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Skill · pattern · parallelization

Fan-out / reduce

Run independent subtasks in parallel and reduce in code (sectioning or voting). Use when subtasks do not share mutable state: multi-file review, scatter-gather research, or LLM-as-judge voting.

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LangGraph Send, map-reduce, and Anthropic's parallelization catalog. Code aggregates. The model does not 'merge vibes'.

When

Independence is real. Shared mutable state makes this a race, not a pattern.

Do

  1. 01

    Split in code

    Sections, files, or candidates. Each worker gets a sliced window.

  2. 02

    Reduce in code

    Concat, vote, or structured merge. Cap fan-out.

  3. 03

    No shared writes

    Workers write to distinct paths. Parent synthesizes.

Don't

  • Fan-out onto the same ticket row without isolation.
  • Unbounded map over 'all the files'.

Hard rules

  • Independent → parallel. Discovered at runtime → orchestrator-workers.
  • Cap breadth the way you cap steps.

Refuse

  • Shared mega-contextA crew sharing one bloated window. Specialists drown in each other's work.
  • Unbounded loopNo max-turns, token, wall-clock, or money budget. The agent rambles until the bill or the context dies.

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Trigger tests

Should fire

  • Review these 12 files in parallel
  • Vote with three judges

Should not

  • The steps must run in order