tl;dr: Anthropic replaced git with Scion, an in-house version control system built in five months to handle their monorepo's explosion from 2,500 to 30,000 daily merges driven by Claude-generated code. Scion treats clients as always-online, shifts most computation server-side via content-addressed APIs, makes checkouts sparse/optional, and scales horizontally—resulting in 200x less infrastructure and checkouts in seconds instead of minutes. A team of seven built it (and a code review tool) largely with Claude's help, and while Scion isn't a general git replacement, Anthropic is sharing lessons so GitHub/GitLab and the git ecosystem can adapt to agent-scale development.