An MCP server is the right shape for "let an LLM drive my UI" but spinning up Anthropic's SDK in a browser tab is overkill — and you don't want every package owning its own framing of MCP. MicroMcpServer is the in-page server every Fancy UI bridge plugs into.

A 200-line in-page MCP server with register(tool), JSON-RPC framing, and an SSE transport. Every register*Bridge in agent-integrations takes one of these and adds its surface-specific tools.

Instantiate with new MicroMcpServer({ name }), hand it to the bridges you want (registerWhiteboardBridge(server, ...)), and attach the transport — local for an embedded agent, or via ShareControls for a relayed external agent.