How does constrained / structured decoding force an LLM to emit valid JSON or grammar?

Prompting for JSON works until it doesn't, and at scale the tail breaks your parser. Constrained decoding makes malformed output literally unsamplable. The signal is knowing exactly where in the loop the constraint bites. Here is the answer.

Updated Aug 2026 · Grounded in real Applied AI Engineer interview loops and written to a senior-engineer editorial bar.