Lilian Weng leaves Thinking Machines Lab, the fourth co-founder exit from a team of six.

She announced July 27 with July 29 as her last day, publishing the note she sent internally and citing seven months of health problems aggravated by startup pace, writing she's been unwell more than at any other point in her life. She joined Mira Murati's company in February 2025 after nearly seven years at OpenAI leading safety systems. Four of six founders gone in roughly 17 months is a retention signal worth tracking against the $2B seed and the Tinker product line. (Lilian Weng)