Although Andorra’s agreements with France and Spain make it effectively part of Schengen, this can (allegedly) be avoided by requesting entry and exit stamps at the border.  I’m skeptical, due to the near impossibility of being able to prove one spent all the claimed time (ninety days) within Andorra’s borders.  Since Andorra rarely does border checks, it would be easy to get a stamp at the Spanish border control site, immediately leave at another site, and reverse the procedure 90+ days later.  Of course, that would be illegal and would present a (small) risk of being caught—a Spanish official might look through your passport and see that you entered Andorra but don't have an exit stamp.

Although Andorra’s agreements with France and Spain make it effectively part of Schengen, this can (allegedly) be avoided by requesting entry and exit stamps at the border.  I’m skeptical, due to the near impossibility of being able to prove one spent all the claimed time (ninety days) within Andorra’s borders.  Since checks aren't generally done at Andorra's borders, it would be easy to get a stamp at the Spanish border control site, immediately leave at another site, and reverse the procedure 90+ days later.  Of course, that would be illegal and would present a (small) risk of being caught—a Spanish official might look through your passport and see that you entered Andorra but don't have an exit stamp.