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Mods: server-side and client-side

The main rule of mods: which ones go on the server, which ones only on players' PCs, and what to do about dependencies.

Three kinds of mods

  • Server-side: they work only on the server (optimisation, protection, backups). Players don't need them.
  • Shared: needed both by the server and by every player (new items, mobs, mechanics). Versions must match.
  • Client-side: only on the player's PC: graphics, shaders, UI, minimap, translations, FPS boosters. On the server they break the startup.

If the server crashes right after you upload a modpack, a client-side mod is almost always to blame. The panel will tell you which one; move that mod from mods to the mods-client-only folder.

Dependencies

Many mods require libraries: Forge needs Architectury, Cloth Config, GeckoLib; Fabric needs Fabric API; Valheim needs Jötunn. If the console says "missing dependencies" or "requires", the same line tells you what's missing.

Step by step

  1. Make a backup.
  2. Add mods one or two at a time and start the server: that way you see right away which one broke it.
  3. Compare the server's and players' mod lists: shared mods must match by version.