Consistency Illusion: Reasoning-Level Divergence Under Answer Consensus

Properties

family: bypass
severity: systemic
triggerError: A multi-agent system uses answer-level agreement across agents as a reliability signal in a safety-critical context such as medical question answering.
mechanism: Debate reduces detectable contradictions between agents' surface answers while simultaneously decreasing the semantic similarity of their reasoning chains, so the consensus-as-reliability heuristic certifies an answer whose underlying justifications are actually diverging -- the agreement is a statistical artifact, not an epistemic one.
blastRadius: Any downstream stage that gates on the consensus signal (routing, human review escalation, confidence thresholding) inherits the false certification; errors embedded in divergent reasoning chains are never surfaced because the answer-level checkpoint passes.
mast: [Cat 3]
detection: Cross-agent reasoning alignment metrics (e.g., CARA) that measure semantic similarity of reasoning chains independently of answer agreement; flag cases where answer consensus is high but reasoning alignment is low.
mitigation: Require agents to commit to named facts and explicit stances on each other's claims (Grounded Debate Protocol); audit reasoning alignment alongside accuracy before treating consensus as a trust signal.
source: The Consistency Illusion: How Multi-Agent Debate Hides Reasoning Misalignment (arXiv:2606.08457)

family: bypass
severity: systemic
triggerError: A multi-agent system uses answer-level agreement across agents as a reliability signal in a safety-critical context such as medical question answering.
mechanism: Debate reduces detectable contradictions between agents' surface answers while simultaneously decreasing the semantic similarity of their reasoning chains, so the consensus-as-reliability heuristic certifies an answer whose underlying justifications are actually diverging -- the agreement is a statistical artifact, not an epistemic one.
blastRadius: Any downstream stage that gates on the consensus signal (routing, human review escalation, confidence thresholding) inherits the false certification; errors embedded in divergent reasoning chains are never surfaced because the answer-level checkpoint passes.
mast: [Cat 3]
detection: Cross-agent reasoning alignment metrics (e.g., CARA) that measure semantic similarity of reasoning chains independently of answer agreement; flag cases where answer consensus is high but reasoning alignment is low.
mitigation: Require agents to commit to named facts and explicit stances on each other's claims (Grounded Debate Protocol); audit reasoning alignment alongside accuracy before treating consensus as a trust signal.
source: The Consistency Illusion: How Multi-Agent Debate Hides Reasoning Misalignment (arXiv:2606.08457)