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Calibrated Enough to Know, Not Calibrated to Act: Fabricated Evidence Makes LLM Agents Commit to the Unknowable

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TL;DR - A study of 12 frontier models finds that authoritative-looking evidence—even when entirely fabricated—can make LLM agents act on provably unpredictable questions without materially changing their stated beliefs. The failure lies in a trainable but context-fragile decision gate between recognizing uncertainty and refusing to act.

  • Escalating evidence displays increased commitment from 6.5% to 54.0%; fabricated panels induced commitment at a rate statistically indistinguishable from genuine market data.
  • Models correctly classified questions as irreducibly unknowable 90% of the time and committed on only 0.4% of those cases when explicitly asked to assess knowability first.
  • Fine-tuning a 3B model on 540 synthetic examples reduced commitment to 0.0% on the original cases and transferred to three unseen domains.
  • The improvement depended on response formats allowing reasoning; rigid formats undermined the abstention gate and could leave models confidently wrong.

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Calibrated Enough to Know, Not Calibrated to Act: Fabricated Evidence Makes LLM Agents Commit to the Unknowable

arXiv cs.AI Pranav Aggarwal 2026-08-27 arXiv:2608.27167
Public signals Semantic Scholar citations 0 · Semantic Scholar influential citations 0
Providers: Hugging Face · N/A OpenAlex · N/A Publisher · N/A Semantic Scholar · Citations 0 · Influential citations 0 X · N/A Fetched 2026-09-24 14:32:41.708833 UTC

TL;DR - A study of 12 frontier models finds that authoritative-looking evidence—even when entirely fabricated—can make LLM agents act on provably unpredictable questions without materially changing their stated beliefs. The failure lies in a trainable but context-fragile decision gate between recognizing uncertainty and refusing to act.

  • Escalating evidence displays increased commitment from 6.5% to 54.0%; fabricated panels induced commitment at a rate statistically indistinguishable from genuine market data.
  • Models correctly classified questions as irreducibly unknowable 90% of the time and committed on only 0.4% of those cases when explicitly asked to assess knowability first.
  • Fine-tuning a 3B model on 540 synthetic examples reduced commitment to 0.0% on the original cases and transferred to three unseen domains.
  • The improvement depended on response formats allowing reasoning; rigid formats undermined the abstention gate and could leave models confidently wrong.
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