OpenAI released o1-preview, the first model in a new series trained with reinforcement learning to 'think' through problems using an internal chain of thought before answering. It was slower than GPT-4o but dramatically better at math, science, and competitive programming.

The launch introduced the reasoning-model paradigm that would define the next two years of AI development: trading inference-time compute for capability. A smaller o1-mini arrived the same day as a cheaper, faster alternative.

The full o1 followed on December 5 alongside the new $200/month ChatGPT Pro tier, cementing reasoning as a premium capability rather than a research curiosity.