Thinking Machines co-founder Lilian Weng departs citing health reasons, rejoins OpenAI

Lilian Weng, co-founder of Thinking Machines Lab, announced this week that she would step down from her role, citing health issues. In an internal Slack message she later shared on X, Weng wrote: “I don’t feel I’m able to continue at the pace a startup requires. 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 confirmed to TechCrunch that Weng would be rejoining the company, where she previously served as VP of AI Safety Research. According to a company spokesperson, Weng will lead a top-level team focused on accelerating OpenAI’s internal research, specifically in recursive self-improvement — a process that would allow an AI system to iterate on itself to become more powerful.

The timing — leaving a startup for health reasons while immediately joining another high-intensity organization — may seem contradictory at first. But the distinction lies in role scope: as a co-founder, Weng bore responsibility across every aspect of the business, whereas at OpenAI she will lead a focused research team with more structured support.

Notably, Thinking Machines co-founder and former OpenAI CTO Mira Murati responded to Weng’s post with support: “We’ll miss you, it’s been wonderful building Thinky together. I’m glad that you’re putting your health first.” It remains unclear whether Murati was aware that Weng would rejoin OpenAI.

This high-profile move underscores the intensifying talent war in AI. Thinking Machines was itself founded by former OpenAI researchers as a competitive venture, and Weng’s return marks a significant talent recapture for OpenAI.

The focus on recursive self-improvement is particularly noteworthy. By assigning Weng to lead this area, OpenAI signals that it is betting on AI systems that can autonomously enhance their own capabilities — a line of research that could reshape the competitive landscape if breakthroughs are achieved.

For Thinking Machines, losing a co-founder is a setback. The company has been working to differentiate itself in a crowded market of foundation model builders, and the departure of a key leader could slow its momentum at a critical juncture.