Lilian Weng, co-founder of Thinking Machines, announced she is stepping down from her role, citing health reasons.

In an internal Slack message she later shared on X, Weng wrote that the consistent stress and workload required by a startup had pushed her beyond what her health could sustain.

Shortly after, OpenAI told TechCrunch that Weng would be rejoining the company, where she previously served as VP of AI Safety Research.

A spokesperson said she will lead a top-level team focused on accelerating internal research, specifically on recursive self-improvement — a process enabling an AI system to iterate on itself to become more powerful.

The move may appear contradictory to her stated health concerns, but the article notes she will likely face less direct pressure as an employee rather than a co-founder.

Thinking Machines co-founder and former OpenAI CTO Mira Murati replied to Weng’s post, expressing support for her decision to prioritize health.

It is unclear if Murati knew at the time that Weng would rejoin OpenAI. The departure comes amid fierce competition for talent among AI labs.