What is the serving overhead of structured (JSON/grammar-constrained) output, and how do you cut it?
Forcing valid JSON sounds free, but the mask computation can stall every decode step. The fix is precompiling the grammar into a fast automaton. Here is where the cost hides and how to strip it out.
Updated Sep 2026 · Grounded in real GenAI, LLM, and AI/ML engineering interview loops and written to a senior-engineer editorial bar.
Forcing valid JSON sounds free, but the mask computation can stall every decode step. The fix is precompiling the grammar into a fast automaton. Here is where the cost hides and how to strip it out.
Lead with where the obvious approach breaks, because that is the judgment they are screening for — most candidates jump straight to the happy path and lose the room.
Then walk the failure back through the pipeline in order, naming the one metric the customer's exec sponsor actually cares about before you propose the fix.
