Structure Tax: How Structured Output affects LLMs Performance
This study addresses the "structure tax" problem, wherein enforcing structured outputs in large language models degrades accuracy, by conducting a systematic evaluation across multiple models and datasets. Methodologically, it employs Centered Kernel Alignment (CKA) to analyze representation separability in intermediate Transformer layers, alongside confidence calibration and hidden-layer geometric measurements. The findings reveal that accuracy degradation stems from schema design rather than structural constraints per se, proposing a paradigm shift from "whether to structure" to "how to structure." Experiments demonstrate that a reasoning-prioritized field ordering strategy matches or surpasses free-text performance while significantly improving model calibration.