Redesigning source control at Anthropic
Simon Willison’s Weblog Simon Willison
Anthropic redesigned its source-control system by building Scion and replacing Git after code-change volume increased. In May 2026 it averaged about 2,500 merged code changes per day, rising to over 30,000 per day by September. The switch introduced the “Antique” code review system and a new migration completed in five months.
Why it matters
Redesigning source control at Anthropic Interesting piece from Anthropic describing why and how they built their own custom alternative to Git, based on the dramatic increase in rate of changes to their code after the past six months. In May 2026, Anthropic averaged around 2,500 code changes merged per day; as of September, we merged over 30,000 per day. They included a chart showing those changes. I had Claude modify their chart (code here) to add markers for the public releases of each of their models in 2026: This shows public release dates, not the date when models became available for Anthropic's internal use. My guess is that the introduction of Fable/Mythos class models is a key factor in the increased coding velocity. "Antique" is their new code review system. The source control system is called Scion, and sounds to me like a reinvention of centralized source control along the lines of Subversion. They had some pretty extreme requirements: The history of our main branch alone i