every delivery from footwork, shot and timing. The language model never touches the outcome. It sits on top as a paid commentary layer, and free players make zero model calls.