Model router
A control layer that picks which model variant or mode to run for each request, for example fast chat versus deeper reasoning, instead of leaving the choice to the user.
A model router sits in front of several models or modes and decides which one handles each prompt. GPT-5 uses one to switch between a fast large language model and a deeper reasoning model based on conversation type, complexity, tool needs, and explicit user intent.
Routers simplify products by hiding the menu, but they also reduce transparency: users may not know which system answered them. The GPT-5 rollout showed how strongly people react when a familiar model disappears behind automatic routing.