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Client Mod Installed on Server

The server crashes on startup trying to load a class that only exists on the client. A client-only mod was placed in the server's mods folder, and the server has no client rendering code for it to hook into.

What does this error mean?
Mods declare which 'side' they run on. The error about loading a net/minecraft/client/ class 'for invalid dist DEDICATED_SERVER' means a mod tried to use client-only code on a server that does not have it. The mod simply does not belong on the server. 
Attempted to load class net/minecraft/client/... for invalid dist DEDICATED_SERVER
Most Common Causes
  • A purely client-side mod copied into the server mods folder.
  • A whole client mods folder mirrored onto the server.
  • A mod that renders UI/graphics with no server function.
  • Assuming every mod must be on both sides.
  • A modpack that did not separate client-only mods.
How To Diagnose
  1. Read the crash for the mod whose client class failed to load.
  2. Check that mod's page, client-only mods say so explicitly.
  3. Ask whether the mod does anything server-side at all.
  4. Compare your server mods against a known server-safe list.
Recommended Fixes
  • Remove the client mod
    Delete the client-only mod from the server mods folder, keep it only on clients.
  • Split client and server mods
    Maintain separate mod lists: shared gameplay mods on both, client-only mods on clients only.
  • Use the pack's server files
    Most modpacks ship a server-specific mods set, use that on the server.
  • Verify each mod's side
    Before adding a mod to the server, confirm it has server-side functionality.
Analyze My Log
Frequently Asked Questions

Its page usually states client-side, and it provides visual/UI features with no server logic (shaders, minimaps, HUDs).

Yes, client-only mods stay on each player's client, they just must not be on the server.

The mod tries to load client rendering classes that do not exist on a dedicated server, which aborts loading.