Why is TOOL:LeftClick() only called serverside in singleplayer?

After some frustration of trying to get TOOL:LeftClick() to draw a derma menu, it seems it’s not meant to be called clientside, at least in singleplayer mode.
The wiki advertises it as being “shared”, (which I believe means to be called both server and client side), however a bulleted note mentions “Doesn’t seem to be called on the client side in single-player mode, for whatever reason.” The CLIENT variable also comes back false when printed in a tostring() function.

Is this a GBug, on purpose, or am I doing something wrong?