Lilian Weng, cofounder of Thinking Machines, is returning to OpenAI after leaving the startup earlier this week. The departure was attributed to stress and illness from startup pressures. She will focus on developing new AI models using recursive self-improvement techniques. Multiple observers note the rapid reversal and comment on talent movement patterns at leading AI labs, with some framing short exits as part of career cycles in the sector. The return underscores OpenAI's continued emphasis on advanced model-building approaches amid competitive industry conditions.