MORTAR: Multi-Turn Metamorphic Testing for LLM-Based Dialogue Systems

With the widespread application of LLM-based dialogue systems in daily life, quality assurance has become more important than ever. Recent research has successfully introduced methods to identify unexpected behaviour in single-turn testing scenarios. However, multi-turn interaction is the common realworld usage of dialogue systems, yet testing methods for such interactions remain underexplored. This is largely due to the oracle problem in multi-turn testing, which continues to pose a significant challenge for dialogue system developers and researchers. In this paper, we propose MORTAR, a metamorphic multi-turn dialogue testing approach, which mitigates the test oracle problem in testing LLM-based dialogue systems. MORTAR formalises multiturn testing for dialogue systems, and automates the generation of question-answer dialogue test cases with multiple dialogue-level perturbations and metamorphic relations (MRs). The automated MR matching mechanism provides MORTAR more flexibility and efficiency in testing. When applied to eight LLM-based dialogue systems as test objects, MORTAR reveals 51% more bugs per test case than the best-performing single-turn metamorphic testing baseline, with 24.6% higher precision and over 42% of detected bugs being unique.