Lilian Weng, a co-founder of AI start-up Thinking Machines Lab, has announced her return to OpenAI. In her new role at OpenAI, Weng will lead a team working on accelerating internal research, part of the company's larger effort to create AI that can improve itself. Weng had previously announced her departure from Thinking Machines this week, citing that she couldn't keep up with the fast-paced environment of a start-up, stating, "I don't feel I'm able to continue at the pace a start-up requires." She admitted the decision was difficult as she didn't want to disappoint her team but was constantly worried about her health. Before co-founding Thinking Machines Lab, Weng worked at OpenAI from 2018 to late 2024, where she helped build the Applied AI Research team, led its Safety Systems work, and contributed to the development of OpenAI's GPT-4 model. Weng's departure from Thinking Machines Lab is the latest in a string of exits; nearly a third of the company's original founding members have left since its launch. Of the start-up's six co-founders, only two remain: CEO Mira Murati and chief scientist John Schulman.