A self-hostable GitHub alternative built for agents. Server-centric version control for monorepos of any size and any file type, from source code to multi-GB game assets. C++23 core, web forge on Ultimate.

Status (2026-11): design stage. Docs only, no code yet. "Cairn" is a working title.

Git puts all history in every clone and merges through queues. With hundreds of agents landing changes, that breaks. Cairn takes the best idea from each version control system (catalog) and builds one model:

• Server owns history. Commits, rebases and lands run server-side. Clients fetch only what they need.
• Land by compare-and-swap. No serial merge queue. Concurrent lands batch into one ref update per round; verification runs after merge, with flake-aware bisect and auto-revert.
• Cost scales with the change. A changeset is a delta against its parent. Root trees, blame, path history and the commit graph are derived indexes.
• Immutable, content-addressed objects (BLAKE3, zstd, FastCDC chunks). Any cache tier serves anything.
• Every file type. Versioned typemap: locks for unmergeable assets, chunked multi-GB files, previews, purge and archive.
• Optional working copies. Sparse checkout, a virtual filesystem (FUSE, NFS/FSKit, ProjFS), or none at all: query the API.
• Self-hostable. One box on PostgreSQL + disk; scale to YugabyteDB + S3 + regional caches.

cairn CLI ─▶ cairnd (node) ─▶ cairn-cache (regional) ─▶ cairn-server ─▶ PostgreSQL / YugabyteDB
                                                           ▲   └──────▶ disk / S3-compatible
                      cairn-web forge (Ultimate) ──JSON────┘

The repo is set up so coding agents, including smaller models, can do great work:

• CLAUDE.md: the contract (rules, layout, commands, boundaries).
• .claude/rules/: path-scoped rules that load automatically (C++, hot paths, determinism, formats, storage, web, docs).
• .claude/agents/: architect, cpp-implementer, reviewer, perf-engineer, dst-hunter, security-auditor, web-forge, docs-keeper.
• .claude/commands/: /planx, /feature, /check, /review, /bench, /hunt, /adr.
• .claude/skills/: dev loop, formats cheat sheet, deterministic simulation, performance patterns.

Cairn builds on published work from Meta's Mononoke (Bonsai data model), Sapling (segmented changelog) and EdenFS; Google's Piper, CitC and TAP; Jujutsu; Dolt (prolly trees); Perforce and Unity Version Control (typemaps, locking); Epic's Lore; and Anthropic's description of Scion. It is independent and not affiliated with any of them. Sources: prior art.