[EntrePrescott]

You can (if you're the bigger coalition partner with a big power imbalance), and in fact it's not that rare at all and it's exactly what the CDU did quite blatantly last time they were in coalition with the FDP at the federal level under Merkel. Of course, the smaller coalition party could theoretically react to such an infringement against the coalition contract by ending the coalition…

… but that would usually hurt them enormously more than their infringing bigger coalition partner, especially if the infringement of their partner against the coalition contract is about something that the majority of voters doesn't care enough about, if it's something that the bigger coalition partner can publicly spin as TINA(*)-necessary due to changed conditions or anything they can sell as a "crisis" or if the polls are currently less favorable for the smaller party than at the last elections.

Ending a coalition is something that would typically come at an extremely high price for the smaller party who'd do it. Which is why it happens much more rarely than unilateral infringements against coalition contracts by the bigger coalition partner.

(*): https://en.wikipedia.org/wiki/There_is_no_alternative