Unexpected Custom Data from Client
A player is disconnected with 'Unexpected custom data from client'. During the connection, the client sent a custom network payload the server did not recognise, so the server rejected it. This is a mod/networking mismatch, not a world or hardware problem.
What does this error mean?
Mods and the game exchange data over named 'plugin message' channels. When a client sends a custom payload on a channel the server does not expect (or cannot parse), the server treats it as unexpected data and closes the connection.
Unexpected custom data from client
Most Common Causes
- A client mod sending data on a channel the server does not have.
- Different versions of a mod negotiating the channel differently on each side.
- A client-only mod that registers networking the server lacks.
- A modpack out of sync between client and server.
- A malformed or oversized custom payload from a buggy mod.
How To Diagnose
- Note when it happens (at join) and whether only modded clients are affected.
- Compare the client and server mod lists and versions.
- Identify any client mod that uses custom networking.
- Test joining with that mod removed from the client.
Recommended Fixes
- Sync mods and versions
Run the identical mod set and versions on client and server so channels match. - Remove the offending client mod
Take the client-only networking mod off the connection if the server does not support it. - Update the mod
Use a build where client and server agree on the network protocol. - Distribute the pack
Share the modpack so every client negotiates the same channels as the server.
Frequently Asked Questions
No, the server stays up, it just disconnects the client sending the unexpected data.
A client mod using custom networking, compare mod lists and test by removing client-only networking mods.
Yes, mismatched versions of the same mod can negotiate channels differently and trigger this.