Anthropic replaces git with in-house Scion: daily merges grew from ~2,500 to 30,000+, and a 7-person team migrated the monorepo in five months

Anthropic's Oct 13 engineering post explains why it left git for an in-house system. Merges to its monorepo rose from about 2,500 per day in May 2026 to more than 30,000 in September; main-branch history is nearly 100 GB and CI performs roughly 25 million checkouts a day, which would be 600+ PB of git traffic if fetched naively. Shallow clones, sparse checkouts and several hundred internal git mirrors were no longer enough.

Scion assumes clients (mostly agents and jobs) are always online. The server handles history, blame, commits, merges and rebases; clients keep a bounded cache and use sparse or virtualized checkouts. Commits are server-side changesets in a write-ahead log that list changed files with content-addressed pointers, backed by Spanner and blob storage, with no packfiles and no repo maintenance. A daemon, regional cache and server tier all speak the same RPCs and each keeps a 99%+ hit rate. The design borrows lessons from Google's Piper and Meta's Sapling.

Anthropic reports a 200x smaller CPU and memory footprint than its git mirrors, checkouts two orders of magnitude faster, and a 25% CI CPU win. Seven engineers built Scion, the Antique review tool and related systems in five months; an early Claude Mythos Preview rewrite of the ingestion pipeline used 20x less memory and ran 4x faster. Scion will not be released, but Anthropic suggests git forges add server-side RPCs and better-composed sparse features, and says it wants to work with GitHub, GitLab and the OSS community. These figures are self-reported.