unsup

Modpack authors can customize unsup as a simple updater for private packs. It works as a generic working directory syncer: it compares the files in a directory against a manifest and fetches what has changed. The tool is attached to a launch as a Java agent, which means it runs before the program it is attached to starts. The working directory has to hold an unsup.ini file, and the JVM argument -Dunsup.bootstrapUrl points to where that file can be found.

The updater is written to avoid leaving a working directory in an inconsistent state. A short power loss during the brief window in which changes are applied is handled so that the directory is not corrupted. Files are checked with hash-based validation, and SHA-256 is the recommended choice. Before an updated file overwrites a local one, the user is warned that the file was changed. Manifests can be signed with OpenBSD Signify. Prism Launcher can import the unsup component through the com.unascribed.unsup.json release published on Forgejo, and unsup can update MultiMC components, including itself once added. Because mod loaders and Minecraft itself are components, they can be updated too.

The mod is open source under the GNU Lesser General Public License v3.0 or later. Its Java Agent files target the range rd-132211 – 1.21.10, with 167 release versions in the catalog, and snapshot builds are published. The latest file is 1.2.0-pre4 for 1.21.10, dated 2025-12-05 on the beta channel. Signing support is still described as somewhat experimental and has not been deployed at large scale.