Redesigning Source Control at Anthropic

Anthropic rebuilt its source-control stack because development velocity exploded: daily merged changes rose from about 2,500 in May to over 30,000 by September, with a peak above 30,000 on September 9. The main branch history approaches 100 GB and a compressed working copy exceeds 25 GB, while CI alone performs on the order of 25 million checkouts per day - traffic that would amount to hundreds of petabytes if served by naive Git. To meet those scale and performance constraints, a seven‑person team created Scion, a custom system that leans toward a centralized, server-backed model and adopts the concept of derived data (building on ideas similar to Google’s Piper and Meta’s Sapling/Mononoke). They also introduced a new review workflow called “antique” which rapidly replaced pull requests as the dominant change type during the summer.

The redesign traded some traditional Git features for operational efficiency: migration from Git completed in five months, and Scion intentionally omits offline support because the largest consumer (their Claude models) is always online. Anthropic makes extensive use of adversarial review - having models find flaws - which speeds discovery but can be myopic and requires experienced engineers to avoid over‑engineering. Model release timing (notably the Fable/Mythos class) correlates with the surge in commits, suggesting product shifts drove much of the increased coding velocity.