All plain text output from the last backend CLI command executed is captured and returned to the client. If the command executed was not a show command and the requested operation succeeded, then the response body is empty. This is consistent with the CLI when executing a command via SSH or the serial console in that status is only rendered to the console upon error.
A JSON status object is guaranteed to be returned in the response body when performing a POST, PUT, or DELETE operation or upon error(s) encountered when processing a request.