Wrong mod loader

Fabric mod dropped into a Forge or NeoForge instance

What the log looks like

noclassdeffounderror: *net/fabricmc/
file [^\n]{0,80} is not a valid mod file
does not contain a mods\.toml
the following mods? (?:file|are) not valid

How to fix it

A Fabric jar has fabric.mod.json; Forge/NeoForge want META-INF/mods.toml
Download the Forge or NeoForge build of the same mod
Sinytra Connector can run some Fabric mods on NeoForge, but it is not a drop-in fix

Loaders	Forge, Neoforge
Severity	Stops the game
Category	Wrong mod loader

Check your own log

Related errors

Wrong mod loaderAccessWidenerFormatException: Namespace (named) does not match current runtime namespace (intermediary)Check the crash log for the line 'Failed to read accessWidener file from mod <modname>' to identify the broken Wrong mod loaderForge mod dropped into a Fabric instance (and the other way round)Forge and Fabric jars are not interchangeable — the file names often look identical