The builtin item renderer API (#563) also allows replacing the builtin renderers of vanilla items (intended feature), but there will be a crash if multiple mods want to replace the renderer of one item.

Should we make the implementation use a Multimap instead, and render the item with all registered builtin renderers?

I'd say hard fail if multiple mods register a renderer for the same item (but allow querying ). Maybe throw a special exception.

Rendering with all registered builtin renderers isn't a great idea since it makes things look weird.

Hard fail with an IllegalArgumentException is the current and documented behaviour. Querying could be a good idea.