Experience being a cruel teacher, my toolkits typically don't err on the sparse side, even on the racer. And I used to think that carrying a chain tool was unnecessary, until I had a chain do its best sleepy snake impression on the road behind me. And then again on a different bike for good measure. ("I checked that those pins were centered! Centerrrred!")

At a minimum, a foldout multi-Allen tool, a teeny chain tool, a Quik-Stik, Rema patch kit, 2 tubes, a small pump, and a 'Murican-made adjustable wrench (4" or 6").

My "weekly ride leader" toolkit, though, has a 6" adjustable wrench, 1/4 hex drive bits in all common Allen & screwdriver sizes, a cool 1/4" hex ratchet, 8/9 open end, 10 open end, compact chain tool, small needlenose mini-tool, donut spoke wrench, Quik-Stik, metal and plastic levers for tires that need additional persuasion, Rema patch kit, boot material, 3 Presta tubes (700x25, 26x1, 26x1.75) with Schrader adapters, metal valve cap with Schrader valve core remover end, good classic hand pump, Fiber Fix emergency spoke. mini bungees, mini bungee net, super-compact folded musette bag, small USB charger, zip ties, tie wire, spare tandem-length gear and brake cables, a few common 5 & 6 mm bolts/nuts/washers, quick links in widths from 5.5 to 7.3, first aid kit with gauze, tape, chamois cream, Ace bandage and antibacterial gunk, sunscreen, more sunscreen, a few bucks for boot / yard sales / other purposes, and a teeny bungee-on LED headlight just in case and to find everything else in the dark. Most of the wrenches and bits are either covered in reflective tape or painted fluorescent orange - again, experience being a bitter teacher when making repairs at night. It all fits in one side pocket of the Jandd rear pannier. And nearly all of it has seen action in the past, either for a person on the ride or someone we encounter along the way.

For El Tour bike patrol I swap the hand pump for a super-light floor pump, and add channelocks (to fix disintegrating headsets and 1-piece bottom brackets on bikes that haven't been ridden much all year) and 13/15/17 cone wrenches for bikes with woobly wheels due to too much axle slop (or vice versa).

Every time I obtain another 6" adjustable wrench, I weigh it to assess whether it rides in the home toolbox or qualifies for bike duty. Typically a weight under 100 grams will get it a slot in an on-bike toolkit.

Your experience, needs, and carrying preferences may vary. Enjoy the ride!