TanStack Start server/client code boundary patterns. Use when: debugging virtual module resolution errors (tanstack-start-injected-head-scripts, tanstack-start-manifest, #tanstack-router-entry), importing from @tanstack/react-start/server, using getCookie/getRequestHeaders/getRequestUrl, adding providers to router.tsx, or when Vite fails to resolve TanStack Start imports. Covers createServerFn patterns, dynamic imports, and proper placement of server-only code.