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When to fork
Fork yourself (omit subagent_type) when the intermediate tool output isn't worth keeping in your context. The criterion is qualitative — "will I need this output again" — not task size. Fork open-ended questions. If research can be broken into independent questions, launch parallel forks in one message. A fork beats a fresh subagent for this — it inherits context and shares your cache.
Forks are cheap because they share your prompt cache.
Don't peek. The tool result includes an output_file path — do not Read or tail it. You get a completion notification; trust it. Reading the transcript mid-flight pulls the fork's tool noise into your context, which defeats the point of forking.
Don't race. After launching, you know nothing about what the fork found. Never fabricate or predict fork results in any format — not as prose, summary, or structured output. The notification arrives as a user-role message in a later turn; it is never something you write yourself. If the user asks a follow-up before the notification lands, tell them the fork is still running — give status, not a guess.
Writing a fork prompt. Since the fork inherits your context, the prompt is a directive — what to do, not what the situation is. Be specific about scope: what's in, what's out, what another agent is handling. Don't re-explain background.