Running a modpack on Fabric or NeoForge

The one thing that trips everyone up

A modpack usually ships two downloads: the client pack, which you install in your launcher, and the server pack, which goes here. They are not interchangeable. The client pack contains mods that only exist on your computer, and several of them will stop a server from starting.

If the pack’s page has no server pack, that pack is not meant to be hosted. Some packs are genuinely client-only.

Whatever version of the pack you install here, every player has to run exactly the same version of the same pack. Not the same mods assembled by hand. The same pack, same version.

1. Pick the right server software

• Fabric for packs built on Fabric. Lighter, faster to start, common for performance-focused packs.
• NeoForge (or Forge) for packs built on Forge or NeoForge. Most large kitchen-sink packs.

You choose this when you order, and it is fixed for the life of that server: our billing records which software your server runs, and a reinstall on the panel alone would leave the two disagreeing. If you need to move from Fabric to NeoForge, or the other way, ask support and we will do it properly. Take a backup and download it first either way, because a software change means replacing the server files.

2. Size it honestly

Modpacks are the heaviest thing you can run on a Minecraft server, and the pack’s own recommendation is usually written for a single player on their own machine.

• Small Fabric performance packs: 6 GB is usually comfortable.
• Mid-size packs, roughly 150 mods: 8 GB, and 12 GB once a few people are exploring at once.
• Large kitchen-sink packs, 300 mods or more: start at 12 GB and expect to want 16 GB.

Loading new chunks is the expensive part, and a modpack generates far more per chunk than vanilla does. Four people exploring in four directions is the worst case, not the busiest build session. You can move up a size from your account at any time and it applies to the same server, so start one size below what you think and watch the memory graph.

3. Install the server pack

1. Stop the server.
2. In the file manager, delete the contents of the server directory if this is a fresh install. Do not delete the world folder if you are upgrading a pack and want to keep the world; check the pack’s own upgrade notes first, because some pack updates cannot be applied to an existing world.
3. Upload the server pack .zip and unarchive it in place.
4. Look at what it produced. You are looking for one of:
   • a run.sh or startserver.sh, which is a script the pack authors want you to run once to download the loader,
   • a .jar named after the loader, such as fabric-server-launch.jar or neoforge-*.jar,
   • a libraries folder plus a unix_args.txt, which is how modern NeoForge packs start.
5. Set the server’s startup jar to match what you found. On our panel this is the SERVER_JARFILE variable in the startup settings. Leave the memory flags alone: we set the Java heap from your plan automatically, and overriding it is the most common way people end up with a server that will not start.
6. Accept the EULA if the pack did not include an accepted eula.txt.
7. Start the server, and be patient. A large pack’s first start takes several minutes while it generates configs and the world. Two minutes of no console output is normal; ten is not.

4. Check the Java version

A pack for Minecraft 1.20 usually wants Java 17 or 21. A pack for 26.1 and newer wants Java 25. Our panel picks a Java image when the server is created; if the pack’s own notes disagree with what is running, tell us and we will switch the image. A Java mismatch shows up immediately as UnsupportedClassVersionError in the console, so you will know.

5. Give your players the pack

Send them the pack’s name and exact version, and the server address from your account page. Nothing else works: a player one version behind is rejected at the door with a mod list mismatch, which is annoying but is the pack doing its job.

When it will not start

Read the console from the top, not the bottom. The first error is the real one; everything after it is fallout.

• Missing or unsupported mandatory dependencies lists mod ids and the versions it wanted. Usually a half-applied pack update.
• Incompatible mods found naming a client-only mod means a client pack was installed. Start again with the server pack.
• The server starts and immediately stops with no error: almost always eula.txt.
• Out of memory during world generation: move up a size and try again before concluding the pack is broken.

If it is still not starting after that, open a support request with the pack name, its version, and the last 50 lines of console. That is enough for us to tell you what is wrong; without the console we are guessing.