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4. Planner-executor separation

One model call makes a plan; cheaper calls execute the steps.

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For multi-step work, splitting planning from execution improves both cost and debuggability. A strong model writes a typed plan once; a cheap model (or plain code) runs each step.

ts
const Plan = z.object({
  steps: z.array(z.object({
    id: z.string(),
    tool: z.enum(["search", "fetch_page", "summarize"]),
    args: z.record(z.string()),
    depends_on: z.array(z.string()).default([]),
  })),
});

const plan = Plan.parse(await planWithStrongModel(goal));      // one expensive call
const done: Record<string, unknown> = {};

for (const step of topoSort(plan.steps)) {                     // deterministic execution
  done[step.id] = await tools.run(step.tool, resolve(step.args, done));
}

Benefits: the plan is inspectable before anything runs, steps without dependencies run in parallel, and a failed step can be retried without re-planning the whole task.

When not to use it: highly interactive tasks where each result changes the goal. There, a plain ReAct loop adapts better than a frozen plan. A common hybrid is *re-plan after every N steps*.

Exercise: add `depends_on` handling and run independent steps with `Promise.all`. Measure wall time against the sequential version.

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Check your understanding

1. Main advantage of a separate planning step?
The plan can be inspected, parallelised, and partially retriedIt removes the need for toolsIt eliminates hallucinationIt shortens the context
2. When is a frozen upfront plan a poor fit?
When each result substantially changes what should happen nextWhen steps are independentWhen tools are slowWhen the model is strong

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