A dynamic customization system provides custom behavior for an application that has client code with an extensibility point and that has server code. An entity that uses the application provides a specification of custom behavior for the extensibility point to an extensibility server. The specification specifies an operation to perform after an occurrence of an extensibility event of the extensibility point during execution of the client code. A client executing the client code retrieves an indication of an extensibility server. The client then retrieves from the extensibility server a specification of the custom behavior for the extensibility point. After the occurrence of the extensibility event, the client sends to an operation server a request to perform the operation. The client may then receive from the operation server a response indicating the status of the request.