What
A subagent is a helper that Claude starts to work on part of a task. Starting one in the background can now go through a step where another part of Claude Code confirms the launch and takes the agent over. If that step refuses, the launch is cancelled. Subagents launched this way cannot show permission prompts. Subagents that are copies of the main conversation are now marked as such.
Why
The way background subagents start has changed. One effect you will notice is that some of them can no longer stop to ask you for permission.
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The writer flagged doubt
It is not clear what performs the confirmation or when that step applies.