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arXiv · 2610.01184

ReCast: Contract-Preserving Protection for Fixed-Interface Multimodal Reasoning

Bingchen Pei·Lichong Chen·Bingxi Zhao·Ziang Wu·Sirui Wang·Min Zhang·Yanhao Chen·Qingxu Liu·Qiang Gao·Chang-Tien Lu·Bo Gao
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Abstract

Remote multimodal models offer strong numerical reasoning capabilities over charts and speech, but sending private inputs risks exposing sensitive content. Text-only sanitization cannot directly satisfy fixed media interfaces, while identity anonymization leaves the underlying task content exposed. We introduce ReCast, an agentic plug-in framework that replaces source-specific content while preserving task-relevant relations and the required input modality. ReCast locally converts inputs into a shared textual evidence-query record, jointly rewrites entities and topics with a distilled 4B model, and substitutes values through a locally invertible, role-aware numerical map. A reconstruction agent generates and validates the required media from the protected record. The remote solver returns a program whose protected operands are restored locally before execution. On 4,000 held-out ChartQA and NMSQA examples, ReCast achieves 75.10% accuracy, retaining 92.43% of unprotected remote accuracy, while a model-based audit flags source-content leakage in 7.95% of solver-bound requests. It outperforms all evaluated local baselines, preserving the benefit of remote reasoning while reducing source-content exposure under existing media interfaces.

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Bingchen Pei, Lichong Chen, Bingxi Zhao, Ziang Wu, Sirui Wang, Min Zhang, Yanhao Chen, Qingxu Liu, Qiang Gao, Chang-Tien Lu, Bo Gao. 2026-10-01. ReCast: Contract-Preserving Protection for Fixed-Interface Multimodal Reasoning. https://arxiv.org/abs/2610.01184

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