I ran into this issue when I was updating the Fodor's Costa Rica guidebook this year. I could not get consistent answers from anybody about passport validity. Even Costa Rican government sources gave me different answers. I finally had to phrase it: "Because of inconsistent enforcement, we recommend that your passport be valid for at least six months when you travel to Costa Rica." And maybe that's the best answer here. As you get down to the past year on your passport, start thinking about renewing it and you won't have to worry.

That should be "as you get down to the LAST year on your passport" in my final sentence above.

" "Because of inconsistent enforcement, we recommend that your passport be valid for at least six months when you travel to Costa Rica." And maybe that's the best answer here."

Probably so. Never wise or worth the time to argue with a bureaucrat or clerk.

Jeff_CR,
It is indeed a problem to get proper info regarding these matters. I just looked at the official link that Samba posted and saw at least one inaccuracy. FMM visas are no longer issued in Mx.
Immigration, states of emergency and alert, airline regs and a slew of other issues seem to change on a regular basis. Most of these agencies don't seem to keep up with new info or communicate with other affiliates. Somethings are written in stone (for the moment, at least) while other things are up for interpretation. It's a crap shoot.
As an example, I've had some airline clerks almost not allow us to fly into the US because we only had a one way ticket while others don't care. I've also had some make us get special entry permission at an on sight airport office because of our Mx residency and others let us go through without a hitch. We were wrong when we figured that having temporary residence in Mx would mean less hassle than having a plain old tourist visa.