The ease of smartphone communications has created an expectation of constant connectivity. While the adoption of virtual assistants has improved, their capabilities for handling proactive communication tasks remain underexplored. We present the design, implementation, and evaluation of a Contextual Auto-Response agent to communicate users’ situational awareness. The agent creates auto-responses by modeling availability using smartphone sensors and sharing contextual information on behalf of the user. In a two-week study with 12 participants, we evaluated the perception of this agent and its impact on device usage behavior. Many participants found the agent useful for signaling unavailability, with some caveats. Participants also reported altering device and agent usage based on their understanding of its functions. Our findings indicate the importance of transparency in proactive agent designs and the need for personalization to enable an enhanced and cooperative human-agent interaction.