Structured Outputs vs function calling in the OpenAI API

Chris Rodriguez — Tue, 29 Sep 2026 03:53:59 +0000

Structured Outputs and function calling are not interchangeable, even though both can involve schema-like structures.

Structured Outputs are the better fit when the model should answer the user in a predictable schema. That keeps the output contract close to the UI and avoids inventing a tool layer that does no real work.

Function calling is the better fit when the model should ask your app to perform an action or fetch data. In that case, the model is proposing a call and the application still owns execution, authorization, and the next turn.

The article shows a small request-shape helper you can run locally, then uses that fixture to prove the difference without needing a live API call. It also calls out the failure cases: using Structured Outputs for a tool-first workflow hides the app boundary, while using function calling for a plain UI payload adds unnecessary orchestration.

The decision rule is simple: if the model owns the final user-facing shape, use Structured Outputs. If your application owns the next action, use function calling.

Read the full article: https://chriseugenerodriguez.com/blog/when-should-you-use-structured-outputs-instead-of-function-calling-in-the-openai-api

DEV Community: Chris Rodriguez

Structured Outputs vs function calling in the OpenAI API