Playing modded Minecraft with friends requires matching the right background libraries across everyone's setups. While 
Alexandria Lib does not add blocks, items, or mobs of its own, it provides the underlying structure for mods that often do.
Deciding Between Client and Server
Whether Alexandria Lib belongs on the client, the server, or both depends entirely on the mods in your list that rely on it:
1. Identify which of your installed mods require Alexandria Lib as their base.
2. If those dependent mods run on the server—such as mods that change gameplay, world features, or shared mechanics—place Alexandria Lib in both the server's mods folder and every player's client mods folder.
3. If you are opening a singleplayer world to LAN, have every joining friend place Alexandria Lib into their client mods folder alongside the matching gameplay mods.
Avoiding Connection Rejections
Before opening your server to players, start the server console once and verify it finishes loading without halting on a missing base error. Keeping Alexandria Lib present on both sides ensures that every dependent mod has the shared code it needs to run stable multiplayer sessions.