What changed
In an October 13, 2026 engineering disclosure, Anthropic said it replaced Git for its main monorepo with Scion, a bespoke server-centric version control system, and migrated the repository in five months. Anthropic says daily merges rose from about 2,500 in May to more than 30,000 by September; its monorepo's main history approached 100 GB and CI alone requested roughly 25 million checkouts daily. The seven-person team also built Antique code review, code search and source-control access controls. Scion is internal infrastructure, not a public replacement for Git.
Why it matters
This is a concrete case of AI coding throughput shifting bottlenecks from code generation to repository, review and CI infrastructure. The transferable lesson is that always-online automated workers may need cheap server-side history and diff APIs, incremental commits and sparse or virtualised working copies rather than repeated full local clones. Most builders should not migrate away from Git: Anthropic explicitly says its architecture only makes sense for extreme monorepos and agent swarms, and its measured gains are self-reported.
Why Git mirrors stopped scaling
Anthropic reports nearly 100 GB of main-branch history, a compressed working copy above 25 GB, and around 25 million daily CI checkouts. The company used shallow clones, sparse checkouts, snapshot seeds and several hundred Git mirrors, but freshness and history queries became expensive as merge volume accelerated. Its clients were predominantly online agents and jobs, so the offline independence central to Git was less valuable for this particular workload.
Scion makes the server do the source-control work
Scion stores changesets as write-ahead-log entries with content-addressed file references. Servers handle merges, rebases, blame, diff, ancestry and commits; clients keep bounded caches and materialise files only when requested. The design minimises global refs and packfile operations. CI jobs can often query the source-control service directly without a working copy, while regional caches and node daemons fetch immutable blobs on demand.
The performance numbers need attribution
Anthropic reports a 200-fold reduction in CPU and memory infrastructure footprint compared with its former Git mirroring fleet, checkouts two orders of magnitude faster, and a 25% CI CPU-capacity gain. It also reports cache-hit rates above 99%. These are internal vendor measurements without published independent benchmarks or a like-for-like general Git comparison. They should not be treated as predictions for typical repositories.
AI changed the engineering process, not just the tools
Anthropic says a seven-person team built Scion, Antique and related services within five months with heavy Claude assistance. Engineers continued to review changes, often focusing on intent, invariants and architecture; they report correcting agents that pursued brute-force approaches or unnecessary complexity. The company shifted attention toward post-merge and pre-deploy tests because conventional global merge locks are difficult at its rate of change.
Not a general-purpose Git replacement
Scion is neither open-sourced nor offered as a product. Anthropic says it would require organisations to replace surrounding code review, search and CI systems and that its economics are atypical. The company proposes working with GitHub, GitLab and the wider ecosystem on more scalable Git-compatible server APIs and sparse workflows instead of asking everyone to adopt Scion.