LatentRefusal: Latent-Signal Refusal for Unanswerable Text-to-SQL Queries

TL;DR

LatentRefusal introduces a novel latent-signal refusal mechanism for text-to-SQL systems, effectively identifying unanswerable queries by analyzing intermediate model activations, thereby enhancing safety with minimal overhead.

Contribution

It formalizes answerability gating and proposes a lightweight probing architecture to detect unanswerable queries from hidden model states, improving safety in text-to-SQL systems.

Findings

Achieves an average F1 of 88.5% across four benchmarks.

Adds only about 2 milliseconds of probe overhead.

Effectively detects unanswerable queries with high accuracy.

Abstract

In LLM-based text-to-SQL systems, unanswerable and underspecified user queries may generate not only incorrect text but also executable programs that yield misleading results or violate safety constraints, posing a major barrier to safe deployment. Existing refusal strategies for such queries either rely on output-level instruction following, which is brittle due to model hallucinations, or estimate output uncertainty, which adds complexity and overhead. To address this challenge, we formalize safe refusal in text-to-SQL systems as an answerability-gating problem and propose LatentRefusal, a latent-signal refusal mechanism that predicts query answerability from intermediate hidden activations of a large language model. We introduce the Tri-Residual Gated Encoder, a lightweight probing architecture, to suppress schema noise and amplify sparse, localized cues of question-schema mismatch that indicate unanswerability. Extensive empirical evaluations across diverse ambiguous and unanswerable settings, together with ablation studies and interpretability analyses, demonstrate the effectiveness of the proposed approach and show that LatentRefusal provides an attachable and efficient safety layer for text-to-SQL systems. Across four benchmarks, LatentRefusal improves average F1 to 88.5 percent on both backbones while adding approximately 2 milliseconds of probe overhead.