Redesigning source control at Anthropic

Anthropic's coding workload has grown immensely, with code changes merged over 30,000 times per day in September 2026. This rapid growth has put immense pressure on their source control system, making it a bottleneck in development. To address this issue, Anthropic developed Scion, an in-house version control system, as part of an effort to migrate their monorepo from Git in just five months.

Scion's design differs from Git, which is a decentralized system allowing clients to work offline and compute diffs, read history, and check out any revision. In contrast, Scion is a client-server system where clients are always online and fetch data as needed. The server handles questions about file history, blame, commits, merges, and rebases, while clients maintain a bounded local object cache and lay out working copy files on disk.

By embracing the fact that all of their clients are online, Scion eliminates the need for a complete local copy of the repository, reducing the data strain caused by the massive monorepo. This change in design has allowed Anthropic to experience a newfound agility in their development processes.