Anthropic's Scion, rebuilt in the open — on top of Gitaly

What it does

open_scion takes the design Anthropic described in its “Redesigning source control” post — a server-centric VCS built for hundreds of agents reading, writing and landing code concurrently — and implements it as open source. Clients stay online and fetch only what they need; history queries, blame, and search run server-side and get memoized; changes land via compare-and-swap instead of a merge queue. Underneath it all sit ordinary Git repositories, so your history isn’t held hostage by a new format.

The interesting bit

It’s a fork of Gitaly, GitLab’s Git RPC server, which supplies the battle-tested Git plumbing — every upstream Gitaly RPC still works. The storage core moves authoritative state into SQLite or PostgreSQL plus object storage (S3, GCS, Azure, or plain files), with local Git repos demoted to rebuildable caches, so server nodes are stateless and you scale by adding them.

Key highlights

• UserLand rebases onto the tip and lands with compare-and-swap; concurrent lands on a branch batch into one reference update — no merge queue.
• Conflict resolution is pluggable: any LLM provider, a deterministic tool, or nothing, via a versioned vendor-neutral protocol (the server itself never calls a model).
• Agents are first-class users: per-agent tokens, RPC allowlists, branch policies, and review rules where a comment-only change needs fewer approvals than a logic change.
• graft-verify tests the tip, bisects failures, and reverts the culprit change automatically.
• “Superfan” monitoring agents watch the changeset log and report what matters, including through Slack.

Caveats

• Version 0.1.0; interfaces may change before 1.0, and known gaps (e.g. garbage collection of packs in the storage core) are tracked in LIMITATIONS.md.
• Not affiliated with or endorsed by Anthropic — it’s an implementation of the publicly described design, not the real Scion.
• Building it requires Go 1.26 plus Meson and Ninja, since the build compiles a bundled Git.

Verdict

If you’re running swarms of coding agents against shared branches and Git’s clone-push-merge cycle is the bottleneck, this is the most complete open attempt at the Scion design you’ll find. Solo humans pushing once a day can safely ignore it.