When I go for longer rides I take a keg that fits in a water bottle cage. It contains:
Patch kit, multi tool (has chain breaker on it), levers, quick link, CO2 inflator.
To this I may add (if I have a bag):
Pump, spare tube.
If on my FG, which is an older bike, I'll add:
Small adjustable wrench.
For bike tours I'll add:
Spare derailer and brake cable, spare M5 bolt, small roll of duct tape, pair of nitril gloves
I don't think chain failures have anything to do with using a chain tool. I've used one for many years and never had a failure as a result. They are almost always from old worn out chain that snap due to metal fatigue or wearing away of material. A chain in good condition s pretty tough. While I do carry a chain breaker as part of my multi tool, I think the better strategy is to look at the chain once in a while and replace when it gets worn.