I heard a long time ago someone said
Only use the tools in your tool kit to do any work on your bike.

Pretty soon your kit will have everything you need.

While I don't use the kit as the only source of tools for working on the bike, I do carry an extensive one that doesn't take up much room
It's in three bags, two zippered pouches and a tool rool.
In the tool roll I have a BMW tool kit that has had Japanese bike sizes added
I'm too lazy to make a jap bike and a euro bike kit
so I just use one.

I've narrowed down the kit to what follows
I ride old bikes that need a lot of work...
A lot of the time so I need to bring my tools with me.

Wrenches
Combo and box ends
6,8x2,9,10x2,11x2,12x2,13x2,14,17,19,22,24
Adjustable end wrench
Slip joint pliers
Locking needle nose
5 way screwdriver (wide"-" narrow "-" #2"+", #3"+" 1/4bit)
spark plug wrench
Small section of hacksaw blade
12v test light
Small section of clear hose for brake bleeding(just enough to keep brake fluid off the paint)
lug wrench (it's a BMW thing)
hook spanner (for shock preload)
tire irons
small section of sandpaper (400 girt)
bicycle tire pump
tire pressure gauge
hex keys
Front axle multi nut tool
$20 dollar bill and a couple of singles
Leatherman multi tool
Wiring diagram of the bike I'm on.
If I need it.

I'm sure I'm missing something.
I don't carry sockets in the road bike tool kit
I do in the offroad kit.

In the two other bags
I have a tire kit
With a little cut down compressor
and a tire kit for tube and tubeless tires
Along with that is a 6foot section of fuel line
and I soon will be added an off-road style tow rope.

In the other bag is the "bike first-aid" kit
It contains the stuff needed to fix the kind of things you need to fix
on a bike by the side of the road.

Fuses
Master link
Section of wiring one in red one in black
Small selection of crimp connections
Handful of
Nuts, bolts, washers, screws, clips, and hose clamps
Safety wire, duct tape, electrical tape,fiber packing tape
zip ties
Section of a coke can to patch holes in cases
Small bottle of tri-flow oil as a last resort chain and other lube.
(double baggie this it stinks!)
Sparkplugs.

I always keep a basic kit on the bike
with a compass, lighter, firestarter, knife, e-blanket, whistle, first aid kit, heat packs

I overpack on tools and stuff
But then I'm most often the one who gets to help out others.

It's all well and good if you know how to use the stuff
If you don't know how to fix the bike that's the best thing you can do. Learn. Seek out an experienced wrench in your area
and see if they could help you out
Here in LA there are a number of ways you can get some wrenching skills if you need them.

In my experience the things that have stopped people most often
other then crashing
Is electrical trouble that's the first area to learn as much as you can about the basic idea of how your bike's system works.


Squish
The opinions I post are mine solely,