Why does constrained decoding break on token boundaries, and how do you handle the edge cases?
Grammar-constrained decoding seems like a solved problem until tokenization stops lining up with the grammar. The signal is the token-vs-character boundary mismatch and how prefix automata and token healing address it.
Updated Sep 2026 · Grounded in real GenAI, LLM, and AI/ML engineering interview loops and written to a senior-engineer editorial bar.
Grammar-constrained decoding seems like a solved problem until tokenization stops lining up with the grammar. The signal is the token-vs-character boundary mismatch and how prefix automata and token healing address it.
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.
