Anthropic replaced Git with a custom system as code changes surged 12-fold

Anthropic has disclosed that it built its own version control system to replace Git, saying the switch was forced by a sharp jump in how many code changes its engineers and AI systems submit each day. In a post describing the project, the company says it averaged about 2,500 merged code changes per day in May 2026. By September, that figure had passed 30,000 per day, a roughly twelvefold increase in four months.

The new system is called Scion. Anthropic pairs it with a separate code review tool called Antique. According to the company, the scale of its codebase made standard Git increasingly hard to manage: the history of its main branch is close to 100GB, and even a compressed working copy of the code runs past 25GB. Anthropic says it performs around 25 million code checkouts a day for continuous integration testing, or CI, automated checks that run on every code change, not counting the checkouts needed for developer machines, AI agent sandboxes, and research jobs. Fetched the normal way, the company estimates that traffic would add up to more than 600 petabytes of Git data moved per day.

Built in five months, by seven people

Anthropic says the Scion team numbered seven engineers and finished moving the company off Git in five months. The company credits Google’s internal system Piper and Meta’s Sapling as influences, and says the “derived data” concept at the core of Scion was shaped heavily by Mononoke, the server component behind Sapling.

Offline support, a staple of Git, was left out. Asked about this on Hacker News, Scion engineer Nick G said: “Since Claude is by far the heaviest user of Scion and is necessarily online, this just wasn’t important to do.” That framing points to who Anthropic is building for: much of the surge in commits is coming from Claude itself writing and submitting code, not only from human engineers.

Anthropic also described leaning heavily on adversarial review, where one model checks another’s work, while building Scion. The company says this caught real problems but also had a cost, noting that “while this is incredibly effective in many ways, it can also be myopic, so skilled engineers needed to steer Claude away from over-engineering parts of the system in response to hypothetical concerns.”

The commit-rate numbers come from Anthropic’s own post, so they have not been independently verified. Simon Willison, who wrote up the disclosure, annotated Anthropic’s chart of the change with the public release dates of the company’s 2026 models and said he suspects the introduction of Claude’s Fable and Mythos model families, used internally before their public launch, is a major reason the numbers climbed so fast. Anthropic has not said how much of the daily change volume comes from its own models versus human engineers, which is the detail worth watching for next.