---
title: "Redesigning source control at Anthropic"
description: "At Anthropic, Claude’s ability to iterate on code at high speed has caused git-based source control to become the bottleneck in their development workflow. To handle the growth from 2,500 to over 30,000 daily code merges, the team built Scion, an in-house version control system, fully migrating their monorepo from git in five months. Scion diverges from git by assuming clients are always online, shifting computation to the server, and using horizontally scalable data structures without any fully materialized copy of the repository. The system stores changes as entries in a write-ahead log, supports sparse and virtualized checkouts, and enables cheap, lock-free commits that scale with change size rather than repository size. Anthropic shares Scion’s design principles to help the broader ecosystem prepare for AI-driven scaling challenges, noting that similar pressure points will emerge for other organizations as AI coding tools become more common."
---

# Redesigning source control at Anthropic

[Anthropic](https://yomu.fyi/company/anthropic) · Anthropic · Oct 13, 2026

**Type:** Explainer

## Summary

At Anthropic, Claude’s ability to iterate on code at high speed has caused git-based source control to become the bottleneck in their development workflow. To handle the growth from 2,500 to over 30,000 daily code merges, the team built Scion, an in-house version control system, fully migrating their monorepo from git in five months. Scion diverges from git by assuming clients are always online, shifting computation to the server, and using horizontally scalable data structures without any fully materialized copy of the repository. The system stores changes as entries in a write-ahead log, supports sparse and virtualized checkouts, and enables cheap, lock-free commits that scale with change size rather than repository size. Anthropic shares Scion’s design principles to help the broader ecosystem prepare for AI-driven scaling challenges, noting that similar pressure points will emerge for other organizations as AI coding tools become more common.

## Context

Anthropic’s Claude generates code at unprecedented speed, causing git-based source control to become a bottleneck. The monorepo grew to 100 GB history and 25 million daily checkouts, making git’s distributed model unsustainable. The team needed a system optimized for always-online agents and massive merge rates.

## Approach / What changed

Anthropic built Scion, an in-house version control system designed for agent-scale source control. Key principles include assuming clients are always online, client-server asymmetry where the server handles computations, horizontally scalable data structures, cheap lock-free server-side commits, and sparse/virtualized checkouts. The system uses a write-ahead log of changesets and distributes repository data without a fully materialized copy.

## Takeaways

- Scion uses a write-ahead log of changesets instead of git’s object model, enabling cheap server-side commits that scale with change size.
- Scion assumes clients are always online and shifts computation to the server, allowing thin clients with minimal local data and eliminating the need for full repository clones.
- Anthropic migrated from git to Scion in five months, finding that git’s distributed offline design was counterproductive for their agent-heavy workload and that removing bottlenecks immediately increased code production.

## FAQ

### Why did git break at Anthropic's scale?

Git's decentralized design assumes offline clients, but Anthropic's agents are always online. The monorepo's history of nearly 100 GB and 25 million daily checkouts caused unsustainable traffic, and workarounds like mirrors and seeds became unmanageable as merge rates increased.

### How does Scion handle large checkouts?

Scion uses sparse and virtualized checkouts that materialize files on demand. CI jobs can query the server for diffs without creating a full working copy, reducing the 25 GB compressed checkout to only the needed files.

### What is a changeset in Scion?

A changeset is an entry in a write-ahead log, similar to a git commit but stored with delta compression relative to the first parent. Scion stores only the changed files and their hashes, with the server computing full file contents on request.

**Tags:** [Agents](https://yomu.fyi/topic/agents), [AI](https://yomu.fyi/topic/ai), [Architecture](https://yomu.fyi/topic/architecture), [Git](https://yomu.fyi/topic/git), [Scalability](https://yomu.fyi/topic/scalability)

- Source: [Anthropic](https://www.anthropic.com/engineering/redesigning-source-control)
- Source URL: https://www.anthropic.com/engineering/redesigning-source-control
- Ingested by Yomu: 2026-10-13T18:00:18.341Z

[Read original post](https://www.anthropic.com/engineering/redesigning-source-control)