Fix missing mods on a modded server

A modded server or a player's game stops with a message about a missing mod. Some mods only work with others installed beside them, so a missing library, such as Fabric API, is a common cause. The message names what is missing, and this guide shows where to read it and how to fill the gap on your panel.

Checked against Forge 1.20.1 and 26.3, NeoForge's FancyModLoader, Forge 1.12.x, Fabric Loader 0.19.5 and the NeoForge 1.21 language file, October 2026.

Find your message

Forge and NeoForge, on the server. The log says Missing or unsupported mandatory dependencies:. Under it come lines like this, one per problem:

Mod ID: 'examplelib', Requested by: 'examplemod', Expected range: '[1.2,)', Actual version: '[MISSING]'

Requested by is the mod that needs help, Mod ID is what it needs, Expected range is the versions it accepts and Actual version is what is installed. [MISSING] means no copy at all. The loading screen words it as "Mod examplemod requires examplelib ... Currently, examplelib is ..." and then "not installed" or a version number.

Fabric. The server stops under the heading Incompatible mods found!. It first suggests a fix, for example Install fabric-api, any version., then gives the detail, such as Mod 'Chunky' (chunky) 1.5.3 requires any version of fabric-api, which is missing!. In Fabric's output fabric-api is Fabric API. A line containing "but only the wrong version is present" means the mod is there at the wrong version.

Old Forge (1.12.x). The error is MissingModsException, worded "Mod examplemod (Example Mod) requires" followed by the missing mod, with a Missing Mods: list showing each one's accepted range and whether you have it.

A player on joining. Forge 1.20.1 and later name the gap in plain words. "Your client is missing the following mods, install these mods to join this server" lists mods the server has and the player lacks. "The server is missing the following mods, remove these mods from your client to join this server" lists the other direction. "The following mod versions do not match" is a version problem, covered in Fix the mismatched mod channel list.

Fix it on the server

1. Open the Mods tab, under Content, and switch to Installed. After a failed start the tab reads the log and puts a badge on the mod that asked for help. For a Fabric or Paper log that names the missing mod, the badge reads Needs and that name, such as Needs fabric-api. For any other line, including a Forge or NeoForge failure, it reads Needs another add-on. The warning "Some add-ons didn't load at the last start" lists any line it could not match to a mod.
2. Switch to the Store, search for the mod the log names, and check that its Loader and game version match your server. The store sets both for you.
3. Press Install. If a window titled Needs 1 more add-on, or a higher count, opens, press Install all so they go in together. Hangar and GitHub do not publish what a mod needs, and the panel says so in that case, so read the mod's own page.
4. Restart from the Console.

If the log gives an Expected range, the installed copy may simply be the wrong version. Update the library with Check for updates, or install the version in the range from the mod's page.

Fix it for a player

1. In the Installed view press Export for players, then Download zip. The zip holds the mods a player needs, with instructions that name the loader.
2. Players put those files in the mods folder in their game folder (%appdata%\.minecraft\mods on Windows and ~/.minecraft/mods on Linux, with the standard launcher) and use the loader named in the instructions, at the same Minecraft version as the server. Join a modded server from your launcher has the player's steps.
3. For "The server is missing the following mods", the player removes the listed files from their own mods folder.

If it keeps happening

Take a backup, then remove the newest mod you added and start the server again. Fix mod and plugin errors on Minecraft goes through the wider set of load messages, and Find Minecraft server logs and crash reports shows where the files are.