ANTHROPIC has replaced Git with a proprietary internal version control system named Scion, designed to cope with its large monorepo and the heavy output from Claude-style AI development. The migration, completed over five months, involved a small team of seven engineers who built custom code review, search, and permission-management tools to support the switch.
Anthropic stated there are no plans to open-source Scion and published an architectural overview to share server-side scaling insights with the wider software community. The move was prompted by growing daily code activity and the limits of traditional Git in their environment.
Evidence cited by Anthropic indicates the scale of the shift: May 2026 averaged around 2,500 daily merges, rising to about 30,000 by September 2026; the main branch history approached 100 GB, with a compressed working copy over 25 GB. To manage this, Scion uses a client-server model where clients request file trees and commit histories on demand, while the server cluster handles resource-intensive tasks such as commits, merges, and rebases.
Regional proxies cache frequently accessed objects, achieving cache-hit rates above 99%. Reports claim dramatic efficiency gains, including central CPU and memory usage reduced to 0.05% of previous levels and CI/checkout speeds improved by around 100×, allowing Anthropic to reclaim roughly a quarter of total CI processor capacity.
Anthropic emphasises Scion is not a universal replacement for Git but a tailored solution for massive corporate monorepos and high-density agentic workflows, with migration required for accompanying search, review, and integration pipelines.