Lilian Weng, a co-founder of Mira Murati's AI startup Thinking Machines Lab, has stepped down from the company citing severe health issues and rejoined OpenAI, her former employer, just days after her departure.
In an internal Slack message, Weng wrote she could no longer sustain the pace a startup requires, saying the amount of consistent stress and workload had pushed her beyond what her health could sustain physically. She previously served as OpenAI's VP of AI Safety Research before leaving to co-found Thinking Machines in February 2025.
At OpenAI, Weng will lead a new top-level team focused on accelerating the company's internal research, specifically recursive self-improvement — the effort to let AI systems iterate on themselves to grow more capable. Her exit leaves only Murati and chief scientist John Schulman from Thinking Machines' original six co-founders.