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OptiFine Installed on Server

OptiFine was placed on a server and it crashed. OptiFine is a client-only optimization and graphics mod, it has no server-side component and cannot run on a dedicated server.

What does this error mean?
OptiFine enhances client rendering (FPS, shaders, connected textures). On a server it tries to load client rendering classes for the DEDICATED_SERVER dist, which do not exist, so loading fails. OptiFine belongs only on the client. 
Attempted to load class net/minecraft/client/... for invalid dist DEDICATED_SERVER
Most Common Causes
  • The OptiFine jar added to the server mods folder.
  • Mirroring a client mods folder onto the server.
  • Believing OptiFine improves server performance (it does not).
  • Following a guide that mixed client and server mods.
  • A pack copied without removing OptiFine.
How To Diagnose
  1. Confirm the crash names OptiFine / a client render class.
  2. Check the server mods folder for the OptiFine jar.
  3. Recall that OptiFine is purely client-side.
  4. Look for other client-only mods alongside it.
Recommended Fixes
  • Remove OptiFine from the server
    Delete the OptiFine jar from the server, install it only on clients that want it.
  • Use server-side performance forks
    For server performance, run Paper/Purpur or Fabric with Lithium, not OptiFine.
  • Keep OptiFine client-side
    Players install OptiFine locally, it never goes on the server.
  • Audit the mods folder
    Remove any other client-only graphics mods from the server.
Analyze My Log
Frequently Asked Questions

No, it is client-only rendering. Server TPS is helped by Paper/Lithium, not OptiFine.

Yes, on their own clients. It simply must not be on the server.

Sodium (rendering) plus Lithium (server logic) and Iris (shaders) are common client choices.