Getting your friends into a modded Minecraft server

What has to match between server and players

When a player connects, the server and the player's game compare notes. Blocks, items, mobs and network messages that a mod adds have to exist on both ends, or the two sides cannot talk about them. Three things need to line up before anyone gets past the loading screen.

Not every mod belongs on both sides, and that is where most confusion starts: mods that only change what a player sees can stay on the player's computer. If you have not picked a loader yet, read Fabric, Forge or NeoForge first.

Joining a server that runs a modpack

A modpack makes joining easy, because the pack author has already decided which files go where. The one thing players must get right is the version of the pack. A newer release can carry different mods, and the join screen treats that as a mismatch.

1. 1

   Write down the pack version when you install it

   For a CurseForge or Feed the Beast pack, the confirmation before installing names the pack and the version you picked. A Modrinth pack takes the newest release that fits your server type and version, or the newest build when there is no release, and the message after installing names that version. Note it then: the list under Versions on the Mods tab shows every version you could install, newest first, not the one your server runs.

2. 2

   Send your friends the name and the exact version

   The name alone is not enough, because a launcher offers the newest version first.

3. 3

   Install that version in a launcher

   The table below shows where each launcher lets you choose it. Start the pack once on its own, so a broken install shows up as a broken install and not as a connection problem.

4. 4

   Join with the address from the panel

   Add the server under Multiplayer and connect.

In the CurseForge app, a profile with Allow content management switched on cannot change versions; turn it off in Profile Options first.

Joining with a mod list you built yourself

1. Install the same loader for the same Minecraft version. In the Fabric installer, choose the version and click Install with Create Profile ticked, then pick the Fabric profile in the Minecraft Launcher and give it a game directory of its own. In the NeoForge installer, choose Install client and Proceed, then create an installation for that NeoForge version with a game directory of its own.
2. Start the game once, then open that profile's game directory. If there is no mods folder yet, create one.
3. Copy in the server's content mods. Use the files the server has, not the newest ones you can find. On Fabric that includes Fabric API.
4. Leave out the server-only tools, and add mods for your own screen if you like.

On a Fabric server, the console right after a start is the easiest reference. Fabric Loader prints Loading with the number of mods, then one line per mod with its id and version. Skip java, minecraft and fabricloader, and the indented lines, which come inside the mod above them. What is left, minus the server tools, is the list a player compares with.

Which mods go on the server, and which only on players' games

Modrinth records for every mod whether it is required, optional or unsupported on the client and on the server. A few well-known examples, as Modrinth listed them when this guide was checked:

Side information can change between releases, so check the mod's page for the version you run. More on the speed mods in Fabric performance mods.

The panel sorts for you when you install through it. The Verified Modrinth catalog leaves out mods that only run in a player's game, a Modrinth pack install puts only the pack's server mods in /mods, and a Feed the Beast pack skips files its author marked as client only.

That also means the server's mods folder is not the whole pack, and a Modrinth pack arrives without its config folders, so the server runs each mod's defaults. Players install the pack itself, not a copy of the server's folder.

Why a client-only mod can stop the server from starting

A dedicated server has no screen, so it does not carry the parts of Minecraft that draw the world, the menus or the HUD. Shader mods and minimaps are built around exactly those parts. Started on a server, such a mod reaches for something that is not there and the start ends in an error.

Fabric has a safeguard: a mod can declare in its metadata that it is for the client, and Fabric Loader then leaves it out on a server. Only a mod without that declaration gets loaded and fails. On NeoForge and Forge it depends on how the mod was written, and one that is not built to sit out on a server ends the start with an error about a class the server does not have.

1. 1

   Find the name in the console

   The error usually names the mod, or a word from its file name, and a crash report names the mod that failed. The startup guide shows where to look.

2. 2

   Remove the file

   Click Remove next to the jar under Installed mods on the Mods tab, or tick it inside mods on the Files tab and delete it.

3. 3

   Start again

   Repeat until the console reaches Done. Players can still keep that mod in their own game.

What players see when something does not match

The wording below comes from the current releases of the game and each loader. Most of these screens also name the mods involved, and that list is the useful part.

Minecraft version

Incompatible client! Please use 26.2

The number is the server's version, and the game checks it before a loader gets a say, so fix this first. Other connection errors are in can't connect to a Minecraft server.

Fabric

This server requires Fabric Loader and Fabric API installed on your client!
This server requires Fabric API installed on your client!
Contact the server's administrator for more information!

The first line goes to a game without Fabric, the second to a game with Fabric Loader but no Fabric API. Fabric lists up to four mod namespaces under it. A server whose mods add nothing that needs synchronising lets such a player in anyway.

Received a registry entry that is unknown to this client.
This is usually caused by a mismatched mod set between the client and server.

With several entries the first line gives a count. The namespaces listed below it usually match the ids of the missing mods.

NeoForge

You are trying to connect to a server that is running NeoForge, but you are not. Please install NeoForge Version: <version> to connect to this server.

The version in the real message is the one the server runs. A NeoForge server whose mods only use optional network channels lets such a player in. A player who does have NeoForge but a different set of mods gets a Connection Lost screen with a table of Channel name and Reason. The reasons read like this:

This channel is missing on the client side, but required on the server!
This channel is missing on the server side, but required on the client!
The client wants the payload to be version: <version>, but the server wants it to be version: <version>!

A reason can start with Channel of mod and the mod's name. Missing on the client side means the player lacks a mod the server has; missing on the server side means the player carries one the server does not; a version line means the two copies of a mod differ.

Forge

A Forge player with the wrong mods gets a screen headed by one or more of these lines, each above a table of mods:

Sharing the mod list with friends

For a modpack, the name and version are all you share. For a list you assembled yourself, hand over the actual files.

1. 1

   Zip the mods folder on the Files tab

   Download works on files, not folders. In the top folder, tick the box next to mods and click Zip. A .tar.gz archive appears beside it.

2. 2

   Download and share it

   Click Download next to the archive. 7-Zip and most archive tools unpack it.

3. 3

   Delete the archive afterwards

   It takes up the server's disk. Tick it and click Delete once everyone has a copy.

4. 4

   Players unpack it into their mods folder

   In a profile with the same loader and Minecraft version. Take out the server tools such as spark and Chunky, and anything whose Modrinth page says it is not for the client, then add their own screen-side mods.

Built the list in the CurseForge app? Share Profile saves it with mods and configs as a .zip. Friends use Import and Choose .zip file, or you generate a profile code, which works for seven days. Prism Launcher's Import page takes a .zip or .mrpack from a file or a link.

A test run before you hand out the address

1. Create a fresh profile or instance, the way your friends will, instead of joining from the one you built the server with.
2. Write the Minecraft version, loader and pack version, or the archive name, where your group talks.
3. Before you remove a mod that adds blocks or items, keep a copy of the world: what it added is gone after the next start. Then tell everyone, because the panel warns that players who still have it may be refused until they remove it too.
4. Big packs need memory on both ends. The RAM guide covers the server side.