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A client-only mod uses rendering, menus, input, or other code that is unavailable on a dedicated server. If one is uploaded to /mods, the server may stop during startup.

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Identify the incompatible mod

  1. Start the server once and read the first relevant error in the console.
  2. Look for a mod name near messages such as requires environment type CLIENT, invalid dist DEDICATED_SERVER, NoClassDefFoundError, or ClassNotFoundException.
  3. Compare the named file or mod ID with the files in /mods.
  4. Check the mod author’s page to confirm whether it is client-only or has a separate server-compatible release.
A class-loading error can also be caused by a missing dependency or incompatible version. Do not remove unrelated mods merely because their names appear later in the stack trace.

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Remove and test

1

Stop the server

Leave it stopped while changing mod files.
2

Keep a copy

Download the suspected .jar or move it outside /mods so you can restore it if the diagnosis is wrong.
3

Start once

Start the server and watch the full startup. If it fails again, diagnose the new first relevant error rather than removing several files at once.
4

Check player clients

Keep client-only mods on player devices when desired. Server-required mods and dependencies must remain compatible on both sides when the mod author requires them.
For other startup failures, follow Troubleshoot a server that will not start.