What Scion does
Scion is the source-control system Anthropic built after merges grew from about 2,500 to more than 30,000 a day. Clients are always online, so the server handles history, blame, commits, merges and rebases. A changeset log with content-addressed blobs replaces packfiles, derived indices serve blame and path history, and a daemon, regional cache and server tier keep checkouts fast. Anthropic reports cache hit rates above 99 percent and an infrastructure footprint about 200 times smaller than its git mirrors.
What Nool does
Nool is a coordination and governance layer for coding agents. Before an agent edits shared code it announces intent and takes a lease on the files, symbols or contracts it will touch. Its proposal is checked against that intent, validated, and solidified into a knot that carries the reason for the change, attestations and a causal history. Findings recorded by one session are available to the next, and blast radius can be computed before an edit rather than after.
How each handles two agents colliding
Scion lets concurrent changes land server-side without a global merge lock and relies on post-merge and pre-deploy testing to catch conflicts, with Claude repairing them afterwards. Nool prevents the collision up front: each agent announces intent and takes a lease under its own identity, conflict discovery shows the overlap before any edit, and the proposal is checked against what the agent said it would touch.
Where each fits
Scion is built for one very large monorepo: no fully materialised copy anywhere, sparse and virtual working copies, every operation exposed as an API and directory-level access control. It has carried a tenfold rise in load at Anthropic. Nool is built for the many repositories most companies run, where the questions are which agent may change what, why a change was allowed and what the next agent should know. It keeps a full local store per repository, mirrors to git and nests into a workspace, and it installs today.
Where Nool is ahead
Availability and the coordination layer. Scion is internal and, in Anthropic's words, difficult to offer in a form broadly useful to other organisations. Nool is an installable CLI that works on existing git repositories with any agent. It adds intent-carrying changes, leases, governance in the write path, signed approvals and attestations, agent memory and blast radius, and it nests from one repository up to a workspace of many projects with rolled-up health and cost.
Using both ideas together
They compose. A Scion-style store gives you fast, scalable storage and serving. A coordination layer decides who may use it, why a change was allowed and what the next agent should remember. If you are running on git today, Nool is the part you can adopt without building the storage layer first.
Scion vs Nool, side by side
| Criterion | Scion | Nool |
|---|---|---|
| Primary problem | Source control at very large scale with many agents | Coordinating and governing agents on a repository |
| Availability | Internal to Anthropic, not offered as a product | Installable CLI with a free Community tier |
| Storage model | Server-side changeset log, content-addressed blobs, no packfiles | Local knot store with a git mirror |
| Concurrent agents | Changes land without a global lock; conflicts caught by tests and repaired | Intent, leases and conflict discovery before the edit |
| What a change records | Files changed relative to the first parent | A knot with intent, evidence, attestations and causal history |
| Review and governance | A separate review tool with semantic approval constraints | Gates, invariants and signed approvals in the write path |
| Agent memory | Not described | Findings and decisions recalled by later sessions |
| Working with existing git | Replaces it for the monorepo | Mirrors to git; GitHub or GitLab stay the remote |
| Hierarchy | One very large monorepo | Nests from repository to workspace of projects |
| Access control | Directory-level permissions in the monorepo | Org identity login, scoped leases and approvals |
| Built for | Tens of thousands of merges a day on one monorepo | Teams and enterprises with many repositories, nested into one workspace |
| Evidence | Anthropic's reported cache hit rate and checkout times | Pre-registered 35-agent benchmark: 98 to 100 percent of tickets accepted vs 33 to 62 percent for a plain git fleet |
Frequently asked questions
- Is Nool a replacement for Scion?
- No. Scion is a storage and serving system built for one organisation's monorepo. Nool is a coordination and governance layer that runs on ordinary git repositories. They address different problems and could be used together.
- Can I use Scion?
- Anthropic describes Scion as internal and says it is difficult to offer in a form that would be broadly useful to other organisations. Nool is available today as an installable CLI.
- Is Nool built for monorepos like Scion's?
- Scion is designed for one very large tree served centrally. Nool coordinates the repositories you already have, with a full local store and a git mirror per project, and scales by composing many coordinated projects into a workspace with rolled-up health, goals and cost.
- How does Nool handle agents colliding?
- Agents announce intent and take leases before editing, and Nool checks proposals against that intent and against other agents' work. In a pre-registered, vendor-run benchmark at 35 agents it accepted 98 to 100 percent of tickets against 33 to 62 percent for a plain git fleet. Landing a change through Nool adds a few hundred milliseconds.