Install Custom Server Software

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Install Custom Server Software

Install custom server software when the required software or build is unavailable through the UltraServers Change Version installer. This guide covers directly executable Minecraft Java server JARs, including Paper, Purpur, Pufferfish, and the Fabric server launcher.

Warning: Only download server software from its official website or another trusted source. A malicious JAR can access or damage your server files.

Important: This process is intended for directly executable server JARs. Forge installers, modpacks, and proxy software such as Velocity or BungeeCord require different installation steps.

Prerequisites

Confirm that the required software is unavailable under Change Version.

Use the panel installer when it offers the required software and build.

Create a server backup.
Confirm that the custom software supports your Minecraft version, plugins, mods, and existing world.
Confirm the Java version required by the server software.

See Which Java Version Do I Use for My Minecraft Server?.

Download a directly executable server JAR from its official source:

Warning: Do not rename a Forge installer to server.jar. Use the panel's Change Version installer or the appropriate Forge or modpack installation guide.

Stop the Server

Log in to the UltraServers panel.
Open the server on which you want to install the custom software.
Open the Console tab.
Click Stop.
Wait until the server shows as offline before changing its files.

Upload the Server JAR

Open the Files tab.
Locate the existing server.jar.
Rename the existing file to a descriptive backup filename, such as old-paper.jar.

You can delete the old JAR instead if you already created a server backup and do not need to retain it.

Click Upload and select the custom .jar file.

You can also drag the file into the file manager.

Wait until the upload finishes.
Confirm that the complete JAR appears in the server's root directory.

Important: Do not start the server while the upload is still in progress. An incomplete JAR cannot start and may produce an invalid or corrupt JAR error.

Configure the Server JAR

Choose one of the following methods:

Rename the uploaded file to server.jar.
Keep the original filename and update the server's startup setting:

Open the Startup tab.
Locate Server JAR File.
Enter the complete filename, including the .jar extension.
Save the value.

For example, if the uploaded file is named paper-1.21.11.jar, enter:

paper-1.21.11.jar

Note: The filename is case-sensitive and must exactly match the file in the server's root directory. If you renamed the file to server.jar, leave Server JAR File set to server.jar.

Configure Java

Remain on the Startup tab.
Locate the Docker image or Java version setting.
Confirm that the selected image uses the Java version required by the Minecraft version and server software.
Change the image if the software's official documentation requires a different Java version.
Save the setting.

Minecraft versions generally use the following Java versions:

Minecraft version	Java version
1.16.5 and older	Java 8
1.17–1.17.1	Java 16
1.18–1.20.4	Java 17
1.20.5–1.21.x	Java 21
26.1+	Java 25

Note: Some custom software may have different Java requirements. Check the software's official documentation before starting the server.

Start the Server

Open the Console tab.
Click Start.
Monitor the console while the server starts.
Wait for the server to finish loading.
Confirm that the console identifies the expected server software and version.
Connect to the server and verify that the world, plugins, and mods load correctly.

Note: The first startup may take longer while the software generates files. Do not repeatedly restart the server during this process.

Troubleshooting

Invalid or Corrupt JAR File

Download the server JAR again from its official source.
Upload the new copy and wait for the upload to finish.
Confirm that the file has a .jar extension.
Confirm that Server JAR File exactly matches the uploaded filename.

Unable to Access the JAR File

Confirm that the JAR is in the server's root directory.
Confirm that the filename under Startup → Server JAR File includes the .jar extension.
Check the filename for incorrect capitalization, spaces, or spelling.
Save the corrected value and start the server again.

UnsupportedClassVersionError

An UnsupportedClassVersionError indicates that the selected Java version is incompatible with the server JAR.

Check the Java version required by the server software.
Open the Startup tab.
Select a Docker image that uses the required Java version.
Save the setting and start the server again.
See Which Java Version Do I Use for My Minecraft Server? for additional guidance.

Server Fails After Changing Software

Check the console for plugin, mod, configuration, or world-version errors.
Confirm that every plugin and mod supports the new server software and Minecraft version.
Remove or update incompatible plugins and mods.
Restore the server backup if the server or world no longer starts correctly.

Warning: Avoid opening a world with an older Minecraft version after it has been saved by a newer version. Downgrading a world can cause missing data or permanent world corruption.

If you require further assistance, you can open a support ticket here.

Feedback and suggestions can be submitted here or further discussed on our Discord server.

Install Custom Server Software
Prerequisites
Stop the Server
Upload the Server JAR
Configure the Server JAR
Configure Java
Start the Server
Troubleshooting
Invalid or Corrupt JAR File
Unable to Access the JAR File
UnsupportedClassVersionError
Server Fails After Changing Software