This book constitutes the proceedings of the First International Workshop on AI Agent for Information Retrieval: Generating and Ranking, Agent4IR 2025, which took place in Philadelphia, USA, during March 2025. The 11 full papers included in the book were carefully reviewed and selected from a total of 38 submissions. They focus on the latest theoretical advancements and practical implementations of AI agents in information retrieval, advance the field and enhance its societal impact.