Constrained decoding engines: grammars, token masks and jump-forward
How engines force valid JSON, regex and grammar-shaped output: logit masking, compiling a schema into a state machine, per-token overhead, jump-forward decoding, batching effects, and when constraints hurt quality or speed.
15 min·Verified 2026-111 animated diagramPro+50 XP
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Applications want output that parses: JSON that matches a schema, a regex-shaped identifier, a function call with the right argument types. Prompting alone gets most of the way and then fails at scale. Constrained decoding makes failure impossible by changing how tokens are sampled, and engines now compete on doing it with almost no overhead.
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