Anthropic Moves Its Monorepo From Git to In-House Scion, Saying It Is Not a Replacement for Git for General Use
Anthropic says it fully migrated its monorepo from git to Scion, an in-house version control system, in five months, after daily merges rose from about 2,500 in May to over 30,000 in September. Anthropic says Scion is not a suitable replacement for git for general use.
Anthropic says it "fully migrated our monorepo from git" to Scion, an in-house version control system, in five months, according to a post on its engineering blog published Tuesday, October 13. The company says coding is "no longer the bottleneck"; the developer infrastructure built for human scale is.
The numbers Anthropic gives
Anthropic says it averaged around 2,500 merged code changes per day in May 2026 and over 30,000 per day as of September. It says its main branch history is almost 100 GB, a compressed working copy is over 25 GB, and it performs on the order of 25 million code checkouts per day for CI. All figures are Anthropic's own.
How Scion differs
Anthropic says Scion is designed around clients that are always online, since nearly all of its clients are agents and jobs. Commits, merges and rebases happen on the server, and checkouts are sparse or virtualised, according to the post. Anthropic says its Scion infrastructure footprint is 200x smaller in CPU and memory than its previous git mirroring infrastructure, and that CI and research jobs check out code in seconds rather than minutes.
A team of seven built Scion and a code review tool called Antique and completed the migration in five months, the company says.
What Anthropic says it is not
"Scion is by no means a suitable replacement for git for general use," Anthropic writes, adding that its approach only makes sense for very large monorepos. It says Scion would be difficult to offer in a form broadly useful to other organisations, and that it wants to share lessons so that parts of Scion's approach can be applied to git, working with GitHub, GitLab and the open-source community.
Simon Willison, who summarised the post, describes Scion as sounding like "a reinvention of centralized source control along the lines of Subversion."
Limits
- Merge counts, infrastructure ratios and timelines are self-reported by Anthropic.