In LLM agent systems, many critical system behaviors no longer originate from hard-coded logic but emerge from model inference, strategy selection, and runtime state. This raises a fundamental question: when behavior is not in the code, what are we actually architecting? Traditional architecture diagrams often omit prompts, yet prompts determine system behavior. They often omit memory, yet memory determines long-term system drift. They often omit policies, yet policies determine whether systems remain safe. We may be drawing incomplete diagrams.

This paper argues that LLM agent architecture requires reasoning about five structural dimensions: control structures (who decides what happens next), knowledge structures (how memory is organized and governed), responsibility structures (who is accountable for what), constraint structures (what behaviors are permitted), and evolution structures (how systems change over time). While components and connectors remain foundational, they may be insufficient to fully characterize these systems; we must also architect the behavior space. We offer not a solution but a coordinate system—a vocabulary to discuss, debate, and develop these architectural foundations.