Anthropic replaces Git with Scion as merges hit 30,000 per day

Anthropic's decision to build Scion came after code changes merged per day rose from around 2,500 in May 2026 to over 30,000 in September. The company said its main branch history alone is almost 100 GB and that a compressed working copy is over 25 GB. It also performs on the order of 25 million code checkouts per day for CI, which it said would amount to over 600 PB of git traffic per day if fetched naively.

Simon Willison, writing on his blog, noted that a chart of those changes showed markers for public model releases in 2026 and guessed that the introduction of Fable/Mythos class models was a key factor in the increased coding velocity. He also described Anthropic's new code review system, called Antique, and said Scion sounded to him like a reinvention of centralized source control along the lines of Subversion.

Anthropic also makes extensive use of adversarial review. Willison noted that while this is incredibly effective, it can be myopic, and skilled engineers needed to steer Claude away from over-engineering parts of the system in response to hypothetical concerns.