Install Cardboard on a Fabric server

Cardboard is a Fabric mod that adds the Bukkit, Spigot and Paper plugin API to a Fabric server, so plugins written for those platforms can run next to Fabric mods. It lets you try both on one server. It is also unfinished: the project says it will not support every plugin.

Checked against Cardboard 26.1-14 (a beta build) on Fabric 26.1.2, October 2026.

What to know first

• It is a server-only mod. Its Modrinth page lists it as unsupported on the client, so players do not install it.
• It is beta software. Modrinth lists no full release of Cardboard: the newest builds for 26.1.2 and 1.21.11 are beta, and some older builds are alpha. The project's wiki answers "Is this mod stable?" with "No" and tells you to use it at your own risk. Take a backup before you start. How to create and restore backups has the steps.
• Versions are limited. Modrinth lists 26.1.2 and 1.21.11 as the actively developed versions, with 1.20.1 and 1.21.1 as older ones. The 26.1 build asks for Fabric Loader 0.19.0 or newer, Java 21 or newer, Fabric API and iCommon API. Our installer picks the newest stable Fabric Loader, and the panel chooses Java for the Minecraft version.
• Paper is the supported route for plugins. If you do not need Fabric mods, Paper runs these plugins without a compatibility layer. CraftBukkit vs Spigot vs Paper explains the family.

Put the server on Fabric

1. Open JAR Manager under More.
2. Set Server software to Fabric and Minecraft version to 26.1.2 (or 1.21.11).
3. Press Apply change, keep Take a backup first ticked and hold Confirm & reinstall.
4. When the install finishes, press Start in the Console, accept the EULA if asked, and wait for Done.

Install Cardboard and its libraries

1. Open the Mods tab, which is under Content, and search for Cardboard. The default source is Modrinth.
2. Press Install. If a window titled Needs 1 more add-on, or a higher count, appears, press Install all. Modrinth lists Fabric API and iCommon API as required on the newest 26.1 builds (26.1-11 to 26.1-14).
3. Open the Installed view and check that Cardboard, Fabric API and iCommon API are all there. The 1.21.11 build lists no requirements on Modrinth, yet its mod file asks for both, so add any that is missing from the Store.
4. Restart from the Console.

The store installs the newest release of a mod and, when a mod has only betas, the newest beta, so Cardboard goes in as a beta build.

Add plugins

1. Download the plugin's .jar from its own page: Hangar, Modrinth, BukkitDev or GitHub.
2. In the File Manager, open the plugins folder. Cardboard creates it at start. If it is missing, create it with New folder.
3. Press Upload, choose the jar and restart the server.
4. Type plugins in the Console command box. Cardboard registers that command, and it lists what loaded. version shows the server version, and version <plugin name> shows one plugin.

The Mods tab only manages the mods folder, so plugins in plugins do not show in its Installed view. Work with them in the File Manager.

When something goes wrong

• A plugin does not load or does nothing. Read the Console. The project's wiki says plugins made for CraftBukkit derivatives will work, but some events are still unimplemented. Report problems on Cardboard's issue tracker with the latest log.
• EssentialsX breaks. The Cardboard wiki says EssentialsX breaks when other mods add blocks or items, and links to its own build of EssentialsX.
• The server stops at start with "Incompatible mods found!". Fabric is naming a missing or mismatched mod, for example Fabric API or iCommon API, or a Cardboard build for another Minecraft version. Fix missing mods on a modded server shows how to read the lines.
• You moved to a newer Minecraft version. Each current Cardboard build targets one Minecraft version, so update it in the Installed view with Check for updates and restart.