Does climb speed from other sources stack with the climb speed from the Spider Climb spell?

Question

Spider climb doesn't say how your base climb is affected, so I’m confused. Does it stack, does the larger amount override the smaller, or does spider climb just overwrite it, period? For example a summoner's eidolon with the Climb and Major Spells evolutions, or the summoner himself with the Climb evolution that has his wizard friend cast spider climb on him. Not sure if that classifies as an ability and thus excluded from your options.

The spider climb spell description in the Pathfinder SRD says:

The subject can climb and travel on vertical surfaces or even traverse ceilings as well as a spider does. The affected creature must have its hands free to climb in this manner. The subject gains a climb speed of 20 feet and a +8 racial bonus on Climb skill checks; furthermore, it need not make Climb checks to traverse a vertical or horizontal surface (even upside down). A spider climbing creature retains its Dexterity bonus to Armor Class (if any) while climbing, and opponents get no special bonus to their attacks against it. It cannot, however, use the run action while climbing.

Answer 1

It does not stack, no. Spider climb specifies that you get a 20-ft. climb speed, not that you increase any existing climb speed by 20 feet. Your other source of a climb speed could conceivably stack—saying that it increases another climb speed by however much—but if both are written like spider climb, they don’t stack. There are examples of abilities that provide a speed or improve an existing one if you already have it, but they have to say that explicitly, and spider climb does not.

Rules as written, though, you could use an existing (presumably faster) climb speed in conjunction with spider climb and still benefit. That is because the spell says “The subject [...] need not make Climb checks to traverse a vertical or horizontal surface (even upside down).” Unlike the other effects of the spell, which are all tied to “climbing in this manner” or “the spider climbing creature,” this effect applies to the “subject,” that is, the subject of the spell. RAW, that means you get this benefit even if you do not use any of the other features that the spell provides. Having a free hand is probably still required to benefit from this even when using another form of climb speed, however. Note that this should probably be run by a GM before learning or preparing spider climb, as it is arguably a technicality and some GMs may object.

On the other hand, personally I would have no problem with someone who wants to cast spider climb on top of their existing climb speed in order to benefit from the best of both worlds. I probably still wouldn’t allow them to stack, however, again unless the other source of climb speed supported that.