Under 2.5" long and .5" wide really limits the options. We're down to Case Peanut territory here. And that's what I'd suggest, actually. Or something like it.
2.5" blade means you're going to be dealing with a 3-3.5" handle. Yes, Spyderco makes/made a bunch of knives 2.5" and under, but unless there's one I'm not thinking of, all will be wider than .5". There's some other dinky knives that come to mind, but they'll all be wider than .5" too. Mostly though, companies don't design knives that small with blades that narrow that also have locks and/or clips.
I do have a couple that lock, but neither have clips. One is an A.G. Russell 2.5" lockback (the baby brother to the 3" mentioned earlier, which is 5/8" wide).
Excuse the craptactular pic.
Unlike the 3", which was available in CF or titanium, the 2.5" was only made in titanium. No clip, or even a lanyard/keyring hole. Blade is just under 2", and is of VG-10. Very high quality, very thin knife. Very hard to find too.
The other knife I have is the Boker Titan 2. I honestly wouldn't recommend this knife outside of this scenario. The blade is indeed half an inch wide, and barely under 2.5" long. The blade to handle ratio sucks, or at least appears to (the ricasso slants towards the handle rather than the tip, meaning you lose maybe 3/8" that could be cutting edge), although the design does give you (me, anyway) a full 3 finger grip. The steel is also rather bottom-of-the-barrel at either 4034 or 440A (reports vary), and I can confirm that it is quite soft. However, it fits the parameters, minus the clip. Don't confuse this with the Titan 2 Drop. Newer knife, with a clip, but I don't think it's small/narrow enough.
I wouldn't be too sad about not having a clip with whatever knife you end up with. A knife that small fits easily in a pocket, or on a keychain. You can buy leather slip sheaths with clips too. I also wouldn't be too sad about not having a lock, like with a small slipjoint such as a Case (or other brand) peanut. Face it, you're not going to be doing anything particularly rough or (*snort*) "tactical" with a knife that small, and I'd be guessing that a job that limits personal pocket knives to that size doesn't really require anything larger or more robust either. So, a Case Peanut, Tiny Trapper, Baby Butterbean, small stockman, 3.25" medium stockman, small jack, sway back jack, etc. The Case/Bose teardrop jack is a particularly elegant knife with a 2.5" blade and a longer handle (4 fingers for me). Plus, nobody bats an eye at a traditional knife with bone or wood handles.