This sounds to me like a breakdown in communication. There is a lot that happens during a transaction and it's easy to become overwealmed. Now, that being said, it is your agent's responsibility to make sure his commission is covered. It is just not good business practice to surprise you at the last minute. Believe me, every agent knows the co-op commission on every home they show, much less write a contract on. Many homes out there have discounted commissions and your agent should have known and disclosed it to you when you wrote the offer. You are correct, if you had know about the extra costs you would have negotiated differently. If your agent knows what is good for him/her, they will eat the 1/2% and learn their lesson. That's what I would do in order to close the deal and keep you from stringing me up! Your good word and future referrals would be much more valuable to me in the long run than a measly 1/2%. But, if you did sign a buyer/broker and your agent wants to hold firm, then chances are that yo will be responsible for the extra commission. Talk to your agent, if they don't feel like they missed an important detail and aren't willing to accept responsibility, then you may want to involve their broker. Best of luck to you and congrats on the purchase!