What to Forget in Unlearning? Forget Set Curation for Language Models

• URL: http://arxiv.org/abs/2608.14855v1
• Date: Fri, 14 Aug 2026 19:51:53 GMT
• Title: What to Forget in Unlearning? Forget Set Curation for Language Models
• Abstract summary: We show how a requester may ask a model to stop reproducing a song or book without knowing which spans, documents, quotations, or near-duplicates support that behavior.<n>We introduce CleanSlate, a benchmark for verbatim output suppression over songs and books, with model-specific extraction profiles, content-grounded QA, and capability-retention evaluations.
• Score: 24.23267387545857
• License: http://creativecommons.org/licenses/by/4.0/
• Abstract: Machine unlearning aims to remove targeted data or behaviors from a trained model without retraining from scratch. Yet most evaluations assume that the examples to forget are already known. In realistic language-model deployments, a requester may ask a model to stop reproducing a song or book without knowing which spans, documents, quotations, or near-duplicates in a trillion-token corpus support that behavior. We study this missing upstream problem, forget set curation: mapping a suppression request to the data passed to an unlearning algorithm. We introduce CleanSlate, a benchmark for verbatim output suppression over songs and books, with model-specific extraction profiles, content-grounded QA, and capability-retention evaluations. CleanSlate exposes two failure modes. Natural lexical and exact-substring curators often yield forget sets that lead to weak suppression. An evaluation-aware curator suppresses requested continuations almost completely, but causes collateral regression on non-requested content and model-dependent capability loss. These results show that practical unlearning is not only an optimization problem once a forget set is given: the data chosen for forgetting determines both what can be unlearnt and what else is damaged.

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