The agent protocol for passive checks is simple and efficient, but at the same time it's very difficult to extend. Especially, when it comes to preserving compatibility with some very old agents. We should create a better, JSON-based protocol for passive checks (used by the new agents in 7.0) but keep support for the old protocol intact. One way to do it is to replace item key with the new JSON struct but fall back to the old proptocol if the agent returns error about invalid key format.