FabricQuiltMC 1.19.xMC 1.20.xMC 1.21.x

Mod requires fabric-language-kotlin (missing Kotlin classes)

Mods written in Kotlin need the Kotlin runtime at load time. On Fabric that runtime is the fabric-language-kotlin library mod, and it is a separate download — a mod depending on it fails with NoClassDefFoundError naming org.jetbrains.kotlin classes when it is missing or version-mismatched.

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How to fix it

  1. 1

    Install fabric-language-kotlin

    Download the version matching your Minecraft version from Modrinth/CurseForge and drop it in mods/. It is a library — it does nothing visible by itself.

  2. 2

    If it is already installed, version-match it

    An old fabric-language-kotlin with a new Kotlin mod fails the same way as a missing one. Update the library to the current release for your MC version.

  3. 3

    Watch for bundled duplicates

    Some mods bundle their own Kotlin runtime — two runtimes in one mods folder can clash. If crashes started after adding a specific mod, that mod's bundled copy is the suspect.

  4. 4

    Restart and confirm

    The Kotlin classes resolve at boot; if the log gets past the failing mod's load, the fix landed.

Alternative causes

These can produce the same error message — worth ruling out if the steps above don't resolve it.

kotlin-for-forge confusion

NeoForge/Forge's equivalent library is Kotlin for Forge. Installing it on Fabric (or vice versa) does nothing — loaders require their own runtime.

Frequently asked

Why do mods need a separate Kotlin library?

Kotlin code needs its runtime classes present. Bundling a full runtime in every mod would bloat every jar, so the ecosystem shares one library mod.

Can I just install the newest fabric-language-kotlin?

Usually yes — it targets a range of MC versions. If your MC version is older, match the release notes; very old packs may need an archived build.

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Last reviewed 2026-09-06. If a step is wrong or out of date, tell us — we'll fix the article and the auto-pattern at the same time.