When you ask Claude to run tasks in parallel, it starts a nested session for each one. Each reports its result back into your main session when it's done, so the main thread stays readable.
Claude runs parallel tasks in nested sessions that report back to the main thread
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Anthropic says that when users ask Claude to run tasks in parallel, Claude starts a nested session for each task. Each nested session reports its result back into the main session when finished, keeping the main thread readable.
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Source: ClaudeDevs · x.comPublished · added here