think you mean str and CON, not AGI. you can mine all the way to 0 before being too tired to mine, its been this way as long as I've been mining(late W5). you're also too tired to walk at that point, but thats a different problem.
I talk about mining with a stoneaxe as i don't have a pickaxe yet
In the last world the message "You're too tired.." appeared when mining with pickaxe when stamina was down to a quarter - my question is just: message not appearing because stoneaxe or because it was changed generally...
You've always been able to mine until 0 stamina. The "too tired" message only deals with the initiation of the mining. If you've got over 25% stamina and you start mining then you'll mine until you hit 0. Same goes for building items.