JSON Mode Makes Your LLM Dumber: The Constrained Decoding Trap
This story is from 2026-09-02. It is preserved in the archive; the latest stories are on the live feed.
I shipped a classifier that was more reliable and more wrong at the same time. Same model. Same prompt. Same temperature. The only diff in the commit was a response_format block. Parse errors went to zero, my retry loop went quiet, and my accuracy quietly fell off a cliff. That was the day I learne…
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- 2026-09-02 16:12 · DEV Community — AI
JSON Mode Makes Your LLM Dumber: The Constrained Decoding Trap