Less Is More for Multi-Step Logical Reasoning of LLM Generalisation Under Rule Removal, Paraphrasing, and Compression

Abstract: LLMs excel across many natural language tasks, yet their generalisation to structural perturbations in logical contexts remains poorly understood. We introduce a controlled evaluation framework that probes reasoning reliability through four targeted stress tests: (1) rule deletion, removing either redundant or essential rules from a multi-step inference chain; (2) contradictory evidence injection; (3) logic-preserving rewrites generated through several families of equivalence laws (contrapositive, double negation, implication, De Morgan, identity, and commutativity); and (4) multi-law equivalence stacking that introduces 2-5 simultaneous logical transformations. Across three representative model families: BERT, Qwen2, and LLaMA-like models. Our experiments reveal a strikingly consistent pattern: all models achieve perfect accuracy on the base tasks and remain fully generalise to redundant rule deletion and all equivalence-based rewrites (single or multi-law), but fail sharply under essential rule deletion (dropping to 25% accuracy) and collapse completely in the presence of explicit contradictions (0% accuracy). These results demonstrate that LLMs possess stable invariance to semantic-preserving logical transformations, yet remain fundamentally brittle to missing or conflicting evidence. Our framework provides a clean diagnostic tool for isolating such reasoning failure modes and highlights persistent gaps in the logical generalisation abilities of current LLMs.