I built a persistent world your agent can join over MCP, with the same permissions as a human player

Disclosure up front: I built this, and it's free. Per rule 3 I'm keeping links out of the post - I'll put the endpoint in a comment. I've been building a server-authoritative persistent world. It's online now. The part I actually want to talk about here is the permission model, because it's what makes agents behave differently in it. An agent connects over MCP and gets exactly what a human player gets through the browser client. No admin tools, no privileged side door, no separate "agent API" with extra reach. It calls join, takes a body, and from then on it's a client like any other: it has to walk somewhere to see what's there, it has to own or be granted land before it can build on it, and it can be refused. The world carries on between its calls, and nothing acts on its behalf while it's thinking. The other half is that nothing in the world was compiled in. There is no House class and no Robot class. A thing is a "design" - named primitives, materials, components, scripts - so anything an agent can describe that way can be built, walked into, programmed, and taken down again. Give an agent goals and it designs a town out of primitives and builds it, and the server never learns what a house is. It has to invent the vocabulary rather than pick from a menu I wrote, which turns out to be a much harder and more interesting test than a tool-calling benchmark. Two design decisions that might be useful if you're building agents against a live environment: Polling is the wrong shape. There's a catch-up token instead: the agent asks for everything that has happened since the token it last got, and receives that plus a new token. A minute later or a day later, same call. It never has to hold a connection open or diff two readings to work out what changed, which matters when your agent is an intermittent process rather than a game loop. Reversibility is first class. Anything built can be demolished, any land claim released, any script stopped. An agent that can undo its own work can afford to try something and look at the result instead of having to get it right in one pass. You can also leave autonomous agents behind to keep working while you're gone, hire and be hired, form groups, and own things collectively - so an agent turns up to a world other people's agents have been changing. What I'd most like from this sub: point an agent at it with a goal I didn't anticipate and tell me where the primitives run out. Happy to answer anything about how it's put together.