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Large Language Models specialized for the medical domain achieve high performance on static benchmarks, but remain vulnerable to sycophantic confabulation, where the models generate medically spurious rationales to justify incorrect user hints. This robustness gap poses severe risks in clinical environments, as models may prioritize contextual faithfulness to a biased prompt over their internal parametric medical knowledge. This study introduces a mechanistic approach to identify and mitigate these failures in MedGemma-27B, isolating hint integration circuits using Sparse Autoencoders and geometric manifold analysis. Our findings reveal that sycophantic bias is a highly distributed and polymorphic concept, with biased reasoning routed through shifting dimensions across transformer layers. We identify a specific causal bottleneck for bias formation and demonstrate that cluster-conditioned dynamic steering tailored to the geometric subspace of the prompt outperforms static global interventions, though it reveals a fundamental tension between bias resilience and the retention of internal parametric knowledge.This work proposes a principled framework toward clinical AI systems that are more robust and aligned with expert medical logic, demonstrating the potential of cluster-conditioned geometric interventions while characterizing the inherent trade-offs in clinical knowledge retention.

ACL 2026

Probing Bias Formation in Medical LLMs through Activation Steering

### Welcome to 2025 Annual Conference of the Nations of the Americas Chapter of the Association for Computational Linguistics

Welcome to the 2025 meeting of the Nations of the Americas Chapter of the Association for Computational Linguistics! I am proud to help organize the first NAACL conference to carry the new name of our organization, one that emphasizes inclusion for all of the Americas. I am also pleased to welcome you to Albuquerque, New Mexico, a state whose unique blend of cultural influences will make for an excellent backdrop for NAACL 2025, especially with this year’s special theme on NLP in a Multicultural World. 
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NAACL 2025

Welcome to 2025 Annual Conference of the Nations of the Americas Chapter of the Association for Computational Linguistics

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It is my great pleasure and honor to welcome you to the 64th Annual Meeting of the Association for Computational Linguistics (ACL 2026), held in San Diego, California, USA, July 2–7, 2026. ACL 2026 continues our fields tradition of excellence in scholarship, innovation, and inclusivity, and I am deeply grateful to the many volunteers who have worked tirelessly to bring this event to life.

It is exciting to see the many developments in the field reflected at our conference, such as the advancement of large language models (born out of our community) into many application directions. This is not without challenges, partly due to unsolved problems, and partly due to misuse of the technology. All this is reflected well at ACL 2026. The conference also reflects that the field is not only about advancing technology, but also the science of fundamentally understanding language and how it is use by many different speaker communities. 
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Sycophancy, the response pattern of language models to flatter and agree with users statements rather than correcting them, is widely studied but benchmarked inconsistently. To enable better interpretations of the contested concept of model sycophancy, we evaluate eight models on eight popular sycophancy benchmarks across five random seeds (32,000 evaluations). Cross-benchmark correlations average near zero, benchmarks fail to agree on which models are most sycophantic, and combining them yields negative internal consistency. Only 2 of 28 model-level benchmark pairs correlate significantly, one of which is negative. Yet, individual benchmarks show high test-retest reliability, confirming they measure their specific operational definition consistently, just not the same construct. We recommend multi-benchmark evaluation designs spanning at least three benchmark families before global interpretations about model sycophancy.

Identifying the Convergent Sycophancy Gap in Model Evaluations

Patient health literacy is a critical determinant of health outcomes, yet medical discharge summaries remain inaccessible to many patients due to jargon and complex language. Large language models (LLMs) offer a promising means of bridging this gap, but their deployment in resource-constrained hospital environments demands lightweight, privacy-preserving solutions. We evaluate a range of open- and closed-source LLMs on the MeDiSumQA dataset, comprising real patient discharge summaries paired with lay questions and clinician-verified answers, and demonstrate that open-source models achieve performance comparable to GPT-5. We then introduce LAMP-MedQA, a lightweight multi-agent framework for patient-oriented medical question answering. LAMP-MedQA decomposes the task into an information extraction stage and a text simplification stage, each governed by an automated metric-driven feedback loop that enables iterative self-correction without human-in-the-loop supervision. Implemented using two small open-source models (Qwen2.5-7B-Instruct and Phi-3.5-mini-instruct), the system achieves significantly improved readability (FKGL) and simplification quality (SARI) compared to zero-shot GPT-5, whilst maintaining competitive semantic similarity and clinical accuracy, suggesting that collaborative lightweight agents represent a viable approach to improving health literacy in clinical settings.

LAMP-MedQA: A Lightweight Multi-Agent System for Patient-Oriented Medical Question Answering

Scientific NLP systems often require outputs that satisfy strict, machine-checkable constraints. In this work, we study structured-generation controllability along three axes: structural control, iterative correction, and decoding dynamics. Diffusion decoding is of particular interest because its iterative refinement may improve global structure and revision behavior, but may also introduce distinct failure modes such as termination instability and repetition. To quantify controllability, we evaluate compliance with five machine-checkable constraints: (i) required headings and (ii) correct ordering, which reflect global structural control; (iii) explicit end markers and (iv) per-section bullet constraints, which probe local constraint adherence; and (v) repetition avoidance, which captures generation stability under different decoding dynamics. We use these metrics to assess both single-pass generation and changes under iterative correction. Our goal is to isolate structural reliability under parser-facing requirements rather than to directly measure scientific correctness. Across our benchmark, diffusion models tend to better preserve global structure, while iterative improvement substantially improves explicit termination and other local control constraints. Hybrid systems show mixed behavior depending on decoding order. These results suggest that machine-checkable controllability can be usefully decomposed into global structure versus local control, and that the two may benefit from different inference-time strategies.

Inference-Time Feedback for Reasoning Controllability in Diffusion Language Models

Thinking Mode Fusion (TMF) enables large language models to support both concise responses and long-form reasoning by unifying a non-thinking mode and a thinking mode within a single model. However, its training dynamics, including the \emph{data ratio} and \emph{training schedule} between the two modes, remain underexplored. In this work, we present a systematic study of TMF by analyzing the effects of the training schedule and data ratio between thinking and non-thinking modes. Focusing on mathematical problem solving, we construct a benchmark with multiple thinking-to-non-thinking data ratios and three training schedules. Our results reveal an asymmetric interaction between the two modes: increasing the ratio of non-thinking supervision reduces the accuracy of the thinking mode. We further show that different training schedules modulate this trade-off and that the optimal schedule depends on the data ratio. Finally, we quantify a negative correlation between non-thinking and thinking mode supervision, highlighting an inherent tension between these two modes. These findings provide practical guidance for designing effective TMF training settings. All code and data are released to support further research at: \href{https://anonymous.4open.science/r/Fusion-Bench-2D62}{\textbf{Fusion Bench}}.

Fusion Training for Mathematical Generalization in Large Language Models

We ask whether topic sentiment has a causal effect on perceived political ideology, and whether the answer depends on who assigns the ideology label. Using the AllSidesV2 test dataset (N=11,516 articles), we hold sentiment annotations constant and compare ideology labels from expert human annotators, GPT-4o-mini (baseline and finetuned), and Llama-3.3-70B. We apply Double Machine Learning (DML) and community-level mediation analysis across all four annotation paradigms. Human annotations yield no significant causal effects at the community level. Fine-tuned GPT-4o-mini achieves the highest classification accuracy (F1=72.48) and is the only annotator paradigm that produces significant community-level treatment effects and significant natural direct effects (NDEs) in mediation. We interpret this as evidence of shortcut learning: fine-tuning on ideology-labeled data causes the model to internalise a spurious sentiment--ideology coupling not operative in human judgment. This coupling is structurally invisible to F1-based evaluation, with implications for the use of LLM annotations as silver labels and as proxies for human judgment in downstream causal analyses.

Does Topic Sentiment Cause Perceived Ideology? Comparing Human and LLM Annotations in Political News Articles

In conversational implicatures, speakers convey hidden intended meanings beyond the literal content of their utterances, and hearers are expected to infer what is implied. This study examines how Large Language Models (LLMs) interpret conversational implicatures, using human interpretation as a baseline and gold standard for comparison. The same experiments were conducted with two types of participants: humans and LLMs. Two metrics were adopted: a surprisal-based metric and a response-based metric. The results suggest that the response-based metric demonstrates higher accuracy, comparable to human responses, than the surprisal-based metric. In particular, humans and LLMs using the response-based metric performed better in the literal condition than in the implied condition. Additionally, they were more sensitive to capturing implied meanings for some-all trigger than for the other triggers. Overall, LLMs employing the response-based metric tend to exhibit human-like behavior, but still diverge from humans in their understanding of conversational implicatures.

Understanding Conversational Implicatures in Humans and LLMs

Masked language modeling for low-resource ancient languages remains challenging because pre-trained multilingual models lack exposure to these languages. We investigate rule-based linguistic constraints and hard negative mining for Sumerian, a language isolate absent from multilingual BERT's training data. We build a hierarchical validator capturing subword, word, and part-of-speech patterns from 4,545 annotated sequences, using it to filter candidates and identify hard negatives for fine-tuning. Vanilla mBERT achieves 18.0\% hit@10 accuracy. The validator alone improves this to 72.8\%, while hard negative fine-tuning reaches 78.3\%. Combining both yields 86.7\%, a 68.7 percentage point improvement. Temporal generalization evaluation on tablets from 600 years earlier shows that both the hard negative mining and the validator alone improve performance, but the combined approach underperforms due to the validator's period specific rules. These findings demonstrate that hard negative mining transfers across periods while explicit rule-based constraints provide strong in-domain improvements but limited cross-period generalization.

Validator-Guided Hard Negative Mining for Masked Language Modeling in Low-Resource Ancient Languages

LLM routing directs queries to a cheaper model when it suffices and to an expensive model otherwise, reducing inference cost. Existing input-based routers optimize cost-performance trade-offs but provide no formal bound on how often the cheaper model fails among routed queries. We adapt a proactive conformal gate framework to LLM routing. A logistic regression gate trained on text embeddings predicts per-query safety, and Clopper-Pearson conformal calibration selects a routing threshold that guarantees the violation rate among routed queries stays below $\alpha$ (the violation tolerance) with probability at least $1 - \delta$ (the confidence level). On two benchmarks covering math reasoning (GSM8K) and multi-domain knowledge (MMLU), routing between Mixtral-8x7B and GPT-4 (a 24.5$\times$ cost difference), our method maintains the target $\alpha$ within the $\delta$ tolerance across a sweep from 0.05 to 0.50, while a validation-tuned baseline crosses the violation boundary on GSM8K. A feasibility analysis across all 10 RouterBench models reveals that routability is jointly model- and task-dependent. To our knowledge, this is the first input-based LLM router with distribution-free safety guarantees.

Conformal LLM Routing with Distribution-Free Safety Guarantees

Turkish idiomatic light verb constructions (LVCs) are challenging for multiword expression processing because they often share the same surface form as fully literal verb–object combinations while functioning as a single, partially idiomatic predicate. We frame Turkish LVC detection as a binary classification task (literal ditransitive vs. LVC) and evaluate on a manually created controlled set (N=147) with matched negatives: out-of-domain random sentences and in-domain literal controls (NLVC), alongside LVC positives. We compare a supervised Turkish encoder baseline (BERTurk with a classifier head) to three instruction-tuned LLMs from different families under zero-shot, one-shot, and few-shot prompting, and analyze how demonstrations shift error profiles. In zero-shot, LLMs perform well on negatives but show very low LVC recall. One-shot prompting sharply improves LVC detection but can induce strong, model-specific biases (over- vs. under-predicting LVC). A richer master few-shot prompt improves calibration and yields robust overall performance for GPT-OSS-20B and Qwen 2.5. Overall, the results highlight substantial prompt sensitivity in Turkish metalinguistic classification: the supervised baseline remains competitive, while prompted LLMs can match or exceed it on LVCs with carefully constructed demonstrations. We will release code, prompts, and evaluation materials to support reproducibility.

Supervision versus Demonstration-Based In-Context Learning for Multiword Expression Classification

Large language models (LLMs) are increasingly deployed in high-stakes settings, yet reliably estimating when their outputs should be trusted remains an open challenge. Existing uncertainty estimation approaches---such as calibration, token-level probabilities, or semantic entropy---typically require access to model internals, additional supervision, or computationally intensive pipelines. We propose answer instability, defined as the variability of a model's final answer across repeated stochastic generations of the same prompt, as a simple, label-free, and black-box uncertainty signal. Evaluated across three task types — reasoning, multiple-choice QA, and constraint-following — using four LLMs and 520 prompt-model pairs, our approach achieves performance competitive with semantic entropy while requiring no semantic similarity model. Our results show that instability strongly correlates with prediction errors and reliably discriminates correct from incorrect outputs. We further demonstrate its utility for selective prediction and targeted repair, improving reliability without access to internal probabilities or additional training.

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Identifying the Convergent Sycophancy Gap in Model Evaluations

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How Can We Diagnose and Treat Bias in Large Language Models for Clinical Decision-Making?

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Identifying the Convergent Sycophancy Gap in Model Evaluations

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How Can We Diagnose and Treat Bias in Large Language Models for Clinical Decision-Making?

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