Large language models (LLMs) show growing promise for scientific discovery, yet existing benchmarks rely on static, single-turn QA formats that cannot measure the iterative reasoning and strategic planning required in real research. We introduce MolQuest, an agent-based benchmark that reformulates molecular structure elucidation as a multi-turn interactive task grounded in authentic experimental data. Within a simulated laboratory, the model must actively plan experiments, acquire heterogeneous spectral evidence (NMR, MS, IR) on demand, and iteratively refine structural hypotheses while minimizing unnecessary tool calls—mirroring the abductive reasoning workflow of a practicing chemist. We evaluate 16 frontier LLM variants under two settings, yielding 32 model–setting configurations, and find that the best Agent-mode result reaches only 51.51% accuracy, while most remain substantially lower. Furthermore, the dynamic paradigm amplifies performance for some models yet degrades others, revealing strategic planning deficiencies invisible to static evaluations. MolQuest provides a reproducible, extensible framework that exposes the critical gap between pattern recognition and genuine scientific reasoning in current LLMs.