arXiv:2510.04452v1 Announce Type: cross Abstract: Interface agents powered by generative AI models (referred to as "agents") can automate actions based on user commands. An important aspect of developing agents is their user experience (i.e., agent experience). There is a growing need to provide scaffolds for a broader set of individuals beyond AI engineers to prototype agent experiences, since they can contribute valuable perspectives to designing agent experiences. In this work, we explore the affordances agent prototyping systems should offer by conducting a requirements elicitation study with 12 participants with varying experience with agents. We identify key activities in agent experience prototyping and the desired capabilities of agent prototyping systems. We instantiate those capabilities in the AgentBuilder design probe for agent prototyping. We conduct an in situ agent prototyping study with 14 participants using AgentBuilder to validate the design requirements and elicit insights on how developers prototype agents and what their needs are in this process.