Anthropic has moved its entire monorepo from Git to Scion, an in-house version control system built to handle a sharp increase in AI-written code. The migration took five months.

The company reported that daily merged code changes rose from roughly 2,500 in May 2026 to more than 30,000 in September. Anthropic said writing code is no longer the main limitation. Instead, the pressure has shifted to developer infrastructure designed for human-scale workflows, including source control, builds, and continuous integration (CI).

Anthropic expects demand for merging code to remain extremely high. In its description of the problem, the company called that latent demand “practically infinite.”

Why Anthropic Moved Beyond Git

Anthropic said Git remains the right decentralized version control system for most repositories. Its own monorepo, however, had grown beyond the range the company considered practical for Git-based infrastructure.

The main branch’s history is nearly 100 GB, while a compressed working copy exceeds 25 GB. Anthropic also runs about 25 million code checkouts each day for CI, which automatically builds and tests code.

If each checkout fetched data naively, Anthropic estimated that this activity would create more than 600 PB of Git traffic per day. Before adopting Scion, the company operated several hundred internal Git mirrors to manage the load.

The problem was not simply storing a large repository. It was supporting a large volume of automated activity around that repository. As AI-generated code increased the number of changes moving through the system, the surrounding source-control, build, and CI infrastructure became the limiting factor.

How Scion Handles Source Control at Anthropic

Scion is built around a different set of assumptions from Git. Anthropic said that nearly all Scion users are agents and automated jobs, and those users are assumed to be online.

Rather than pushing much of the work to local clients, Scion places more responsibility on the server. The server manages:

  • Commits
  • Merges
  • Rebases
  • File-history queries
  • Questions about who changed a specific line

Scion also uses sparse or virtualized checkouts. A job downloads only the files it reads instead of retrieving a full working copy.

According to Anthropic, this approach has reduced the CPU and memory needed for its source-control infrastructure. The company said Scion uses 200 times less CPU and memory than its previous Git mirror fleet.

Faster Checkouts and More CI Capacity

Anthropic said that Scion has changed the speed of its automated workflows. Checkouts that previously took minutes now take seconds.

The company also reported that the change freed 25% of its CI CPU capacity. For an environment completing around 25 million daily code checkouts, reducing checkout work can directly reduce the amount of infrastructure needed to support automated builds and tests.

Scion is not the first purpose-built source control system for a very large monorepo. Anthropic pointed to Google’s Piper and Meta’s Sapling as earlier examples of custom systems designed for that type of scale.

A seven-person team developed Scion, a code-review tool called Antique, and related systems while daily merge volume increased tenfold.

Anthropic also described a case where an early internal version of Claude Mythos Preview rewrote a handwritten pipeline that moved data from Git into Scion. The rewritten pipeline used 20 times less memory and ran four times faster.

The company said Claude now writes code better than its engineers do, while the engineers continue to make the system-design decisions. In practice, Anthropic described engineers as frequently needing to tell Claude “no.”

That distinction matters in the company’s account of the project. AI assistance contributed to implementation work, but the system’s design decisions remained with the engineering team.

Scion Is Not a General Git Replacement

Anthropic has been clear that Scion is not intended as a replacement for Git in general use. The company said the system only makes sense for enormous monorepos and is “by no means a suitable replacement for git for general use.”

Scion will not be released as open-source software. Instead, Anthropic is publishing its design ideas.

The company expects comparable scaling issues to emerge at other companies within six to 12 months. It said it hopes to work with GitHub, GitLab, and the open-source community to adapt relevant ideas for Git.

What Anthropic’s Scion Migration Shows

Anthropic’s move from Git to Scion reflects the infrastructure strain created when automated systems generate and merge code at a far higher volume than traditional developer workflows.

For Anthropic, the answer was not to replace developer work with more code generation. The company said the constraint had already shifted away from writing code and toward the systems that store, build, test, review, and merge it.

Scion was built for that environment: an enormous monorepo, millions of CI checkouts each day, and users that are primarily agents and automated jobs. Anthropic does not present it as a tool for ordinary repositories, but as an internal system shaped by its own scale and workflow.

FAQ

Why did Anthropic replace Git with Scion?

Anthropic said its AI-driven code volume outgrew the developer infrastructure built around Git. Its monorepo and CI workload required hundreds of internal Git mirrors before the move.

How much did Anthropic’s code merge volume increase?

Anthropic averaged around 2,500 merged code changes per day in May 2026. By September, it reported more than 30,000 daily merges.

Will Anthropic release Scion as open source?

No. Anthropic said Scion is designed for enormous monorepos and is not suitable as a general-purpose Git replacement. The company plans to publish its design ideas instead.