GPT-5.1: faster, more conversational by default, better at following instructions.

GPT-5.1 with Thinking: for more complex problems — adapts how much “thinking time” it uses, quicker on simple tasks, more persistent on hard ones.

GPT-5.1 Codex: optimized for real-world software engineering — smarter code generation, stronger debugging/review, and more efficient token use on simple tasks while going deeper on complex ones.