Days after leaving Thinking Machines Lab due to health concerns and stress, AI researcher Lilian Weng is returning to OpenAI. Weng announced her departure from Thinking Machines on Monday, noting that July 29 would be her last day at the startup. Despite leaving the startup, Weng made it clear she was not abandoning the field of AI. Another major departure from Thinking Machines LabWeng's exit marks yet another departure of a senior figure from Thinking Machines Lab, the AI startup founded in February 2025 by Mira Murati, OpenAI's former chief technology officer. Of the original founders, only Murati and chief scientist John Schulman remain at Thinking Machines.