Anthropic swaps Git for in-house system as AI code surges

Anthropic has replaced Git with Scion, a version control system it built in-house. The company said the switch became necessary after AI-written code overwhelmed its developer infrastructure. It moved its entire monorepo to the new system in five months. In a Tuesday, Oct. 13, engineering blog post, the company wrote that "in May 2026, Anthropic averaged around 2,500 code changes merged per day; as of September, we merged over 30,000 per day".anthropic+1

The company said writing code is no longer its main constraint. The slowdowns now come from "developer infrastructure built for human scale: source control, builds, and CI". Anthropic also said it does not expect that growth to slow, calling the latent demand for merging code "practically infinite".anthropic

Why Git buckled

Anthropic said Git's decentralized design is the right choice for most repositories, but its own codebase is far outside that range. The history of its main branch alone is almost 100 GB, and a compressed working copy is more than 25 GB. The company runs about 25 million code checkouts a day for continuous integration (CI), the automated building and testing of code. "Fetched naively, that would be over 600 PB of git traffic per day," it wrote. Before the switch, it ran several hundred internal Git mirrors to handle the load.theregister+1

Scion assumes that its users, which are almost all agents and automated jobs, are always online. The server does the heavy work: it handles commits, merges and rebases, and it answers questions about file history and who changed which line. Checkouts are sparse or virtualized, so a job only downloads the files it actually reads. Anthropic says Scion uses 200 times less CPU and memory than its old Git mirror fleet. Checkouts now take seconds instead of minutes, and CI freed up 25% of its CPU capacity. Anthropic noted that Google's Piper and Meta's ↘ 3.32% Sapling are earlier examples of custom systems built for giant monorepos.anthropic+2

Built by seven people and Claude

A seven-person team built Scion, a code review tool called Antique, and several related systems while daily merge volume grew tenfold. In one case, an early internal version of Claude Mythos Preview rewrote a handwritten pipeline for moving data from Git into Scion. The new version used 20 times less memory and ran four times faster. The engineers said Claude now writes code better than they do, but they still decide on the system's design. "Very often, we found that the role of the engineers was to tell Claude 'no,'" the company wrote.gigazine+2

No open-source release

Anthropic called Scion "by no means a suitable replacement for git for general use." It said the system only makes sense for enormous monorepos, and the company will not release it publicly. Instead, it is publishing its design ideas. It expects similar scaling problems to become common at other companies within six to 12 months, and it said it hopes to work with GitHub Microsoft Corporation ↘ 1.75%, GitLab ↗ 1.64% and the open-source community to adapt them for Git.theregister+2