Fix "Missing or unsupported mandatory dependencies" on a Minecraft server

Open the Console tab after a failed start and look for this heading. The lines right under it are the ones that matter, one line per missing mod:

[main/ERROR] [net.neoforged.fml.loading.ModSorter/LOADING]: Missing or unsupported mandatory dependencies:
	Mod ID: 'uranus', Requested by: 'iceandfire', Expected range: '[2.3.2,)', Actual version: '[MISSING]'

On Forge the heading starts with net.minecraftforge instead of net.neoforged; the rest is the same. A moment later the loader says it once more in plain words, Mod iceandfire requires uranus 2.3.2 or above, and the server stops.

Ten mods without their library give ten lines. Several of them can ask for the same mod ID, and you only install that one once.

The range is written with brackets. A square bracket includes the version next to it, a round bracket leaves it out, and an empty side has no limit.

In the plain words line, a range with an upper limit reads like Mod lowlustworldboss requires pixelmon 9.3.14 or above, and below 9.4.0.

When Actual version shows a number instead of [MISSING], the mod is there but outside the range. Replace that file with a version inside the range. Adding a second copy next to the old one gives you a different error on the next start.

The loader writes mod IDs: the short, lower case names mods use internally. They often differ from the name on the download page. Some you will meet:

If the name is not obvious, open the download page of the mod after Requested by. Its required mods are listed there, under Dependencies on Modrinth and under Relations on CurseForge.

1. Open the Mods tab of your server.
2. Under Verified Modrinth catalog, type the name of the missing mod in Search mods.
3. Click Verify & install, and check that the version falls inside the Expected range.
4. Start the server and read the Console again. A library can need a library of its own, so a second, shorter list sometimes follows. Repeat for each line.
5. Give your players the same mod, or update the modpack they play with. A client without it cannot join.

A mod that is not in the catalog goes in through the Files tab: open the mods folder and use Upload, or From link for a direct download address. Pick the file for your loader and your Minecraft version.

You can also go the other way. If you do not need the mod after Requested by, find it with Filter installed mods and remove it; its missing library then no longer matters.

Now and then the missing mod ID is the loader or the game itself. No extra mod fixes that: the mod was built for a newer loader or for another Minecraft version.

Typical causes are a mod downloaded for 1.21.4 on a 1.21.1 server, or a mod made for NeoForge on a Forge server. Fabric, Forge or NeoForge explains which mods run on which loader.