When tools go red in hot bar and can't be used.

[RedneckPilgrim]

It's a weird problem, I look all over and people basically state to drop it in the world and pick it up. That didn't work for me, but I did manage to go in the inventory and equip them without the hotbar, making the hotbar worthless. Then it worked for me then.

[Askannon]

The only things I can think of are:
1) Was your tool in your backpack?
2) Did you hold a large tool (like the pickaxe) and did you have room for it to be placed?

[RedneckPilgrim]

Under normal circumstances it would go back in inventory when the tool is unused in the hotbar, going yellow. However even when I had inventory space, it wouldn't go back. Both hands would be occupied sometimes, but other times it wouldn't. So putting it back into the inventory and off the hotbar, equipping it through the inventory, worked for me.

[Garth Zader]

Apart from the things askannon said, the only other time this happens is with building tool, one is temp other isn't. Don't know why it's a thing.
Try emptying the entire hotbar and repopulating it with the tools/weapons from inventory, just don't be flying when you do because it sometimes drops them instead.

[Askannon]

Other option could be custom item with a large component in it (e.g. a propellant tank in a pocket pump), which might block to the right of the custom item.
Also a slight possibility is ghost items (items that don't show up in your backpack, but are, in
weight at least, there). Just go to the market place, find the terminal where you can sell items and empty out your backpack and all other equipment slots and see if you have items there. Those you can just sell, as they aren't accessible in any other way and just weigh you down, though I don't know if they are duplications of items past or just items that got lost.

I don't know if this might have an influence, but another suspect could be you being in easy build mode, but that is a far fetched idea, based solely on the idea of EBM being the punching bag of the game.

And finally: maybe it is just a bug or something. In those cases it often helps to close the game to let it clear its cache as maybe some variable got corrupted or wrongly applied.

[Garth Zader]

There is also that weird bug of dropping items into the world and them not existing until you move them, have no idea if that would cause issues with tools.

[RedneckPilgrim]

I found out one reason it was happening, for some odd reason my tools were duplicating. I ended up with two pickaxes and buzzsaws.

[marty44]

I had a similar issue of my tools not holstering or unholster. I fixed the problem by exchanging the tools in the large tool slot and then back again. Then I was able to once again pull out my pickaxe and put back into my backpack again.

Hope that helps.