Right now, the raw op mode API for show interfaces always returns a list, even when the request is about a single interface.
vyos@vyos# sudo /usr/libexec/vyos/op_mode/interfaces.py show_summary --raw --intf-name eth0 [ { "ifname": "eth0", "oper_state": "up", "admin_state": "up", "addr": [ "192.168.56.107/24" ], "description": "", "mtu": 1500, "mac": "08:00:27:64:ce:12", "vrf": null } ]
That's an annoyance from the client perspective since clients need to unwrap a single-item list.