An assistant refers to a (human or software) agent that helps a user accomplish her goals, ideally with minimal instructions from the user. In this paper we describe a software assistant agent that can proactively assist human users situated in a time-constrained environment. We specifically aim at assisting the user’s normative reasoning–-reasoning about prohibitions and obligations–-so that the user can focus on her planning objectives. In order for proactive assistance, the agent must be able to 1) recognize the user’s planned activities, 2) reason about potential needs of assistance associated with those predicted activities, and 3) plan to provide appropriate assistance suitable for newly identified user needs. To address these specific requirements, we develop an agent architecture that integrates user intention recognition, normative reasoning over a user's intention, and planning, execution and replanning for assistive actions. This paper presents the agent architecture and discusses practical applications of this approach.