A Minecraft mod needs another mod that is not installed

Many mods need another mod to run, often a library such as Fabric API. When that mod is missing, or installed in a version the first mod does not accept, the server stops at start and names both. Install the mod it asks for, in a matching version, or remove the mod that needs it.

What you see

The Why your server stopped box on Overview and above the Console says one of these, with your mods' names:

• A mod needs another mod , which is not installed. and Install it from the Modding page, then start the server again. Or remove the first mod.
• A mod needs a different version of another mod, with It needs the versions it accepts , and the version you have is installed. Install a matching version from the Modding page.

The button Find and the needed mod's name opens Modding.

In the Console:

• Fabric: Mod 'Name' (id) version requires a version of the other mod, then , which is missing! or , but only the wrong version is present: and the version.
• Forge and NeoForge: Missing or unsupported mandatory dependencies:, then a line Mod ID: with the needed mod, Requested by: the mod that needs it, Expected range: the versions it accepts, and Actual version: with [MISSING] or the version you have.

Fix it

1. Open your server in the panel and click the Modding tab, then Mods.
2. Under Browse, search the needed mod's name and install it. Pick the build for your Minecraft version and your loader. When you install a mod from Modding, the panel adds the mods it needs at the same time and says so.
3. If the message was about the wrong version, open the needed mod under Installed, choose Choose a version in its menu, and pick one in the range the message gives.
4. Start the server. If another mod is named, repeat.

If you do not want the extra mod, remove the mod that needs it instead.

Version ranges

The message gives the versions the mod accepts. Forge and NeoForge write them as a range in square brackets: [9.1,) means 9.1 or newer, and [1.0,2.0) means 1.0 or newer but older than 2.0. Pick a version inside it.

Good to know

• Library mods often have to be on both the server and every player's game. Check the mod's own page.
• Mods you upload yourself through Files are not checked for what they need. Installing from Modding avoids most of these errors: How to install mods on Forge, Fabric and NeoForge.
• For plugins on Paper, Purpur or Spigot, see A Minecraft plugin needs another plugin that is not installed.
• Other causes the panel names: How to see why your Minecraft server stopped.