July 29 is Lilian Weng’s final day at Thinking Machines after she told staff seven months of illness, worsened by startup pace, left her unable to give 100% to the job.
Weng’s exit makes her the fourth of the startup’s six original cofounders to leave since Mira Murati launched the company in February 2025, leaving only two from the founding group.
Three earlier departures underscored the churn: Andrew Tulloch left for Meta in October 2025, while Barret Zoph and Luke Metz returned to OpenAI in January 2026.
Thinking Machines still shipped Inkling on July 15 — a 975 billion-parameter open-source model tied to its Tinker customization platform — even as cofounder attrition mounted.
The turnover lands heavily at a startup that raised $2 billion at a $12 billion valuation, intensifying scrutiny of whether its culture and operating pace can retain elite AI talent.