Semantic Agreement Enables Efficient Open-Ended LLM Cascades

Duncan Soiffer; Steven Kolawole; Virginia Smith

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arXiv · 2509.21837

Semantic Agreement Enables Efficient Open-Ended LLM Cascades

Duncan Soiffer·Steven Kolawole·Virginia Smith

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Abstract

Cascade systems route computational requests to smaller models when possible and defer to larger models only when necessary, offering a promising approach to balance cost and quality in LLM deployment. However, they face a fundamental challenge in open-ended text generation: determining output reliability when generation quality lies on a continuous spectrum, often with multiple valid responses. To address this, we propose semantic agreement -- meaning-level consensus between ensemble outputs -- as a training-free signal for reliable deferral. We show that when diverse model outputs agree semantically, their consensus is a stronger reliability signal than token-level confidence. Evaluated from 500M to 70B-parameter models, we find that semantic cascades match or surpass target-model quality at 40% of the cost and reduce latency by up to 60%. Our method requires no model internals, works across black-box APIs, and remains robust to model updates, making it a practical baseline for real-world LLM deployment.

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Duncan Soiffer, Steven Kolawole, Virginia Smith. 2025-10-27. Semantic Agreement Enables Efficient Open-Ended LLM Cascades. https://arxiv.org/abs/2509.21837

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