Redesigning source control at Anthropic

Anthropic has unveiled their custom source control system, Scion, to handle the rapid increase in code changes over the past six months. In May 2026, Anthropic averaged around 2,500 code changes merged daily, rising to over 30,000 by September 2026. To visualize this, Anthropic shared a chart showing code changes alongside the public release dates of their models in 2026. It's likely that the launch of Fable/Mythos class models contributed to this surge in coding velocity.

Scion is described as a reinvention of centralized source control, similar to Subversion. With their main branch history nearing 100 GB, a compressed working copy exceeds 25 GB, and the system handles approximately 25 million code checkouts per day for CI processes. This would generate over 600 PB of git traffic daily if fetched naively. The new system was developed by a team of seven engineers and was completed in five months.

Anthropic also employs adversarial review extensively, using models like Claude and GPT to identify potential flaws in their code. While this approach is highly effective, it can sometimes lead engineers to over-engineer solutions in response to hypothetical concerns.