Beyond Surface Similarity: Evaluating LLM-Based Test Refactorings with Structural and Semantic Awareness

Abstract: LLMs are increasingly employed to automatically refactor unit tests, aiming to enhance readability, naming, and structural clarity while preserving functional behavior. However, evaluating such refactorings remains challenging: traditional metrics like CodeBLEU are overly sensitive to renaming and structural edits, whereas embedding-based similarities capture semantics but ignore readability and modularity. We introduce CTSES, a composite metric that integrates CodeBLEU, METEOR, and ROUGE-L to balance behavior preservation, lexical quality, and structural alignment. CTSES is evaluated on over 5,000 test suites automatically refactored by GPT-4o and Mistral-Large-2407, using Chain-of-Thought prompting, across two established Java benchmarks: Defects4J and SF110. Our results show that CTSES yields more faithful and interpretable assessments, better aligned with developer expectations and human intuition than existing metrics.