What
A background session is a Claude Code conversation that keeps running without being on your screen. Sending it a reply now accepts an option that saves the reply for the session's next turn. A reply sent this way always goes through Claude Code's own message channel rather than being typed into the session's terminal.
If the session is blocked, meaning it is waiting on something, and it is not running in exec mode, the reply is no longer typed into its terminal. Claude Code reports that the reply was not delivered.
Why
Replies to a background session can now wait for its next turn instead of being typed in while it is busy.
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It is not clear which part of Claude Code sends these replies or how a reader would use the next-turn option.