Lilian Weng rejoins OpenAI after leaving Thinking Machines for health

Lilian Weng, co-founder of Thinking Machines, announced on July 29, 2026 that she would step down from her role, citing health issues. She later joined OpenAI, where she previously served as VP of AI Safety Research.

Weng shared her decision in an internal Slack message, also posted on X, stating, "I don't feel I'm able to continue at the pace a startup requires." She explained that after months of consideration, she concluded the consistent stress and workload had pushed her beyond what her health could sustain physically.

On July 29, 2026, OpenAI confirmed to TechCrunch that Weng would rejoin the company. According to a company spokesperson, she will lead a top-level team focused on accelerating OpenAI's internal research, supporting cross-research work on recursive self-improvement, a process allowing an AI system to iterate on itself to become more powerful.

This move may appear contradictory to Weng's statements about startup pressures affecting her health, but she likely faces less direct pressure as a non-co-founder. The departure is notable amid fierce competition for talent among AI labs.

Thinking Machines co-founder and former OpenAI CTO Mira Murati replied to Weng's X post earlier that week, expressing support. Murati wrote, "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." It is unclear if Murati was aware Weng would rejoin OpenAI.

Weng's return to OpenAI marks a significant shift, as she takes on a role driving internal research acceleration. The move underscores the dynamic talent landscape in the AI industry, with high-profile individuals navigating between startups and established labs.