EDIT: Yeah for merging posts into threads newer than the posts... the actual OP is here: https://forums.uberent.com/threads/discussion-working-around-unit_list-json.70938/#post-1118958 likely an anti cheat mechanism?
No, the game doesn't know units exist if they aren't in unit_list.json. It's how the game knows what units to load. In order to be able to have composable server mods, the game would need to be able to load differently named unit_list.json files.
Moving from server api requests thread. If you want a unit that only you can build, you can already do that by including it in unit list and only giving yourself the build bar for it. I don't want to avoid buildability checks, I just want them to be checked for dynamically loaded units. You can paste unlisted units and they run fine, so both the server and client are clearly able to load and use a previously unknown unit. However it looks like the buildability information is not updated. p.s. I ran across this while searching for prexisting threads for this; somebody has been thinking about this.
I've tested two more cases: - Fabricators asked to build an unlisted structure get the build outline, but nothing happens when placed. Note that this includes the model and placement size for the unlisted unit, although I'm re-using the model for a listed unit at the moment. - If you paste an unlisted factory, it can build listed units
If you paste an unlisted mobile unit, and then an unlisted factory which can build that unit, the mobile unit won't be built. The server seems to use only listed units (rather than known units) for buildability tests.