Found two (possibly related) apparent issue(s) in the 1.12.2 release.

I bound the cycle left and cycle right options to 'O' and 'P' respectively, and added a vanilla iron pick and shovel. While the GUI interface works perfectly, the cycle keys do not respond.

Also, (and probably just cosmetic) the options/controls menu is showing key.toolbelt.cycle.left and key.toolbelt.cycle.right instead of relevant lang entries.

Am I missing something, or is this a bug? Any info appreciated.

After finding it, I reproduced it using a clean install of:
MC 1.12.2
Forge 14.23.5.2847
ToolBelt 1.9.12

(I'm not mod author)
I thought same thing. but I figured out pressing cycle left/right key will make makes move tool select cursor while tool swap GUI is opening.
I don't know this is only fuction. but default key binding of cycle keys are empty, so I think this is option for game-controller users.

Sorry for not answering earlier, I have been sick, and I'm still recovering.
Yes the cycle keybinds are currently designed to work while holding the key that opens the radial menu, they "move" the mouse to each slot, so that you can simulate the changes without mouse movement.
If people want a quick-cycle option, I could make the same keybinds work outside the radial menu also, but it would be a bit weird because it wouldn't leave the items back where they used to be.

Hope you're feeling better! On the cycling - that didn't occur to me. I'll have a think about it. Being a new fan, I'm used to quick 1-button hotbar switching my tools and just coping with the hotbar clutter that results. It feels 'heavy' to have to hotkey-and-click to swap, so I assumed the action was just insert into slot removed from, and grab the next tool clock and anticlockwise respectively. Personally, I don't mind if the tool order gets a little jumbled occasionally - if you could rapid cycle through the items without finding different keys and without the GUI it would be ok.

But I guess if its working as intended, and everyone else is happy with the action as is, I'll play with it as is for a while longer and see if I get used to the different action. It might feel better with different binds so its easier. I'll play around. Otherwise, awesome mod! I can't believe I took this long to pick it up for our pack.