Gabor Cselle, the former CEO and co-founder of X (formerly Twitter) challenger Pebble, has joined OpenAI to work on a secretive project.

Cselle, who according to LinkedIn has been employed at OpenAI since October, announced the news in a post on X yesterday. “Will share more about what I’m working on in due time,” he wrote. “Learning a lot already.”

Cselle is a repeat founder who sold his first company, the Y Combinator-based mobile email startup reMail, to Google. His second company, the native advertising startup Namo Media, he sold to Twitter before Elon Musk purchased the social network and rebranded it to X.

Nearly a decade ago, Cselle worked at Twitter as a group product manager, focusing on the home timeline, user onboarding, and logged-out experiences. Cselle left Twitter in 2016 for Google, where he was director at the tech giant’s Area 120 incubator for spin-offs.

Cselle began working on Pebble, originally called T2, in 2022 with Michael Greer, Discord’s ex-engineering head. Pebble, whose microblogging service emphasized safety and moderation, grew to a small but engaged community and raised funding from angles including Android co-founder Rich Miner.

Ultimately, though, Pebble struggled to maintain meaningful growth. The company shut down last October, reemerging as a Mastodon instance in November.

In May, Cselle joined the accelerator South Park Commons, where he worked on a range of generative AI prototypes including an homage to the viral HQ Trivia.

Csell’s hiring reveal comes the same weekend as OpenAI rival Anthropic gains its own high-profile recruit: Embark founder Alex Rodrigues. Rodrigues, who led autonomous trucking firm Embark through a SPAC merger in 2021 (and subsequent fire sale to Applied Intuition in 2023), said on Friday that he’d be joining Anthropic as an AI safety researcher.