Blog Post Says Anthropic's Scion Copies Meta's Mononoke, and Anthropic's Post Names Only Sapling

A developer who writes as Rain on the blog sunshowers said on Oct. 14 that Anthropic's new version control system, Scion, copies the design of Mononoke, the source control server in Meta's open-source Sapling project. Rain says they helped build Mononoke at Meta. Anthropic's engineering post credits Sapling and Google's Piper for "hard-won lessons" and does not mention Mononoke.

No Anthropic response appears in either of Rain's posts or in Anthropic's engineering post.

What Anthropic says it built

Anthropic said it moved its monorepo off git in five months, after daily merges grew from about 2,500 in May 2026 to more than 30,000 by September. A team of seven built Scion and Antique, its code review tool, and ran the migration.

Scion is server-centric and assumes clients, mostly agents and CI jobs, are always online. Its unit of work is a changeset, an entry in a write-ahead log that stores the files changed relative to the first parent, with content-addressed pointers to blobs. Refs are compare-and-swap updates on a single database row.

Because changesets record only changes, Scion builds other structures from them, including indices for blame, path history and commit-graph skip lists. Anthropic says these are computed "through one generic mechanism," each defined recursively in terms of its values for a changeset's parents.

Figure	Anthropic's number
Merges per day, May 2026	about 2,500
Merges per day, September 2026	more than 30,000
CI checkouts per day	about 25 million
CPU and memory vs old git mirroring	about 200 times less
Main-branch history	almost 100 GB

These are Anthropic's own figures. Scion is not a product, and Anthropic says it would be hard for other organisations to adopt. It plans to share design thinking so teams can apply parts of it to git, and to work with GitHub, GitLab and the open-source community.

What the sunshowers posts claim

Rain's first post, dated Oct. 14, sets Anthropic's description of derived data beside a 2022 Oxide candidate document that describes Mononoke's design. In that document, blame becomes "O(number of commits that touch the file) rather than O(number of commits in the repository)." Rain says the Anthropic post "borders on plagiarism" and suggests Claude designed the system when Meta's team supplied many of the underlying insights.

Rain cites shared chats in which Opus 5.5 and GPT-6 Astra, asked about prior art, pointed to Mononoke. Those chats are Rain's evidence and were not checked here. Rain does not object to copying a design, writing that credit is "now easier, and more important, than ever."

The follow-up, dated Oct. 15, goes further. Rain says they privately received confirmation that Scion "intentionally and blatantly ripped off Mononoke," with overlap in personnel, and does not name who confirmed it. Rain says Scion changesets are "exactly the same" as Mononoke's Bonsai changesets and that the server-side API is similar, while conceding earlier systems had similar APIs. Treat the confirmation as an unverified claim.

What cannot be checked

Anthropic's post says Scion has dedicated structures for file blame and path history. It does not describe how blame is computed, so the complexity comparison at the centre of Rain's first post has nothing on the Anthropic side to be tested against.

The dispute is over credit rather than licence. Mononoke is open source, and the sunshowers page does not state its licence. Anthropic's post names Sapling and Piper as prior art without detailing what was borrowed, which is the gap Rain objects to. Rain's follow-up says their default assumption is now that Anthropic's public engineering posts are "largely bad-faith ripoffs."

Adjacent coverage on this site tracks other agent tooling that is spreading fast, including the rea reverse-engineering agents repository and a comparison of four Rust agent verifiers.

Neither post shows an update from Anthropic as of Oct. 15, two days after its engineering post.