Yet Another Config Lib and Fabric Language Kotlin: how they differ

FLK role

Kotlin runtime

YACL role

Config screens

Same mod?

No

Install both?

If required

Yet Another Config Lib, usually shortened to YACL, and Fabric Language Kotlin often show up together in the dependency lists of Fabric mods. They are separate projects that solve different problems. Knowing which is which makes dependency errors much easier to read.

Two libraries, two jobs

• Fabric Language Kotlin lets Fabric Loader run mods written in Kotlin and supplies the Kotlin runtime libraries. See the plain-English explainer for details.
• Yet Another Config Lib is a library that mod authors use to build configuration screens, the menus where players change a mod's settings.

One is about the programming language a mod is written in. The other is about a user interface feature that a mod offers. Neither replaces the other.

Why they appear together

A single mod can depend on both: it might be written in Kotlin and also use YACL for its settings screen. Other mods need only one of them. There is no general rule, because each mod declares its own dependencies. The only reliable source is the mod's own requirements list.

How to read a dependency list

1. Open the mod's page and find its required dependencies.
2. Install every required entry for your game version and loader.
3. Treat optional or recommended entries as optional. They unlock extra features but are not needed to launch.

If Fabric Loader stops with a missing dependency message, it names the mod ID it could not find. fabric-language-kotlin refers to this library. A YACL mod ID refers to the config library.

Installing both correctly

• Match each library to your Minecraft version and to the Fabric loader.
• Place both jars in the same mods folder. Neither needs to be unzipped.
• Check the minimum versions requested by your mods and satisfy each one.

Common mix-ups

• Installing YACL to fix a Kotlin error, or the reverse. Read the mod ID in the error before downloading anything.
• Using a build for the wrong loader. Some libraries publish separate files per loader. Fabric setups need the Fabric build.
• Assuming a library adds features. Both are support libraries. The visible features come from the mods that use them.

More error messages and their fixes are listed in the troubleshooting guide.