Anthropic Builds Scion to Replace Git After Merges Hit 30,000/Day

Anthropic has replaced Git in its monorepo with Scion, a custom version-control system built by seven engineers in roughly five months, and the company says it will not be open-sourced. In a post on its engineering blog, the company reports that daily merges on the repo climbed from "around 2,500 code changes merged per day" in May 2026 to "over 30,000 per day" by September, a jump it attributes largely to agents writing code.

The framing is that Git's distributed design cracked under agent-scale load. "The properties that make git excellent for distributed, offline work were actively undermining our ability to scale as the repo grew," the post reads. The main-branch history alone runs to almost 100 GB, the compressed working copy sits above 25 GB, and CI pulls roughly 25 million checkouts per day on its own.

Scion is built around an always-online client-server model with regional caches, with each tier running at over 99% cache hit rate. Anthropic claims the new infrastructure runs with a 200x smaller CPU and memory footprint than its previous Git-mirroring setup, and that "CI and research jobs check out code two orders of magnitude faster (in seconds rather than minutes)." A companion code-review tool, Antique, was built in parallel. This is one of 344 Anthropic items our tracker has logged in the last 90 days.

The post also credits Claude for part of the system itself. On the Git-to-Scion ingestion pipeline, the engineers write: "We asked it to rewrite the whole thing, and with no further instructions, it produced a version that consumed 20x less memory and was 4x faster."

Anthropic is explicit that the tool is not for general release. "Scion is by no means a suitable replacement for git for general use," the post says, and the company instead urges the Git community to adopt compatible server-side improvements, better RPCs, and improved negotiation protocols for agent-scale workloads.