Thinking Machines Lab cofounder Lilian Weng steps down, citing startup

A cofounder of Thinking Machines Lab said she's stepping down, citing stress and illness.

Lilian Weng said in a post on X that Tuesday would be her last day at Thinking Machines Lab, the AI startup she cofounded with OpenAI's former chief technology officer, Mira Murati.

Weng, previously OpenAI's vice president of research and safety, said she had been ill more often in the past seven months than at any other point in her life. She did not disclose a specific diagnosis.

"Despite all the laughter and challenges building a neolab has brought us, I don't feel I'm able to continue at the pace a startup requires," Weng wrote in a message she said she had shared with colleagues at the company, known internally as Thinky.

She said the "amount of consistent stress and workload have pushed me beyond what my health can sustain physically."

It is a hard and sad decision. I shared this message with folks at Thinky. Thank you all for the time together♥️ Just as the last sentence in my message: The future worth building is human. pic.twitter.com/VGb5fM5chX

Weng said she tried to rest and properly take sick leave while Thinking Machines was "sprinting" to release Inkling, the company's first open-weight model, but felt "guilt."

She said she had considered a narrower role that would cause less anxiety, but decided it was not a fit. "If I cannot give it 100% for what I should be responsible for, I fail my job," she wrote.

Take a smarter break in your day - and see how far you get.

Instead, Weng said she would seek a more predictable, scoped role outside a cofounder position while staying in AI.

Weng studied computer science at Peking University in China before completing a Ph.D. at Indiana University Bloomington, according to her LinkedIn profile. She joined Dropbox in 2014, moved to fintech company Affirm in 2016, and then joined OpenAI in 2018.

During nearly seven years at OpenAI, ending in November 2024, Weng worked on robotics, founded the Applied AI Research team, and later led the company's Safety Systems work before joining Thinking Machines Lab as a cofounder in 2025.

"We'll miss you, it's been wonderful building Thinky together," wrote Murati in a reply on X. "I'm glad that you're putting your health first."

Weng's departure follows several recent health-related step-backs by leaders in the AI industry.

Earlier this month, Fidji Simo stepped down from her full-time role as OpenAI's CEO of AGI after a flare-up of postural orthostatic tachycardia syndrome, or POTS. Simo moved into a part-time advisory role and said she would focus on recovery from the chronic illness.

In January, xAI cofounder Greg Yang said he had moved into an informal advisory role after doctors diagnosed him with Lyme disease. Yang said months of unexplained fatigue had left him with less energy and creativity, and that he wanted to "go founder mode on my health."

Weng's departure also comes as Thinking Machines Lab has lost a significant share of its founding team amid Silicon Valley's AI talent war.

A Business Insider review in May found that 13 of the startup's 42 founding members had left since launch, including three of its six cofounders, as rivals — including Meta and OpenAI — offered huge compensation packages.