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

Cselle has been employed at OpenAI since October, according to LinkedIn, but he announced the news in a post on X only 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, a Y Combinator-backed mobile email startup called reMail, to Google. His second company, native advertising startup Namo Media, 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 on-boarding and logged-out experiences. He 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, which emphasized safety and moderation, enjoyed a small but engaged community and raised funding from angel investors, including Android co-founder Rich Miner.

However, Pebble struggled to maintain meaningful growth. The company shut down last October, but reemerged as a Mastodon instance in November.

This 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 on 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.