run0

It is inherently hard to get this right in every case. But IMHO we could get reasonably close by having each agent report whether it is a GUI agent or a text-based agent (and/or its controlling terminal/session). Then you could have the requestor specify the same information, and filter the available agents to ones that are likely to be usable:

* If the requestor is a CLI app, and there is no $DISPLAY variable or similar, then contacting GUIs is probably a bad idea. It's probably also a bad idea to contact text-based agents with a different pty/tty, but that might be appropriate if there is no alternative (e.g. we're running under systemd or otherwise do not have a controlling terminal).
* Similarly, if the requestor is a GUI, contacting text-based agents is probably a bad idea.
* Finally, if we're a CLI app with a $DISPLAY, then it might be OK to contact GUIs on that specific display, as well as text-based agents on the same pty/tty. But it should not contact some random other session that has nothing to do with us.