This mod is for the Fabric mod loader and 
Also works on Quilt!
You can find zephaniahnoah's Forge port here.
Does not require the Fabric API.
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Client and server functionality are independent, meaning clients with Discerning Furnace can connect to servers without it, and clients without it can connect to server with it.
Discerning Furnace
This tiny mod (~32 lines of code) prevents un-smeltable items from inserting into the input slot of a furnace.
This is useful for sorting items and for preventing clogs in furnace arrays.
As of 1.0.8, automated inserted items are handled if the mod is installed on the server, while player inserted items are handled if the mod is installed on the client.
It affects all vanilla furnaces, and modded furnaces that don't have their own way of limiting inputs should work as well.
This mod is only for Fabric (works on Quilt, too!) and I won't be porting it to Forge. The license is MIT, however, so anyone else is free to port it and zephaniahnoah has ported it!
I'd appreciate links back to this page if you port or otherwise modify this project, but links aren't required.