Andrej Karpathy doesn't make random career moves. He co-founded OpenAI in 2015, left to build Tesla's self-driving program, came back to OpenAI for a year, then left again in 2024 to start an AI education company. Every transition has been deliberate and every one of them has turned out to be worth paying attention to. On Tuesday he posted on X that he's joined Anthropic. "I think the next few years at the frontier of LLMs will be especially formative," he wrote. "I am very excited to join the team here and get back to R&D." The "get back to R&D" part is the signal. Karpathy has spent the last several years teaching, building, and explaining. Now he's going back to the frontier. And the specific place he's going says something about where the most important work in AI actually is right now.