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Improving Verbatim Financial Causality Extraction with Supervised Fine-Tuning and Prompt Repetition

This paper investigates the application of generative Large Language Models (LLMs) for strict verbatim span extraction. We evaluate our methodology within the FinCausal 2026 shared task. Because generative LLMs optimize next-token probability rather than strict boundaries, they naturally suffer from over-generation and boundary drift in extraction tasks. To address this, we introduce a generalized structural training constraint, extending prompt repetition from a purely inference-time heuristic to a training-time supervision framework. By incorporating duplicated prompts directly into Supervised Fine-Tuning (SFT), we hypothesize that this encourages the model to internalize a form of unidirectional cross-reading behavior, leading to stronger alignment between generated spans and the source context for exact extraction. Evaluating on open-weights (Qwen2.5-14B-Instruct-1M) and proprietary (GPT-4.1-Nano) architectures, we find this soft attention constraint improves Exact Match scores for open models and helps balance cross-lingual performance disparities. Conversely, the proprietary model exhibited sensitivity to prompt duplication, achieving its highest score without repetition. Ultimately, our deterministic SFT approach secured 4th place in the Spanish subtask (4.73) and 6th place in the English subtask (4.70), indicating the viability of structurally simple, natively fine-tuned models compared to complex multi-stage pipelines.