Troubleshoot Java mod-loader mismatches

Record the required combination

Write down the exact:

Minecraft version

Loader family: Forge, NeoForge, Fabric, or Quilt

Loader version

Mod or modpack release

Dependency versions

Java runtime required by that Minecraft and loader release

Do not compare only the mod name. Two files with the same name may target different loaders or Minecraft versions.

Read the first loader error

Error pattern	What it usually means
Mod requires a different Minecraft version	The mod build and server version do not match.
Mod requires `fabricloader`, `forge`, `neoforge`, or `quilt_loader`	The wrong loader is installed or its version is outside the required range.
Missing mandatory dependency	A library or API mod is absent.
Mod is for a newer or older loader version	The loader must be updated or the mod must be changed to a compatible release.
Client reports an incompatible mod list	Client and server mod sets or versions differ.
Error references client-only rendering or environment code	A client-only mod was uploaded to the server.

Correct the mismatch

Stop and back up

Stop the server and create a backup.

Choose one target combination

Use the modpack manifest or mod author’s requirements to choose the Minecraft version, loader family, and loader version.

Install the matching server software

Select the compatible version in the Purpify installer. Do not mix loader families in the same /mods folder.

Replace incompatible mods

Upload releases for the selected loader and Minecraft version, including required API or library mods.

Match player clients

Install the same required mod and loader versions on player clients. Keep optional client-only mods off the server.

Start and test

Start once and read the first remaining error. Join only after the console reports a completed startup.

Install mods and modpacks

Review manual and installer-based mod setup.

Remove rendering and interface mods from the dedicated server.