I'm guessing Dorymyrmex because of the diagnostic single spine on the propodeum. Further: single-segment petiole, no ocelli, and the apex of the abdomen (as far as I can tell) is more slit-like than circular.
~3 mm body length.
D. bicolor?
Though BugGuide doesn't have any Bay Area records of that species.
Fisher & Cover say the western Dorymyrmex species aren't well understood.
I added three photos of ants on deerweed tending aphid herds; these were a separate population from those in the previous photos, but I think they're the same species. You can just about see the propodeum spine in one of the photos.