Structured Output

Why Structured Output Is Needed

A model's default output is free-form text — humans can understand it, but programs struggle to reliably continue processing.

What you need is:

• Writing results to a database
• Direct rendering by frontend components
• Triggering the next step in an automation flow
• Checking fields one by one

These all require programs to "understand" the model's output. Structured Output transforms the model's output from "a paragraph of text" into "structured data."

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What Is Structured Output

One-line definition: Structured Output is the ability to have models return results in a predefined structure, such as JSON, object fields, or fixed enumerated values, rather than free-form text.

Two common implementations:

Tool Calling / Function Calling (recommended): let the model call a "fake tool," where the tool's parameters themselves are structured data.

JSON Mode: directly request the model to answer in JSON format.

This relies on Function Schema to define the output structure.

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How to Do It: When to Use

Scenarios suited for Structured Output:

• Data extraction: extracting structured data from unstructured text (resume parsing, invoice recognition)
• API responses: Agent's output needs to be further processed by programs
• Multi-field responses: answering multiple fields at once (title, description, tags)
• Classification and routing: judging intent category, priority

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Common Pitfalls

• JSON Mode doesn't guarantee format correctness: models may output incomplete JSON, requiring validation and retry
• Tool Calling is more reliable: because tool parameters have Schema constraints, format is usually more stable
• More complex structures lead to more errors: error rates increase with deeper nesting levels
• Correct format doesn't mean correct semantics: field names may all be right, but field values could still be fabricated

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Remember this: Structured Output transforms the model's output from "a paragraph of text" into "data" — programs can read it to continue processing.

Related terms: Function Schema · Tool Calling