Ilya Sutskever, OpenAI’s co-founder and chief scientist who played a central role in last November’s dramatic firing and reinstatement of CEO Sam Altman, announced his departure from the company on Tuesday.

“After almost a decade, I have made the decision to leave OpenAI,” Sutskever wrote in a social media post. He said he is working on a project that is “very personally meaningful” and will share details later. Sutskever’s exit comes one day after OpenAI unveiled GPT-4o, its latest AI model, which the company said can engage in real-time spoken conversations.

Sutskever voted with the board majority to remove Altman in November, citing concerns that Altman had pushed AI “too far, too fast,” according to reporting. He reversed course days later, signing a staff letter calling for Altman’s return and Altman was reinstated after five days. In a statement, Altman called the news “very sad” and said he is “forever grateful for what he did here.”

OpenAI named Jakub Pachocki, previously director of research, as the new chief scientist.

With his departure, that chapter closes.