When to Fan Out to Subagents
One property — an isolated context — is every benefit and every failure mode; the criterion is not a checklist but a single question
Digging into when to use subagents across three sources — official docs, empirical literature, local skills — they converge on one point: a subagent is defined by a single property — it works in an isolated, fresh context and returns only a summary — and every benefit (parallelism, context protection) and every failure mode (coordination collapse, write-inconsistency) is the two faces of that one property. So the criterion collapses to a single question: does this task want its context isolated, or shared? Reading/gathering/independent search wants isolation, so fan out; writing/synthesis/coordination wants sharing, so keep it in one agent. The cost gate (N×, 15×) and "upgrade the model before adding agents" sit on top. Not a controlled experiment — a synthesis plus a reflexive observation (this very research was itself a read-fan-out).
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