It's built with slateJS and works with any OpenAI-compatible endpoint (bring your own backend).
You pass it a "purpose" prompt & external context to inform completions. E.g. the demo gif[1] has a purpose of "COOL and SLICK announcement post", and was given the CopilotKit release notes as context. (super important, to make suggestions specific).
Part of CopilotKit (open-source copilot/sidekick infra).
IMO it works really well. After working with it in all my apps for the past week it's honestly very frustrating to not have it available universally.