Lilian Weng, co-founder of Thinking Machines, announced this week that she would step down from her role, citing health issues.

“I don’t feel I’m able to continue at the pace a startup requires,” she wrote in an internal Slack message, which she also shared on X. “After thinking about it for several months, I ultimately have to admit that the amount of consistent stress and workload have pushed me beyond what my health can sustain physically.”

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

According to a company spokesperson, Weng will lead a top-level team focused on accelerating OpenAI’s internal research. This team will support cross-research work on recursive self-improvement, a process that would allow an AI system to iterate on itself to become more powerful.

This move could be read as contradictory to Weng’s statements about the impact of working at a fast-paced startup on her health — however, she likely won’t face as much direct pressure while working at a company where she isn’t a co-founder. Still, in a time of fierce competition for talent among AI labs, this high-profile departure is notable.

Thinking Machines co-founder and former OpenAI CTO Mira Murati replied to Weng’s X post earlier this week and expressed support for her decision to focus on her health. It is unclear if Murati was aware that Weng would rejoin OpenAI.

“We’ll miss you, it’s been wonderful building Thinky together. I’m glad that you’re putting your health first. Thank you for everything,” Murati wrote in response to Weng’s post.

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