Short answer
World of ClaudeCraft is a free, open-source MMO by Levy Street that runs in the browser, on desktop and on mobile. Its first playable version was built in about 48 hours in June 2026 with Anthropic’s Claude Fable 5 coding model. Since launch, 60 contributors have added close to 20,000 commits, and the public repository has more than 2,300 stars.
- Repository created 10 June 2026; public launch 12 June 2026.
- 2,303 GitHub stars, 732 forks and 60 contributors as of 19 October 2026.
- 91,094 characters created on the live server by 19 October 2026.
- Six independent creator videos in English, Portuguese and French passed 107,000 YouTube views by 20 October 2026.
- Andreessen Horowitz partner Justine Moore called it “a fun look at the future of vibecoding real games”.
- One deterministic game core runs the server, an offline world and a Gymnasium environment for AI agents.
- What
- A free, open-source classic-era MMO for browser, desktop and mobile
- Built by
- Levy Street and 60 open-source contributors
- First version
- About 48 hours, 10 to 12 June 2026, with Claude Fable 5
- Stack
- TypeScript, Three.js, Vite, PostgreSQL, a Gymnasium RL environment
- Licence
- MIT for code; name and branding not licensed
- Status
- Live; v0.44.6 released 9 October 2026
What World of ClaudeCraft is
World of ClaudeCraft is a classic-era MMO: nine classes with three talent specialisations each, dungeons from five-player elite runs to a ten-player raid, ranked arenas, professions, guilds, a player market and a world that keeps going past the level cap. It runs at worldofclaudecraft.com in any modern browser, as signed desktop builds for Windows, macOS and Linux, and as iOS and Android apps, all in one shared online world.
It is also an unusually public piece of engineering. The full source is on GitHub under the MIT licence, every release is tagged, and the game has a companion wiki generated directly from live game content so it cannot drift from the world it documents.
How the first version was built in about 48 hours
The repository was created on 10 June 2026. Its first commit, co-authored by Claude Fable 5, already contained a 16,589-line game with three faces sharing one simulation core: a Three.js client, an authoritative multiplayer server with Postgres accounts, and a headless Gymnasium environment for training agents. The game went public on 12 June, picked up 406 GitHub stars that day, and was released under the MIT licence the same afternoon.
The speed came from a clear architecture, not just a fast model. The rules Levy Street set for the codebase made it possible to let an AI coding agent move quickly without breaking the game:
- One simulation core
- A fixed 20 Hz tick, all randomness from one seeded generator, and zero browser or rendering imports in the simulation code.
- Server authority
- Every combat roll, loot drop and quest credit resolves on the server. The client only renders.
- Architecture tests
- An automated test scans every simulation file for forbidden imports, browser globals, clocks or stray random calls.
- Agent instructions in the repo
- Root and per-directory CLAUDE.md, AGENTS.md and GEMINI.md files tell coding agents the local conventions.
These are the same disciplines we apply to client systems: hard boundaries, deterministic cores where possible, and tests that catch the mistakes an AI coding agent is most likely to make. Our engineering notes on reliable agents go deeper.
What happened after launch
Open-sourcing the game turned a weekend experiment into a community project. Outside pull requests were merged on launch day. By 19 October 2026 the repository had:
The game was covered by MMORPG.com (“The First Vibe Coded MMORPG Is Free, Open Source, and Surprisingly Complete”), MMOHuts, WinFuture, t3n and RuntimeWire, and discussed on Hacker News. Not all coverage was kind: some reviewers called the early build rough, which was fair. The response was to keep shipping, release by release, in public.
A real environment for training AI agents
World of ClaudeCraft is also a research tool. The game core runs headless as a Gymnasium environment: a Python agent calls reset and step, chooses from a discrete action space (movement, targeting, abilities, interaction), and receives a reward built from changes in experience, damage, kills, deaths and quest progress. Because the simulation is deterministic by construction, an experiment can be replayed exactly.
That matters beyond games. Testing agents against the real system, rather than a mock, is the single biggest improvement most teams can make to their agent evaluations.
What it shows about AI-assisted engineering
World of ClaudeCraft is our most visible answer to a question clients ask often: how far can AI coding agents take a real product? Our honest summary:
- The first 80 percent is fast. A strong model with a good brief can produce a working, surprisingly complete system in days.
- Architecture decides whether it lasts. The rules above, written by engineers, are why 60 contributors and close to 20,000 commits later the game still runs.
- People still own the product. Design, balance, art direction, moderation and release decisions stayed with people. Some art is commissioned or generated with image models, and the project credits it openly.
We use the same approach in custom AI development and when we embed engineers with client teams: let AI accelerate the build, and invest human effort in the structure that keeps it reliable. More in what building games with AI taught us.
Written by the Levy Street team in Wellington. Last updated 19 October 2026.