Lilian Weng, co-founder of AI startup Thinking Machines, has stepped down from the company citing health concerns, before rejoining former employer OpenAI. Weng said in an internal Slack message, also posted publicly, that she could no longer sustain the pace and stress of startup life, having reached a decision after several months of consideration. Her departure marks a notable loss for Thinking Machines and reflects the intense competition for top AI talent among leading labs.

OpenAI confirmed Weng's return, where she previously held the role of VP of AI Safety Research, saying she would now lead a senior team tasked with accelerating internal research, including cross-team work on recursive self-improvement of AI systems. The move appears at odds with her stated reasons for leaving Thinking Machines, though she will no longer carry the added pressures of being a co-founder. Thinking Machines co-founder and former OpenAI chief technology officer Mira Murati responded supportively to Weng's announcement on social media, thanking her for their time working together, though it remains unclear whether Murati knew in advance that Weng would be moving to OpenAI.