Unanimity Without Persuasion: A Single Round of Debate Erases the Disagreement That Verification Needs

Abstract:A debate panel can become unanimous without becoming more correct. This is dangerous for downstream safeguards: a substituted verification ballot can change only narrow-margin votes, while richer arbiters lose disagreement as a natural targeting signal. We show that one debate round can erase that resource without requiring persuasion. Tracking a heterogeneous 7-judge panel through a blind round and three debate rounds on 600 code-correctness candidates, unanimity on a fixed cohort jumps from 39.5\% to 95.2\% in round 1 (93.1\% of the total collapse), while accuracy moves by less than one point and 96.3\% of verdict flips follow the displayed peer majority. An execution-based verification ballot corrects 8 of 2,037 pre-debate candidate-substitution instances but changes zero in every later round; by round 3 every wrong decision is unanimous, erasing dissent that had flagged two-thirds of the panel's errors. Identical-cohort controls explain why: no-peer reconsideration reproduces 79.6\% of the collapse, real labels without reasoning reproduce 91.5\%, and random labels steer flips toward whatever they display; the full-debate condition adds 4.3 percentage points over labels only (clustered 95\% CI 0.7--8.1). The one-round collapse reproduces in two additional real runs and two fake-label seeds, remains under panel sizes 3--7, and appears in MATH-500. Parse failures concentrate on contested candidates ($p<0.001$), making attrition non-ignorable. The design implication is operational: verify before any second-pass evaluation or peer exposure, and never treat post-debate unanimity as independent evidence of reliability.

Artificial Intelligence (cs.AI)

arXiv:2609.26145 [cs.AI]

(or arXiv:2609.26145v1 [cs.AI] for this version)

https://doi.org/10.48550/arXiv.2609.26145 Focus to learn more arXiv-issued DOI via DataCite (pending registration)

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