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A closed-form model of multi-agent LLM pipelines says decomposition does not pay: a planner-coder-reviewer pipeline cost 3.22x a single strong call and solved no more tasks

AspireAI community · arXiv
Multi-agent LLM pipelines are usually justified on capability grounds with runtime cost treated as an unknown overhead, so the authors build a closed-form model of the coordination tax (decomposition overhead plus per-handoff protocol cost) against the heterogeneity dividend from routing work to a cheaper model, giving a break-even threshold and a feasibility bound of alpha < 1/r. Measured on 135 HumanEval and ClassEval tasks across 2430 runs of six architectures with Claude Opus 5.5 and Haiku 5.5, the protocol added 7.2 ms per handoff (0.11% of wall time), while a planner-coder-reviewer pipeline cost 3.22x as much as one strong call, took 3.24x as long and solved no more tasks. Moving the planner and reviewer to the cheap model cut cost to 0.39x but cost 6.7 points of pass@1, and the break-even forecast was off by 19% because cheap roles inflated token volume; the saving the authors actually find is picking one model per task, which an oracle selection solves 94.3% of runs at half the cost per solved task.
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