Custom JAR

By default, the launcher will download and launch the appropriate .jar for the selected Minecraft version, but you can substitute this file with your own.

This allows you to compile in your own .jar mods.

Getting .jars

You can normally get unmodified game .jars for Minecraft from the versions/ folder within its main data folder. Remember that if you modify the .jar, you should remove the manifest and signature files from the META-INF/ folder.

Using Custom .jars

In your modpack's src/ folder, place your custom .jar inside and name it custom_jar.jar. The launcher will automatically use it.

You may want to make the .jar client only (with a _CLIENT folder).

Using .jars a Mojang Launcher Profile

If you are using a .jar that comes from a custom Mojang launcher profile (that was added by running an installer, like the Forge installer), you may need a Custom Version Manifest too.