This paper systematically evaluates LLM reliability on the complex semantic task of Natural Language Inference (NLI) in Farsi, assessing six prominent models across eight prompt variations through a multi-dimensional framework that measures accuracy, prompt sensitivity, and intra-class consistency. Our results demonstrate that prompt design-particularly the order of premise and hypothesis-significantly impacts prediction stability. Proprietary models (Claude-Opus-4, GPT-4o) exhibit superior stability and accuracy compared to open-weight alternatives. Across all models, the ’Neutral’ class emerges as the most challenging and least stable category. Crucially, we redefine model instability as a diagnostic tool for benchmark quality, demonstrating that observed disagreement often reflects valid challenges to ambiguous or erroneous gold-standard labels.