Excalidraw

Works two ways: Kroki API for quick SVG generation with just curl, or the local CLI if you need PNG. The real value is proactive use. Claude will offer diagrams when explaining systems with three or more components or complex data flows, not just when you ask. Comes with a detailed color palette (60-30-10 rule), semantic arrow styles, and font hierarchies so the outputs look professional by default. The macOS keyboard shortcut patch and Firefox requirement for local exports are quirks worth knowing upfront. Best for architectural diagrams, flowcharts, and technical visualization where you want clean, consistent styling without manual tweaking.