Anthropic Replaced Git With Scion as AI Pushed Merges Past 30,000 a Day

On October 13, Anthropic revealed that it replaced Git across its primary monorepo with an in-house version control system named Scion. The catalyst was not human headcount, but autonomous software agents: between May and September 2026, daily merges into trunk jumped from 2,500 to over 30,000.

The engineering strain exposed Git's fundamental design assumptions. Built for distributed workflows where each developer keeps a full local history offline, Git buckled under automated systems. Anthropic reported that its trunk history approaches 100 GB, with a compressed working copy exceeding 25 GB. Continuous integration alone demanded roughly 25 million checkouts per day, which would have consumed over 600 petabytes of daily network traffic using standard fetches. The company had resorted to maintaining hundreds of internal mirrors before the approach became unmanageable.

Instead of patching distributed trees, Scion functions as a strictly centralized engine. It tracks changesets through a write-ahead log pointing to content-addressed blobs, updating head references using compare-and-swap operations in a single row of Google Spanner. Packfiles and repository repacking are eliminated entirely. A local client daemon routes calls through multi-tier regional caches boasting hit rates above 99%.

According to Anthropic, the shift reduced infrastructure CPU and memory consumption roughly 200-fold compared to the mirror cluster, dropping checkout times from minutes to seconds and freeing 25% of CI computing capacity. Seven engineers built the system alongside an internal review platform called Antique in five months. The data migration pipeline itself was rewritten by Claude, cutting memory use by a factor of 20 while running four times faster.

Surging commit frequency also broke traditional merge queues. Global locks proved unworkable at tens of thousands of daily branches, prompting Anthropic to abandon pre-merge locking in favor of post-merge, pre-deployment testing monitored by automated watchdog models. Human engineers reportedly spent much of their review time reigning in model suggestions that violated core system constraints.

Those hoping for an open-source release will be disappointed. Anthropic stated that Scion is not suited for general-purpose use, maintaining that standard Git remains the correct choice for ordinary repositories.

Source: habr

Primary source: Anthropic Engineering