I've done a version of "world" for Sigil, a programming language I was kinda doing but stopped, also for agents. LLM generated article here https://inerte.github.io/sigil/articles/worlds-not-mocks/

But basically world is a bit more narrow, that moment where your code touches the outside world (logging, http, etc), you can swap that. It's sorta like DI but deliberately narrower, only the moments where code touches the outside world are swappable.. With DI in theory you can replace anything, which has its benefits, but at least personally I am not a big fan of mocks, except when they touch the outside world. So that's what's replaceable.