Redesigning source control at Anthropic

Summary

Anthropic explains why it replaced git for its monorepo with an in-house system, Scion, built in five months by seven engineers working with Claude. Agent-driven merge volume grew more than tenfold in four months, and git's full-clone, offline-first design did not fit clients that are almost all always-online agents and CI jobs.

Key points

• Merges per day: about 2,500 in May 2026, over 30,000 by September 2026. About 25 million checkouts per day for CI alone; the repo history is almost 100 GB.
• Design: server-side commits, merges and rebases; clients fetch trees and blobs on demand; source-control queries exposed as APIs, so many jobs need no checkout; sparse and virtualised working copies; tiered daemon, regional cache, and Spanner-backed servers.
• No global merge queue: Anthropic relies on post-merge and pre-deploy testing, with Claude resolving most conflicts.
• Also built: a code review tool (Antique) with semantic approval rules, code search, and per-directory access control.
• Results: about 200x less CPU and memory than the old git mirrors; CI and research checkouts went from minutes to seconds.
• Engineers' role shifted to setting constraints and reviewing intent, often telling Claude "no" when it broke invariants or over-engineered. "Superfan" agents watch each component's telemetry.
• Scion will not be released; Anthropic plans to share ideas with GitHub, GitLab, and the git community.

Why it matters

Concrete numbers on how coding agents change engineering infrastructure: at agent scale, source control, merge queues, and review become the bottleneck.