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Structured JSON

What is structured JSON?

Structured JSON refers to JSON (JavaScript Object Notation) output from an LLM that follows a specific, predefined schema or format. When LLMs are configured to return structured JSON, their responses conform to expected fields, data types, and structures—making the output programmatically parseable and reliable for downstream systems.

JSON is a data format that balances machine-readability with human-readability. It's easy to work with programmatically while still being understandable when you look at it.

Why use structured JSON with LLMs?

Structured JSON enables reliable integration of LLM outputs into programmatic workflows. Instead of returning free-form text that needs to be parsed and interpreted, LLMs can return data in a predictable format that code can immediately work with.

For example, an LLM analyzing a customer interview might return structured JSON like:

{
  "questions_asked": 12,
  "follow_up_questions": 5,
  "timeline_built": true,
  "leading_questions": ["Can you tell me more about that?"]
}

This structured format allows downstream code to access specific fields (questions_asked, timeline_built) without trying to extract values from unstructured text.

How do teams validate structured JSON?

Code assertions are commonly used to validate that LLM responses match the expected JSON structure. Teams write code that checks whether the JSON contains the required fields, whether data types are correct, and whether values fall within expected ranges.

This validation is particularly important for evals—ensuring that the LLM consistently returns data in the correct format is often the first check before evaluating the quality of the content itself.

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