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14,320 LLM-written unit tests: null, negative and empty arguments are barely tested at all

AspireAI community · arXiv (Xiaoyu Shi, Wenqiang Luo, Boyang Yang, Haoye Tian)
Coverage and mutation score say how much of a program a test suite exercises, but not which behavioral obligations it quietly skips. BlindSpotEval adds obligation-based metrics aligned with the Arrange-Act-Assert pattern - Boundary-Value Adequacy, Control-Flow Adequacy and Oracle Strength - and the ObligBench benchmark of 895 complex and 700 low-complexity Java functions. Against 14,320 tests from four LLMs under four prompting strategies, the findings are uncomfortable: incorrect assertions account for 15.6% of test failures, so coverage alone does not establish oracle correctness, and among executable tests the coverage of null, negative and empty arguments reaches only 34.8%, 23.8% and 38.8% against above 93% for booleans. Catch blocks and default groups score a Control-Flow Adequacy of just 0.100 and 0.312. The LLM-generated tests reproduce the same happy-path bias seen in human-written tests, and the blind spots persist even in low-complexity functions - so code complexity alone does not explain them.
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