What
A subagent is a separate helper Claude starts with the Agent tool to work on part of a task. The Agent tool decides whether a subagent runs in the background, meaning it works on its own while the main conversation carries on. That decision now also takes into account whether the caller is itself a subagent in a non-interactive session, such as one started with -p. When that is the case and a further condition holds, it changes whether the subagent is forced to run in the background.
Why
This may change whether subagents started by other subagents run in the background when you use -p.
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The writer flagged doubt
The exact conditions and whether nested subagents end up more or less likely to run in the background are not clear.