Why Anthropic replaced git with Scion: an engineering breakdown
Anthropic went from about 2,500 merged changes a day to more than 30,000. Git's assumptions broke first. This is what they built instead, and when it does not apply.
Anthropic went from about 2,500 merged code changes a day in May 2026 to more than 30,000 a day by September. Writing code stopped being the bottleneck. Git became the bottleneck.
This is a summary of what they reported, what they built, and where it applies. All figures below come from Anthropic's own post.
Where git stopped scaling
The main-branch history of their monorepo is almost 100 GB. A compressed working copy is over 25 GB.
They run about 25 million checkouts a day for CI. Fetched the usual way, that would be more than 600 PB of git traffic a day. At the peak, they ran several hundred internal git mirrors to keep up.
The core mismatch is in git's design. Git assumes clients work offline and each one holds the whole repository. Nearly all of Anthropic's clients are agents and CI jobs that are always online.
What Scion changes
They built Scion, an in-house version control system, and moved off git in five months.
- The server owns history. Blame, commits, merges, and rebases run on the server.
- Clients keep a small cache. They check out only the folders they need, and files load on demand.
- Many jobs skip the checkout entirely and ask the server through an API.
- Commits are cheap and lock-free.
- No machine holds a full copy of the repository.
Each caching layer reports a hit rate above 99%.
Reported results
- About 200 times less CPU and memory than the old git mirrors.
- Checkouts in seconds instead of minutes.
- A 25% CPU capacity gain for CI alone.
The team
Seven engineers built Scion, plus Antique, a new code review tool, while daily merges grew tenfold.
Anthropic says an early internal snapshot of Claude Mythos Preview rewrote their ingestion pipeline to use 20 times less memory and run 4 times faster.
They also say the engineers often had to tell Claude no, when a change broke system constraints or added complexity the system did not need. Reviewers now check the intent of a change, not every line. They keep one Claude per component watching that component's health in Slack, and they call these "superfans."
When this does not apply
Anthropic is clear that Scion is not a general replacement for git. It makes sense for very large monorepos with mostly online, automated clients. Their suggestion for everyone else is that git servers could adopt the same ideas: server-side history, partial checkouts, and files loaded on demand.
The lesson for most teams is narrower. When agents produce most of the commits, the expensive part moves from writing code to moving, checking out, and reviewing it. Measure that path before it becomes the bottleneck.