You can use multiplayer commands on a singleplayer world

Can confirm in 20w21a 20w22a

@Mish Rodic you mean 22a? I can't confirm in 21a, only 22a.

This bug seems to have been added in 22a. When updating fabric it caused our command tests to fail.

The fix seems trivial (Talking in official mojang names):

In Minecraft.selectLevel, ServerResources.loadResources is called with true being passed as the 2nd argument, this should be false, as its being ran on the intergrated server. This boolean is only used when true to enable the dedicated server commands (such as Ban and Stop). I created a mixin [gist|https://gist.github.com/modmuss50/8a65868faa5d4a0ffe60649b16a06153] to change this and validated that it fixed the issue.

Ah, yes. 20w22a. 🤦