Tools that treat diagrams as code, such as PlantUML, are invaluable for communicating complex system behavior. But Unmesh Joshi often wished for an extension to walk through these diagrams step-by-step. Yet, extending established tools like PlantUML often involves significant initial setup - parsing hooks, build scripts, viewer code, packaging - enough “plumbing” to deter rapid prototyping.He narrates now he used an LLM to build a small extension adding step-wise playback to PlantUML sequence diagrams. This illustrates how syntax design, parsing, SVG generation, build automation, and an HTML viewer were iteratively developed through a conversation with an LLM - turning tedious tasks into manageable steps.