Lilian Weng, former OpenAI VP of Research and Safety and co-founder of Thinking Machines Lab, resigned citing health issues from sustained pressure. She said her body could no longer handle the workload. Weng joined OpenAI in 2018, led safety teams for GPT-4, and left in November 2024. In February 2025, she co-founded Thinking Machines Lab, which recently raised $2 billion at a $12 billion valuation. Her departure highlights the high-pressure reality of AI startups.