How can i make an idle animation play for my tool when my character's standing still?

Instead of checking the humanoid's state, try using the velocities of the HumanoidRootPart in the character.

You can place this in a local script inside the tool:

local Player = game.Players.LocalPlayer
local Character = Player.Character
local HumanoidRootPart = Character.HumanoidRootPart

local Running = false -- checks last state

coroutine.resume(coroutine.create(function()
	while wait() do
		 if HumanoidRootPart.Velocity.magnitude >= 1 and not Running then
			Running = true
			idleAnim:Stop()
			runAnim:Play()
		elseif HumanoidRootPart.Velocity.magnitude < 1 and Running then
			Running = false
			runAnim:Stop()
			idleAnim:Play()
		end
	end
end)

Basically the script checks to see whether the player was last running, and if the player was last running and the velocity of the torso is less than 1, then the character idles. However, if the player was not running and the velocity of the torso is greater than or equal to 1, the character runs.

I hope this helps!

Edit: this script is just an example of how to use the velocity of the HumanoidRootPart and not meant to be used in replacement of the script, but if you use this make sure you define what runAnim and idleAnim are