Lilian Weng Leaves Thinking Machines Citing Health, Rejoins OpenAI Days Later

Thinking Machines co-founder Lilian Weng’s exit note, shared on X, read as unusually candid for a founder departure in the AI industry.

She wrote that she had tried resting while the lab raced to ship its first open-weight model, Inkling, but felt persistent guilt, and had considered a narrower role before deciding that staying on in a reduced capacity wasn’t who she was.

Fellow co-founder Mira Murati replied that she’d miss Weng and was glad she was putting her health first, thanking her for helping build the company. However, OpenAI later told TechCrunch that Weng will rejoin the company. 

From Exit Note to New Job in Two Days

The outlet reported that Weng announced her departure on Tuesday and was confirmed as returning to ChatGPT creator OpenAI by Wednesday, prompting questions about her need to step back from startup intensity to taking another high-profile research role. 

TechCrunch noted the tension, saying the move could appear to contradict her stated reasons for leaving, though she likely won’t face the same pressure at a company where she isn’t a co-founder responsible for its trajectory. 

Weng previously spent nearly seven years at OpenAI, from 2018 to 2024, building its safety infrastructure, including its fine-tuning API and moderation systems, before leaving to co-found Thinking Machines with Murati in early 2025.

A Pattern of Health-Driven Exits Among AI’s Second Tier

Business Insider’s reporting placed Weng’s departure within a broader pattern emerging across AI labs this year, noting that her exit note described being ill more often over the past seven months than at any point in her life, and that she did not name a specific diagnosis. 

The report drew a direct comparison to OpenAI’s own Fidji Simo, who stepped back from a top role this month following a flare-up of a chronic illness, and Greg Yang, the cofounder of SpaceX’s merged subsidiary xAI, who moved into an advisory position in January after a Lyme disease diagnosis. 

Business Insider observed that the executives stepping away under these circumstances have tended to sit one level below the very top of their organizations, even as some Silicon Valley AI teams continue running punishing six-day weekly schedules that show no sign of slowing regardless of who burns out along the way.

A Thinning Founding Team at Thinking Machines

Weng’s return to OpenAI also underscores just how much turnover Thinking Machines has absorbed since its high-profile launch. 

TechCrunch’s coverage noted that Weng represents a significant, high-profile departure amid what is already fierce competition for AI talent.

Business Insider added that her exit continues a steady thinning of the startup’s original team, with only two of the six founding co-founders now remaining at the company. 

That erosion comes despite Thinking Machines raising a $2 billion seed round led by Andreessen Horowitz, underscoring that even well-funded, high-profile AI startups aren’t immune to the pull of established labs like OpenAI. 

Source: Lilian Weng left the company citing health reasons, then joined OpenAI