You need this in your rope bag. You need this on your person or within reach every time you tie, without exception.
Rope bottoms are taught to check for this and refuse interaction if the rope top is without safety shears. Why? Because it can be life and death.
Buy one here or elsewhere but be sure to buy this. One for each rope bag.
Hopefully you don't ever need it. But practise to use it, so that you know how when you need it.
Please be safe