Re-evaluating Detection of Equivalent Mutants using LLMs: We Should Properly Measure How Far We Are

Mutation testing is a widely used approach for measuring test-suite quality. A critical problem in mutation testing is equivalent mutant detection (EMD), i.e., determining if a mutant semantically behaves the same as the original code despite some syntactic differences. A recent study has shown that LLM-based EMD techniques hold great promise, reporting substantial improvements over traditional compiler- and machine-learning–based approaches. In this work, we revisit those recent results and evaluate the generalization capabilities of the proposed LLM-based EMD techniques across two additional datasets that differ from the prior dataset in mutation operators, programming languages, or source projects. Contrary to prior findings, the proposed LLM-based EMD techniques suffer substantial performance degradation on the two additional datasets. Through an extensive analysis, we identify a key factor underlying the differences as original-method–level data leakage (i.e., the same original method appearing in both training and testing sets), indicating that prior results under within-method evaluation do not generalize to cross-method evaluation. We find that the studied LLMs tend to rely on a method-wise majority-voting shortcut rather than reasoning about the semantic effects of mutations. Based on these findings, we call for the adoption of realistic cross-method evaluation and the development of mutation-centric semantic reasoning in future LLM-based EMD research.