Constrained Decoding: Forcing LLMs to Respect Your Taxonomy

A structural guarantee for LLM classification output Code: The Problem When you use an LLM for classification across many categories, it sometimes returns a label that was never in your input list. The output is directionally close but structurally wrong. A downstream system that expects exact category strings fails. The standard workarounds do not fully solve this. Adding explicit prompt instructions (“respond only with labels from the provided list”) reduces errors but cannot eliminate them.