> a smart and fast model that answers most questions, a deeper reasoning model for harder problems, and a real-time router that quickly decides which model to use based on conversation type, complexity, tool needs, and explicit intent (for example, if you say “think hard about this” in the prompt).

From https://openai.com/index/gpt-5-system-card/

In the API, there’s no router. Developers just pick whether they use the reasoning model or non-thinking ChatGPT model.