SIGNALAI·May 29, 2026, 4:00 AMSignal85Short term

Leak@$k$: Unlearning Does Not Make LLMs Forget Under Probabilistic Decoding

Source: arXiv cs.LG

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Leak@$k$: Unlearning Does Not Make LLMs Forget Under Probabilistic Decoding

arXiv:2511.04934v3 Announce Type: replace Abstract: Unlearning in large language models (LLMs) is critical for regulatory compliance and for building ethical generative AI systems that avoid producing private, toxic, illegal, or copyrighted content. Despite rapid progress, in this work, we show that \textit{almost all} existing unlearning methods fail to achieve true forgetting in practice. Specifically, while evaluations of these `unlearned' models under deterministic (greedy) decoding often suggest successful knowledge removal using standard benchmarks, we show that sensitive information rel

Why this matters
Why now

The paper is published amidst increasing scrutiny on AI ethics, regulatory compliance, and the practical effectiveness of unlearning mechanisms in LLMs.

Why it’s important

This finding reveals a fundamental limitation of current LLM unlearning methods, challenging the efficacy of compliance strategies and safety measures reliant on them.

What changes

The assumption that unlearning methods effectively remove sensitive information from LLMs is now heavily questioned, especially under probabilistic decoding scenarios.

Winners
  • · AI safety researchers
  • · Adversarial AI developers
  • · Regulatory bodies (in clarifying requirements)
Losers
  • · LLM developers (relying on current unlearning methods)
  • · Companies using LLMs with sensitive data
  • · Ethical AI frameworks (if not updated)
Second-order effects
Direct

Current LLM unlearning methods are demonstrated to be insufficient for true forgetting, particularly with probabilistic decoding.

Second

This will necessitate a re-evaluation and significant innovation in LLM unlearning techniques to meet regulatory and ethical demands.

Third

Increased legal and reputational risks for companies deploying LLMs, potentially leading to new compliance standards for 'true' unlearning or restrictions on LLM deployment in sensitive contexts.

Editorial confidence: 95 / 100 · Structural impact: 70 / 100
Original report

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Read at arXiv cs.LG
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