Anthropic has moved its main code repository off git and onto Scion, a version control system it built itself. The company says it merged about 2,500 code changes a day in May 2026 and more than 30,000 a day by September. The Register ties that jump to Claude writing more of the company's code. Git is built on the idea that every copy holds the full history. That stopped making sense when nearly every user was an always-online AI agent. Anthropic described the system on October 13 but will not release it.
Why git stopped working
"The bottleneck, now, is all of the developer infrastructure built for human scale," Anthropic writes in its engineering post. Writing code is no longer the slow part. Source control, builds and continuous integration (CI), the automated tests that run on every change, are.
The repository is a monorepo, a single repository that holds most of the company's code. Its size explains the strain:
| Measure | Figure |
|---|---|
| History of the main branch | Almost 100 GB |
| Compressed working copy | Over 25 GB |
| Checkouts per day for CI alone | About 25 million |
| Git traffic if each checkout were a plain clone | Over 600 PB a day |
| Internal git mirrors at the peak | Several hundred |
Anthropic tried the usual fixes. It used shallow and sparse clones, which download only part of a repository, plus internal mirrors and pre-built copies. Each got harder to run as merges rose. The pre-built copies drifted from the main branch, and tools such as diff and blame broke on partial clones.
How Scion works
Scion assumes its clients are always online. The server handles history, blame, commits, merges and rebases. Each machine keeps only a limited local cache and its working copy, the files it is actually editing.
Every change is stored as a changeset in a write-ahead log, a running record of changes. It lists the files that changed and points to their contents in blob storage. Files are fetched on demand, so a checkout never downloads a multi-gigabyte bundle first.
Scion runs in three tiers: a daemon on each machine, a cache in each region, and an authoritative server. Anthropic says each cache tier answers more than 99% of requests. Branches other than main and release branches are discouraged.
Scion is not a drop-in replacement for git. Scion engineer Nick G. explained the trade-off on Hacker News, as quoted by Simon Willison. "Since Claude is by far the heaviest user of Scion and is necessarily online, this just wasn't important to do," he wrote. Willison compares the result to older centralized systems such as Subversion.
What Anthropic says it gained
Anthropic reports these results on repositories that run natively on Scion:
- About 200 times less CPU and memory than its old git mirrors.
- CI and research checkouts about 100 times faster, taking seconds instead of minutes.
- A 25% cut in the CPU its CI system uses.
Seven people built Scion, a code review tool called Antique and about half a dozen related systems. The migration took five months. The Register reports that work on Scion began in January 2026.
The other tools include code search, per-directory access permissions and a way to land changes without one global merge queue. The team built them with Claude's help. A Claude-written rewrite of the early git-to-Scion import pipeline used 20 times less memory and ran four times faster, Anthropic says.
People still steered the work. "Very often, we found that the role of the engineers was to tell Claude 'no,'" Anthropic said, as quoted by The Register.
Will anyone else get Scion?
No. Anthropic says Scion is hard to offer in a broadly useful form and is not suited to general use. It is sharing the design instead and says it wants to work with GitHub, GitLab and the open source community.
The post suggests ideas git hosts could borrow without changing how git works. These include doing more work on the server and making sparse and shallow clones easier to combine. Git itself is also changing: we covered the debate over Git 3.0's SHA-256 default earlier this month.
What this means for developers
- Git is still fine for most teams. The Register notes that git remains the right choice for most repositories. Scion solves a problem that appears at tens of thousands of merges a day.
- Watch CI checkout costs as agents scale. About 25 million checkouts a day is what broke git here. If your agents clone the repository for every task, measure that traffic now.
- Treat agents as always-online clients. Offline copies matter to people, not to agents running in the cloud. Tools that fetch files on demand can cut the cost of each agent run.
- Expect git hosts to borrow these ideas. Anthropic wants GitHub and GitLab to adopt parts of the design. Server-side operations and better partial clones are the likely first steps.
- Keep humans in review. Anthropic's engineers spent much of their time rejecting Claude's suggestions. Plan for that review load when agents write most of the code.
This article was first published on Tech AI Wire.
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Sources
- Redesigning source control at Anthropic - Anthropic
- Anthropic reinvents source control as Claude floods the world with code - The Register
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