Atrium

Atrium was the first real consumer of Trellis, and building a dense, interactive app on it showed me which parts of a canvas library matter: controlled nodes, a typed change stream, and stable coordinates under zoom. I learned to keep layout pure and testable, so Arrange (grouping linked cards into clusters and packing them into rows) is a plain function the store, the client, and agents all share. Multiplayer taught me Server-Sent Events end to end: one stream per open surface, versions hashed per surface so unrelated changes don't reload anyone, presence, and draining streams cleanly on shutdown. Documents are stored as Portable Text but agents read and write Markdown, which meant writing converters both ways, including tables and references to other cards. Running the same tools through Mastra for the in-app agent and through MCP for outside agents made me design the store's functions as the single API everyone goes through.

Atrium shows I can design a product model and the interaction work that makes it feel fast: drag, resize, link, lock, arrange, focus, search with ⌘K, and keyboard paths for each. It is a real test of Trellis, since every card type, link, and layout runs on the library I published. The agent work is practical rather than a chat bolted on: the agent creates documents, boards, charts, and UI cards on the canvas, and every viewer watches them appear. The app is covered by 44 Vitest files and 15 Playwright specs, including one that drives two signed-in people on the same surface.