I am currently writing our own zabbix-agent like for our 20 year old c++ framework and noted an oddity in the protocol : Zabbix always answer my program with a message with the ZBXD header whatever the message I send it.

Steps to reproduce:
send a message to update a value through a trapper item in Zabbix

The message is pure JSON :

{"request":"sender data","data":[{"host":"myHost","key":"testalarm","value":"0"}]}

Result:
The response is still in version protocol v1:

ZBXD Z{"response":"success","info":"processed: 1; failed: 0; total: 1; seconds spent: 0.000536"}

Expected:
I would have expected the response in pure JSON, without the header

I looked at the source code and noted that from the trapper there is the zbx_send_response which is redefined as zbx_send_response_ext which use the ZBX_TCP_PROTOCOL constant as parameter for the protocol, which is in the end compared to itself in the zbx_tcp_send_ext which in the end sends the response. So it is always in version 1.