From an AI user perspective, the four big leaps so far in ability: 1. GPT-3.5 (ChatGPT, November 2022) 2. GPT-4 (Spring 2023) 3. Reasoners (starts with o1-preview, but the real deal was o3, Spring 2025) 4. Workable agentic systems (Harness + good reasoner models, December 2025)