Barb is correct. The agreement must be in writing to be enforceable.
Explanation:
A written agreement that is legally binding is a true agreement and can therefore be enforced.
It ensures that the people that signed the agreement must perform their duties under the agreement. We can be penalized if they do not.
Although documents need not be published to be legally binding, it is a good idea to keep a record in writing of what you have agreed. It minimizes the likelihood that you and the other side will find themselves on the same page in a disagreement. The article describes the conditions for a legally binding written agreement.