It's not a new model; it's a router + many, many chains of optimized system prompts. Note the terminology they use to describe it.
> We are introducing GPT‑5, our best AI system yet
> GPT‑5 is a unified system with a smart, efficient model that answers most questions...
They make a special point to call it a system, of which "a model" is a part. I'm betting they didn't pre-train gpt5 from scratch, but did a ton of RL on gpt4 + prompt optimization and the routing engine.
> We are introducing GPT‑5, our best AI system yet
> GPT‑5 is a unified system with a smart, efficient model that answers most questions...
They make a special point to call it a system, of which "a model" is a part. I'm betting they didn't pre-train gpt5 from scratch, but did a ton of RL on gpt4 + prompt optimization and the routing engine.