We highlight the importance of developing the user experience (agent experience) of interface agents (hereinafter "agents") based on generative AI models and address the need for a system that helps diverse individuals, beyond AI engineers, prototype agent experiences. We conducted a requirements analysis study with 12 participants of varying experience to identify key activities for agent experience prototyping and desirable features of an agent prototyping system. We implemented these features into the AgentBuilder design probe and conducted a field agent prototyping study with 14 participants to validate design requirements and gain insight into developers' agent prototyping methods and requirements.