The Bias Tax: From Closure Failure to Verification Overhead in Long-Context LLM Auditing

Abstract

As long-context LLM evaluation shifts from retrieval toward audit-like reasoning, the critical challenge is preserving correct logical closure under prompt-induced pressure and understanding how generation failures propagate into downstream verification cost. We study this in an 80,000-token legislative corpus using six prompting conditions—Control, Management, Chain-of-Thought, Periodic Summary, Union, and One-Shot—within a Reader–Judge architecture (DeepSeek-V3 as solver, DeepSeek-R1 as auditor). We introduce a Logical Needle-in-a-Haystack (L-NIHS) stress test requiring traversal of a five-needle chain from base rule to factual evidence and final closure while resisting a late-stage distractor. Our main result is structural: the dominant failure is not early retrieval loss but late-stage closure failure under behavioral pressure. Early anchors remain recoverable, while interference rejection and closure degrade sharply once persona-congruent distractors enter the reasoning path—confirmed for four of five non-control conditions in a randomized replication (p < 0.01 for N4 and N5 vs. Control). On the auditor side, distorted outputs induce a Bias Tax: persona-conditioned responses (Management and Union) are shorter, yet each token costs the auditor significantly more to verify. Length-normalized verification cost is 44% higher for Management and 27% higher for Union relative to Control (p < 10⁻⁵), while non-persona conditions show no elevation. This overhead reflects the auditor's need to disentangle grounded evidence chains from persona-congruent fabrications—procedural blockers under Management, stacked compensatory clauses under Union—rather than solver-side reasoning depth. We further show that high tone certainty is widespread and unreliable as a proxy for logical fidelity. The results suggest that strong persona induction reshapes not only answer style but the pathway by which evidence is selected, justified, and transformed into both final decisions and downstream audit burden.

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