Structured output from LLMs: JSON mode, function calling, and grammar-constrained decoding

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A practical comparison of three approaches to getting structured data from LLMs: prompt-only JSON, API-level JSON mode and function calling, and grammar-constrained decoding with Outlines, Guidance, and vLLM.

Published 14 Jun 2026

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A practical comparison of three approaches to getting structured data from LLMs: prompt-only JSON, API-level JSON mode and function calling, and grammar-constrained decoding with Outlines, Guidance, and vLLM.
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