Minecraft wiki entry for Mods, associated text: Because the Minecraft server software ignores custom content from the client side, most of the client mods which add new content to the game do not work in multiplayer unless a modification has also been installed on the server. Some authors of client mods have made server versions of their mods available, and others have not. A few of the Loader/API-type client mods and many of the functional client mods (e.g. OptiFine or TooManyItems) have effect in multiplayer straight away, without any modification on server side.