By accident, I used the hostid of the proxy host instead of proxy_hostid for an amount of hosts.

This didn't cause an error and the hosts are shown to be monitored by no proxy.

Trying to change the proxy_hostids for these hosts via the API (hosts.massupdate) again doesn't do anything and also doesn't give any error.

Example request:

{
"method": "host.massupdate",
"params": {
"proxy_hostid": 100100000011256,
"hosts": [

{ "hostid":"100100000011069" }

,

{ "hostid":"100100000011073" }

]
}

I am able to manually assign a proxy via the web interface on the host configuration page.