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Anthropic moved its monorepo off git after daily merges rose from 2,500 to 30,000

A team of seven built the replacement in five months, and every one of them now rates Claude as the better coder.

By Think Facility · October 13, 2026 · Anthropic AI coding Save

On October 13, 2026, Anthropic said it had moved its main code repository off git and onto Scion, a version control system it built itself in five months. The engineering post gives the reason in one number. In May 2026 the company merged around 2,500 code changes a day. By September it was over 30,000.

That's twelve times as much code in four months. Claude writes most of it, so the thing that broke git at Anthropic was, in a sense, its own model. A team of seven built Scion (and a code review tool called Antique) while the daily merge volume went up 10x.

Daily merges, May 2026

around 2,500

Daily merges, September

over 30,000

Built by

a team of seven, in five months

CPU and memory

200x smaller than the old git mirrors

Checkouts

seconds rather than minutes

Why git stopped fitting

Git gives every clone a full copy of the repository, so you can work offline. For most projects that's fine, the post says. Not here. Anthropic's main branch history alone is almost 100 GB, and CI checks out code on the order of 25 million times a day. Fetched naively, that'd be over 600 PB of git traffic daily (a petabyte is a million gigabytes).

It ran several hundred internal git mirrors at the peak. That stopped working. Nearly all of Anthropic's clients are agents and jobs, and they're always online anyway, so Scion puts the work on a server that answers questions about history and blame and does the merges itself. Clients fetch only the files they need. Plenty of jobs never check anything out at all.

What Claude did, and what the engineers did

The sharpest example in the post is a rewrite. The team had largely handwritten the first git-to-Scion ingestion pipeline. Then they gave it to an early internal snapshot of Claude Mythos Preview, with no further instructions.

Every engineer on the project now rates Claude as better at writing code than they are, the post says. Their job shifted to design calls and, very often, telling Claude "no." Agents sometimes broke core system constraints to fit a pattern, and adversarial review (one Claude checking another) pushed Claude into over-engineering that people had to steer it away from.

Humans still review every change to Scion. They just don't read every line now; they read for what the change is trying to do. For anyone running a team, I'd say that's the biggest shift in the post.

Not for sale

Anthropic won't release Scion. It's no suitable replacement for git for general use, the company says. It only makes sense for enormous monorepos, the kind with tens of thousands of engineers or swarms of agents writing code.

The company expects this class of problem to become more common over the next six to 12 months. Instead of a product, it's offering ideas to git hosts: richer server-side APIs and a different fetch protocol. It says it wants to work on that with GitHub, GitLab and the open-source community.