Lilian Weng, co-founder of Thinking Machines, announced this week that she was stepping down from her role, citing health concerns.

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

On Wednesday, OpenAI told TechCrunch that Weng would rejoin the company, where she previously served as vice president of AI security research.

According to a company spokesperson, Weng will lead a senior 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 working at a fast-paced startup has on his health; However, you probably won’t face as much direct pressure while working at a company you’re not a co-founder of. Still, in a time of fierce competition for talent among AI labs, this high-profile exit is notable.

Thinking Machines co-founder and former OpenAI CTO Mira Murati responded to Weng’s X post earlier this week and expressed support for his decision to focus on his health. It is unclear whether Murati knew that Weng would rejoin OpenAI.

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