It’s Agent Week at Understanding AI! This week I’m going to publish a series of articles explaining the most important AI trend of 2025: agents! Today is a deep dive into reinforcement learning, the training technique that made agentic models like Claude 3.5 Sonnet and o3 possible.Today’s article is available for free, but some articles in the series—including tomorrow’s article on MCP and tool use—will be for paying subscribers only. I’m offering a 20 percent discount on annual subscriptions th...