Summarize with AI
Anthropic has moved its monorepo off git onto Scion, a version control system it built itself. Agents pushed merges from around 2,500 a day in May 2026 to over 30,000 a day in September, and git's model of a full copy in every clone could not keep up.
Anthropic described the switch on its engineering blog on October 13, 2026. Scion is internal only: Anthropic says it is "by no means a suitable replacement for git for general use".
Why git stopped working
- Size. Main branch history is almost 100 GB. A compressed working copy is over 25 GB.
- Checkouts. CI runs about 25 million checkouts a day. Fetching naively "would be over 600 PB of git traffic per day".
- Workarounds. Shallow clones, sparse checkouts, partial clones and repository snapshots, served by several hundred git mirrors at peak. They got harder to run as merge rates rose.
What Scion is
Scion keeps commits, merges, rebases and history on the server. Clients ask for what they need and keep a small cache.
| Tier | What it does |
|---|---|
scion-daemon |
Runs on each node and stores objects for the whole node |
scion-cache |
Regional cache, over 99% hit rate |
scion-server |
Source of truth, reads from Spanner and cloud storage |
Alongside Scion the team built Antique, a code review tool. It can set rules on who must approve which changes, so reviewers can focus on sensitive ones.
The numbers
| Item | Value |
|---|---|
| Merges per day, May 2026 | around 2,500 |
| Merges per day, September 2026 | over 30,000 |
| Migration time | 5 months |
| Team | 7 people (Scion, Antique and related systems) |
| Footprint vs old git mirrors | 200x smaller (CPU and memory) |
| CI and research checkouts | about 100x faster, seconds instead of minutes |
| CI CPU capacity saved | 25% |
The team used Claude Mythos Preview to rewrite the git-to-Scion ingestion pipeline. The new version used 20x less memory and ran 4x faster.
Will others get it?
Anthropic has not announced plans to release Scion. It says "it's difficult to offer Scion in a form that would be broadly useful to other organizations." Instead it shares the design so the git community can borrow parts of it, and it suggests working with GitHub, GitLab and open source developers.
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