SEPS: A Separability Measure for Robust Unlearning in LLMs (2505.14832v2)

Abstract: Machine unlearning aims to selectively remove targeted knowledge from LLMs, ensuring they forget specified content while retaining essential information. Existing unlearning metrics assess whether a model correctly answers retain queries and rejects forget queries, but they fail to capture real-world scenarios where forget queries rarely appear in isolation. In fact, forget and retain queries often coexist within the same prompt, making mixed-query evaluation crucial. We introduce SEPS, an evaluation framework that explicitly measures a model's ability to both forget and retain information within a single prompt. Through extensive experiments across three benchmarks, we identify two key failure modes in existing unlearning methods: (1) untargeted unlearning indiscriminately erases both forget and retain content once a forget query appears, and (2) targeted unlearning overfits to single-query scenarios, leading to catastrophic failures when handling multiple queries. To address these issues, we propose Mixed Prompt (MP) unlearning, a strategy that integrates both forget and retain queries into a unified training objective. Our approach significantly improves unlearning effectiveness, demonstrating robustness even in complex settings with up to eight mixed forget and retain queries in a single prompt.

A Separability Measure for Robust Unlearning in LLMs

The paper "A Separability Measure for Robust Unlearning in LLMs" explores the domain of machine unlearning, focusing on the challenges associated with selective removal of knowledge from LLMs. The primary objective is to enable LLMs to forget designated content while preserving essential information. This is particularly complex in real-world scenarios where prompts often contain mixed queries—both retain and forget requests—simultaneously.

Core Contributions

1. Challenges in Current Unlearning Metrics: Existing metrics are inadequate for evaluating unlearning effectiveness in scenarios where prompts consist of both forget and retain queries. They primarily focus on scenarios where these queries appear in isolation, thereby failing to capture the nuanced interactions seen in practical applications.
2. Key Identified Failure Modes:

The research identifies two significant failure modes in traditional unlearning methods: - Untargeted Unlearning: Tends to erase all associated knowledge in a prompt indiscriminately when a forget query is detected. - Targeted Unlearning: Often overfits to single-query scenarios, resulting in poor performance when multiple queries are involved.

1. Mixed Prompt (MP) Unlearning Approach: The paper introduces the Mixed Prompt (MP) strategy to address these issues. MP unlearning integrates forget and retain queries into a unified training objective. This approach is shown to significantly improve unlearning effectiveness, even when dealing with complex prompts containing up to eight mixed queries.

Experimental Framework and Findings

The proposed evaluation framework measures a model's ability to retain and forget within the same prompt. Through extensive experiments across three benchmarks, the MP methodology demonstrates robustness, highlighting its superior performance over existing techniques in real-world-like scenarios.

Results indicate that MP approaches maintain strong separability—distinguishing between forget and retain content—while still optimizing model utility and forget efficacy. Specifically, MP-IDK achieves an unprecedented separability score, reflecting its ability to selectively forget while robustly retaining knowledge across interleaved prompts.

Implications and Future Directions

This research has both practical and theoretical implications. Practically, it offers a pathway to more reliably manage LLM content, aligning with privacy and security requirements by ensuring sensitive information can be effectively forgotten without eroding overall model utility. Theoretically, the paper introduces a novel metric and methodology for evaluating unlearning performance, advancing the discourse in AI and machine learning.

Future research could focus on enhancing unlearning techniques to handle even more complex multi-turn interactions and adversarial scenarios. Additionally, exploring the integration of the MP approach with other unlearning methods might yield beneficial hybrid strategies, further refining the balance between forget efficacy and knowledge retention.

Overall, the paper casts light on the critical need for robust unlearning frameworks within LLMs, underpinning advancements in AI safety and ethical deployment.