Thinking Machines Lab Co-Founder Lilian Weng Leaves the Startup and Returns to OpenAI

Lilian Weng, co-founder of the AI startup Thinking Machines Lab, left the company on July 29, 2026, citing health problems, and immediately afterward joined OpenAI, where she previously served as vice president of AI safety research.

What happened

Lilian Weng left Thinking Machines Lab for health reasons and almost simultaneously announced her move to OpenAI, TechCrunch reported on July 29, 2026. For her, it is a return: she spent a significant part of her career at OpenAI before joining the startup.

• Date of the event: July 29, 2026
• Reason for leaving Thinking Machines Lab: health problems
• New (and former) employer: OpenAI
• Previous role at OpenAI: VP of AI Safety Research
• Thinking Machines Lab was founded in 2025 by former OpenAI CTO Mira Murati

Why it matters for Murati's startup

Thinking Machines Lab was founded in 2025 by Mira Murati, OpenAI's former chief technology officer, who built the team partly from former OpenAI researchers. The startup positioned itself as a research lab focused on more open and customizable model development.

Losing a co-founder less than a year and a half after launch is a notable staffing blow for a young lab that is still building its product and reputation. Weng was one of the most recognizable names on its roster, so her departure, and her move back to a former employer, drew heightened attention across the industry.

Who is Lilian Weng

Lilian Weng is an AI researcher who worked at OpenAI from 2018, where she led teams in applied research and safety and rose to vice president of AI safety research. In the engineering community she is best known for her technical blog, lilianweng.github.io, where she gives detailed breakdowns of how language models work, reinforcement learning, and safety questions.

For OpenAI, her return strengthens the safety team at a time when competition among labs for researchers with this profile remains extremely intense.

Weng left

Thinking Machines Lab, citing health problems, and then joined OpenAI, TechCrunch reports.

What this means

Moves by leading researchers between OpenAI and newer labs like Thinking Machines Lab show how narrow and fluid the market for AI safety specialists is: the same people circulate among a handful of power centers, and any reshuffle immediately becomes industry news.

Frequently asked questions

Why did Lilian Weng leave Thinking Machines Lab?

The stated reason is health problems. According to TechCrunch, Weng herself cited them when announcing her departure from the startup on July 29, 2026.

Where did Lilian Weng go?

To OpenAI. For her, it is a return: before working at Thinking Machines Lab, she had already served as OpenAI's vice president of AI safety research.