Interesting. Based on your idea of installing the docker plugin on the HAOS instance, I first tried to determine which version of Python is running on that. Seems there’s none. That explains why it wouldn’t work, but should not cause check_mk agent to hang in general. But I do recall from another host on which I run docker, that you’d also need to install the python-docker libs. Don’t know if that’s an option with HAOS. And even if you manage to get it working, I believe you’ll have to reinstall them after every HAOS upgrade.
And I totally agree, it would be better to monitor through the HA API, so you can also monitor the state of your entities and devices and all through CheckMK. But I miss the knowledge of the HA API to code something like that myself. (More, I’m missing the time to dive into it). So if you can come up with a more native way that’d be great.
And yes, after writing in this topic, I did move my agent in /config to have it persistent across reboots. Should have done that a lot sooner.