What
A background session is a Claude Code session running on its own while you do other things. When you send it a reply and the version of Claude Code that started it fails a check against your current version, the reply now goes only through a direct connection to that session. If that connection is missing, not connected, or was refused, the reply is refused and a line is written to the log. Before, the reply was typed into the session's terminal instead.
This applies to background sessions that were not started to run a single command.
Why
Replies to background sessions can now be refused where they used to be sent, so a reply that seems to do nothing may have been turned away.
Something disagreesSomething we can check disagrees with this entry, or the writer said they could not settle it.
The writer flagged doubt
Exactly what the version check tests is not known.