A developer posted on r/ClaudeAI about Civitas, an experimental multiplayer 3D city builder that runs in the browser on three.js and TypeScript, where Claude Code wrote the code, tests and tooling while the developer picked features, set priorities and made trade-offs. The free public demo is a read-only London laid out from real coordinates, with the Thames and its bridges, about 30 real landmarks, and roughly 30,000 individually simulated citizens with homes, jobs and commutes. The author says one Claude Code session acted as coordinator, breaking features into kanban tasks via an MCP server and handing them to subagents — Opus for simulation and networking, Sonnet for rendering and UI — each working in its own git worktree and running typechecks, a demo hash-comparison check and load tests before merging.