Don't ask the model for valid JSON — delete the wrong tokens, force every illegal logit to , and it's valid by construction
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You ask a model for JSON and it mostly complies — then prepends "Sure!", wraps the answer in a `` json fence, quotes a number, or adds a trailing comment, and JSON.parse()` throws. Prompting ("return ONLY JSON") lowers the failure rate but never zeroes it, because sampling is probabilistic: there's always some probability mass on the wrong token. Constrained decoding kills that at the source — not by asking nicely, but by making the wrong tokens impossible to sample. I built the whole engine…
1Key Takeaways
- You ask a model for JSON and it mostly complies — then prepends "Sure!", wraps the answer in a `` json fence, quotes a number, or adds a trailing comment, and JSON.parse()` throws.
- Prompting ("return ONLY JSON") lowers the failure rate but never zeroes it, because sampling is probabilistic: there's always some probability mass on the wrong token.
- Constrained decoding kills that at the source — not by asking nicely, but by making the wrong tokens impossible to sample.
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3Why it matters
Coding AI shifts how fast software ships and how much human review each change needs. DEV — ML reports that you ask a model for JSON and it mostly complies — then prepends "Sure!", wraps the answer in a `` json fence, quotes a number, or adds a trailing comment, and JSON.parse()` throws.
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