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Lithium works on both the client and the server, and the two sides are independent. A server running Lithium benefits every player who connects, even those without the mod installed. A client running Lithium only helps that one player.

Server-Side Setup

If you run a Fabric or NeoForge server, place the Lithium jar in the server's mods folder just like on a client. The server generates its own config file on first start. You can tune it the same way — disabling specific optimizations if a plugin or datapack misbehaves. Restart the server after any config change. Because the optimizations target tick-level work like mob AI and chunk loading, the biggest wins appear on busy servers with many players or large farms.

Client-Side on Someone Else's Server

You can run Lithium on your own client while playing on a server that does not have it. This is safe and gives you the client-side performance gains without affecting anyone else. The server does not need to know. Just keep the jar in your local mods folder. If the server has strict mod policies, check whether client-side performance mods are allowed before joining, since some communities restrict any non-vanilla client modifications regardless of whether they change gameplay.