On Forge and Fabric, the server and client compare their mod lists when connecting. If they disagree in a way that matters, the connection is refused — and the message a player sees is often much less informative than the one in your console.
What you are seeing
- The player is disconnected during connection with a mod list error.
Missing or unsupported modswith a list, or a channel mismatch message.- It works for some players and not others.
- It broke for everyone after you added or updated one mod.
What to do
- Read the server console, not the player's screen
The client message is usually truncated. The console names the specific mods and versions that did not match.
- Compare versions, not just names
«They have the mod» is not enough — a different version of the same mod is still a mismatch for anything that registers content.
- Check whether the mod is server-only or client-only
Performance and utility mods often need to be on one side only. Adding a server-only mod to clients, or vice versa, causes its own set of problems.
- Distribute the pack rather than a list
«Install these fourteen mods» produces fourteen chances to get a version wrong. A launcher-installable pack removes the entire class of problem.
Which mods must match
| Mod type | Must be on both sides? |
|---|---|
| Adds blocks, items or entities | Yes, and at the same version |
| Adds a dimension or world generation | Yes |
| Server-side performance mods | No, server only |
| Client-side visual or interface mods | No, client only |
| Libraries required by another mod | Wherever that mod is |
There is no partial rollout. The moment the server has a different version, everyone using the old pack is refused. Announce updates before you make them, and post the new pack at the same time — not afterwards, when twenty people are already unable to connect.
«Our pack is version 1.4.2» in the Discord topic answers most connection questions instantly, and lets people check themselves before asking.
A client with the published pack connects and reaches the world; the console shows no mod list warnings during the handshake.