Liam Fedus
About Liam Fedus
Liam (William) Fedus is a machine learning researcher and entrepreneur based in San Francisco. He studied physics at MIT and later worked as an equity research associate at Fidelity Investments. Crunchbase lists a master's degree in elementary particle physics from UC San Diego and a PhD in computer science from Université de Montréal.
He worked at Google Brain, where he co-authored the Switch Transformer, a mixture-of-experts model scaled past 1 trillion parameters. He then moved to OpenAI, where he was part of the group that developed ChatGPT and became vice president of post-training research. Crunchbase dates that VP role from October 2024 to March 2025.
In 2025 he left OpenAI and co-founded Periodic Labs with Ekin Dogus Cubuk, formerly of Google DeepMind. The company was announced in September 2025 and said it had raised $300 million in seed financing led by Andreessen Horowitz. It aims to build an AI scientist that pairs models with automated laboratory experiments.
Fedus's angel activity is recent and appears in announcements of other companies' rounds. Two AI rounds on the Lounge deals desk are RadixArk (Seed, $100M) and Proximal (Seed, $15M). He has not published a personal investing thesis or a list of his angel deals.
RadixArk is an AI infrastructure company founded by Ying Sheng and Banghua Zhu. It builds on the open-source SGLang and Miles projects for serving, training and post-training models. Its $100M seed at a $400M valuation was announced in May 2026 and led by Accel, co-led by Spark Capital. William Fedus is named among the participating angels. Lounge's desk records the deal as January 2026.
Proximal sells coding training data and reinforcement learning infrastructure to labs and enterprises. Sacra reports a $15M seed at a $300M valuation led by General Catalyst, with Fedus, Kevin Weil and Erik Bernhardsson listed as angels. Sacra says the round closed in late 2025 and was announced in September 2026.
Both known deals are seed rounds in AI infrastructure and training-data companies, aimed at frontier model development and coding agents. Fedus has not publicly stated a check size, preferred stage or geography, so none is given here. Third-party directories list him as a San Francisco angel focused on AI.
- RadixArk: open AI infrastructure for training, serving and post-training models; angel in its $100M seed, announced May 2026.
- Proximal: coding training data and RL infrastructure for frontier labs; angel in its $15M seed led by General Catalyst, announced September 2026.
- en.wikipedia.org/wiki/Liam_Fedus
- crunchbase.com/person/william-fedus-d642
- research.contrary.com/company/periodic-labs
- x.com/LiamFedus
- finsmes.com/2026/05/radixark-raises-100m-in-seed-funding.html
- x.com/radixark/status/2051648113014882586
- sacra.com/c/proximal
- askforfunding.com/investor/liam-fedus-angel-investor
Investing activity
Liam Fedus appears in 2 AI deals tracked by the Lounge deals desk, rounds totalling $115M. The rounds are mostly Seed, with a median round size of $100M. The most recent are Proximal (Seed, $15M, Sep 2026) and RadixArk (Seed, $100M, Jan 2026).
Liam Fedus most often invests alongside General Catalyst, SV Angel, Chemistry and Go Global Ventures, and the companies they back on Lounge are mostly in AI and AI Infrastructure.
Deals
Frequently asked questions about Liam Fedus
Does Liam Fedus invest in AI startups as an angel?
Yes. He is named as an angel in RadixArk's $100M seed and Proximal's $15M seed. Both companies work on AI infrastructure or training data.
What is Liam Fedus's check size?
He has not made his check sizes public. Round announcements list him as a participant without amounts.
What stage does Liam Fedus invest in?
His two known AI deals are seed rounds. He has not published a stage preference.
Who is Liam Fedus?
He is a co-founder of Periodic Labs. He was previously VP of post-training research at OpenAI and worked at Google Brain.
What kind of founders might suit a pitch to Liam Fedus?
His known investments are in AI infrastructure and coding-model training data, and his own work is in post-training and AI for science. He has not said what he looks for, so this is inference from his deals.
Where can founders reach Liam Fedus?
He posts on X as @LiamFedus. No LinkedIn profile was confirmed in the sources used.
Is Liam Fedus an angel investor?
Yes. Liam Fedus invests personally as an angel.
What AI companies has Liam Fedus invested in?
Liam Fedus's AI investments include Proximal and RadixArk. The 2 rounds on Lounge are listed on this page with amounts, dates and co-investors.
What stage does Liam Fedus invest at?
In the rounds tracked on Lounge, Liam Fedus has invested at Seed, with a median round of $100M.
Who does Liam Fedus co-invest with?
Liam Fedus's most frequent co-investors on Lounge are General Catalyst (1 deal), SV Angel (1 deal), Chemistry (1 deal), Go Global Ventures (1 deal) and Kevin Weil (1 deal).