Unclear Nothing was seen choosing the second version, so whether any background session uses it yet is not known.
What
A background session is a Claude Code session that runs while you do something else. Claude Code gives it a set of instructions, under the heading "# Background Session". That text can now be put together in a second way, which uses its own separate text about keeping work isolated. The default way still produces the same text as before.
Why
Nothing changes in the default instructions. The second version is a different mode for background sessions.
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Nothing was seen choosing the second version, so whether any background session uses it yet is not known.