Logical fallacy detection models frequentlyover-flag valid reasoning due to reliance onsurface-level spurious correlations. We in-troduce 703 LLM-generated CounterfactuallyAugmented Data (CAD) pairs—minimally dif-ferentiated valid and fallacious arguments—todebias models through targeted augmentation.Fine-tuning DeBERTa-v3-large on CoCoLoFaaugmented with these pairs yields marginalin-distribution improvement (+0.4% F1) butsubstantial out-of-distribution robustness: 58%relative reduction in false positive rate (64%→ 26.7%) on a 300-sample Reddit-sourcedevaluation set. While recent LLMs (Llama-3.1-8B, Llama-3.3-70B) achieve high perfor-mance under optimized prompts (F1 90–94%),they degrade severely under simple human-like prompts (F1 63–72%, FPR 54–74%).Our lightweight, prompt-invariant approachachieves competitive robustness (F1 85.9%,FPR 26.7%) across all prompting regimes with-out prompt engineering, making it stable forproduction deployment with unpredictable userinput. The dataset and model are publicly re-leased.