How did you determine the subspecies?

it has to do with location. All the P. zelicaon west of the Sierra Nevada are referable to the "nominate" subspecies (the one that bears the same name as the species). It gets a bit fuzzy east from there though. These two subspecies are pretty poorly defined.

In your range, P. zelicaon is definitely the most probable ID for this caterpillar. The best way to be sure is to raise it and see what it turns into. The Guide has many more images of P. polyxenes (black swallowtail) caterpillars then P. zelicaon, so if you don't mind losing some parsley, raising it to an adult and confirming the ID of this caterpillar would be especially useful.

For my P. polyxenes, I buy bunches of inexpensive curly parsley at the supermarket and raise the cats indoors. Parsley keeps a long time in the fridge, and I've never had to buy more than two bunches of parsley for the entire larval period of a dozen caterpillars. Admittedly they do produce a lot of large droppings, I put a basket coffee filter on the bottom of the rearing container for easy cleanup.

(Not that everyone wants to raise caterpillars, of course. I'm just a weird person who grows parsley solely to acquire P. polyxenes larvae.)

Another commenter diagnosed it as Black Swallowtail, but I never see an adult Black. I don't see P. zelicaon adults, either, only Western Tiger. Could it be that the adults of those species don't feed on anything nearby and they leave to find their preferred nectars after pupating? Who doesn't like buddleia?

We're already losing parsley, we're losing it outdoors is all. Don't worry, there's plenty of parsley. The specimen appears to be the last instar. Perhaps unnoticed previous and smaller instars were responsible for the missing carrot tops in the same raised bed.

Over on the East Coast, I see more tiger swallowtails at buddleia too - usually the only P. polyxenes adults I spot are females hovering over parsley patches! Most adult swallowtails feed on both nectar and soft, rotting fruit, and P. polyxenes seem to prefer the fruit. (I offer captive adults flowers and canned peaches, and they usually go for the peaches.)

P. polyxenes and P. zelicaon can be indistinguishable as larvae - P. zelicaon may develop spikey-looking tubercules in the last instar, but not always. Quite possibly many caterpillars identified as P. polyxenes in California are just misidentified P. zelicaon.

Swallowtails that want fruit can get it here, too. Near the parsley, half-eaten loquats are available. Squirrels drop them from the tree. The loquats might not be as tempting as peaches packed in sugar syrup, though. I haven't seen butterflies at the fallen loquats, just ants.