mcp_toolkit extracts the shared MCP-server framework that Smily's apps grew independently: a Streamable-HTTP transport, cache-backed sessions, central-app token introspection (satellite + authority roles), a registry-driven "generic tools over N resources" dispatcher, and an injectable serializer DSL. It wraps the official `mcp` gem as the JSON-RPC core so each app ships only its serializers, resource registrations, and scope blocks.