The Schema Tax Is a Schema Design Tax: When Constrained JSON Decoding Costs Accuracy, and When It Helps

Separates schema enforcement, schema visibility and field order in constrained JSON decoding. Across Qwen3-1.7B/4B/8B on GSM8K, AG News and CoNLL-2003, enforcing a shown, reasoning-first schema costs little, while answer-first field order and hidden schemas cause the losses. Code and the 12,600 model responses: 10.5281/zenodo.23755108 and github.com/alleyxd/SchemaTax.