responses to the lease4/6-get-by-* commands are not documented.

https://jenkins.isc.org/job/Kea_doc/KeaAdministratorReferenceManual/index.html#lease4-get-by-client-id

It's just

{
    "result": <integer>,
    "text": "<string>"
}

although response to those commands are:

{
  "arguments": {
    "leases": [
      {
        "cltt": 1597924174,
        "fqdn-fwd": false,
        "fqdn-rev": false,
        "hostname": "",
        "hw-address": "10:10:10:10:10:10",
        "ip-address": "192.168.51.10",
        "state": 0,
        "subnet-id": 2,
        "valid-lft": 4000
      }
    ]
  },
  "result": 0,
  "text": "1 IPv4 lease(s) found."
}

or

{
  "arguments": {
    "leases": [
      {
        "cltt": 1597923516,
        "duid": "00:03:00:01:09:09:09:09:09:09",
        "fqdn-fwd": false,
        "fqdn-rev": false,
        "hostname": "",
        "hw-address": "09:09:09:09:09:09",
        "iaid": 6,
        "ip-address": "2001:db8:a::6",
        "preferred-lft": 3000,
        "state": 0,
        "subnet-id": 1,
        "type": "IA_NA",
        "valid-lft": 4000
      },
      {
        "cltt": 1597923516,
        "duid": "00:03:00:01:09:09:09:09:09:09",
        "fqdn-fwd": false,
        "fqdn-rev": false,
        "hostname": "",
        "hw-address": "09:09:09:09:09:09",
        "iaid": 1,
        "ip-address": "2000::",
        "preferred-lft": 3000,
        "prefix-len": 96,
        "state": 0,
        "subnet-id": 1,
        "type": "IA_PD",
        "valid-lft": 4000
      }
    ]
  },
  "result": 0,
  "text": "2 IPv6 lease(s) found."
}

so let's write a bit more about response in ARM