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Use function calling when the model needs to request an application capability, such as retrieving data or taking an action. Use Structured Outputs with a JSON Schema response format when the model is answering the user and your application needs that answer in a predictable shape. They are not mutually exclusive: function arguments can also be constrained with Structured Outputs.
What is the difference?
The key distinction is the job the structured data performs. Function calling connects the model to functions your application provides; a structured response format shapes the assistant’s answer for your application to parse or display.
| Decision point | Function calling | Structured response format |
|---|---|---|
| Use it when | The model should choose or request an application capability, such as accessing external data or performing an action. | The model should answer the user in a defined, machine-readable structure. |
| What the model produces | A tool call containing a function name and arguments. Your application handles the call and may return its result to the model. | A response that follows a supplied, supported JSON Schema when Structured Outputs is enabled. |
| Design question | “Should the model invoke one of the capabilities I provide?” | “Should the answer itself have a predictable structure?” |
| What you define | A function’s parameters; strict mode can constrain arguments to the supported schema. | The desired response object, using supported JSON Schema features. |
| What your application must handle | Validate arguments, execute the function safely, and provide tool results as needed. | Check for refusals or incomplete responses before consuming the output. |
OpenAI describes function calling as a way for models to interface with external systems and access data beyond their training data. Structured Outputs instead addresses the form of the assistant’s response. See the function calling guide and Structured Outputs guide.
When should you choose function calling?
Choose a function tool when a useful answer depends on something your application can do or know at runtime. Examples include looking up an order in your database, checking an account balance, or asking your system to schedule an appointment. The model requests the function; your application—not the model—decides whether and how to execute it.
Plan for the tool cycle
- Define the function and its parameters for the capability you want to expose.
- Send the request and tool definition to the API.
- Inspect the model’s response. If it requests a function, validate the arguments before running application code.
- Execute the function and, when appropriate, return its result to the model so it can respond to the user.
Function arguments are not a security boundary. The Chat API reference warns that arguments may be invalid JSON or include parameters not declared in the schema. Parse and validate input against your application’s own requirements before executing anything.
Choose how much control the model has
Tool choice determines whether the model can skip tools, must call a tool, or is directed to a particular tool. With auto, the model can decide whether and which available tool to call; required or forced choices narrow that behavior. Exact options and request shapes vary by API surface, so use the reference for the endpoint you are implementing.
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When should you choose a structured response format?
Choose Structured Outputs when the model’s task is to produce an answer, but your application needs that answer in a known structure—for example, an object with fields your UI can render or your code can process. Define the response schema around the data your application actually consumes, then use Structured Outputs on a compatible model and endpoint.
Structured Outputs and JSON mode are not equivalent. Both can produce valid JSON, but JSON mode does not guarantee that the result matches your intended schema. If downstream code depends on required keys, types, or enum values, use Structured Outputs where supported rather than treating valid JSON as schema validation. The Structured Outputs guide documents the supported approach and its limits.
Can function calling and Structured Outputs be used together?
Yes. Function calling answers the question of whether the model should request an application capability; Structured Outputs can constrain the shape of the arguments in that function call. In other words, you can use a tool for the action and schema constraints for its inputs. The distinction is not “tools versus JSON”—tool arguments are themselves structured data.
What does strict mode require?
Strict mode can enforce function-argument adherence to the supported schema, but it has requirements. OpenAI documents that strict function schemas must set additionalProperties to false and mark all properties as required. To represent a value that may be absent, use a nullable type rather than omitting it from the required property list. Only a subset of JSON Schema is supported, so verify the current supported-schema list before relying on more complex constructs. See the function calling guide and Structured Outputs guide.
Handle refusals and incomplete responses
A schema does not guarantee that every request yields a usable object. The model may refuse a request, and a refusal may not follow the requested response schema. Check the refusal indication and handle incomplete responses before passing output to code that expects a complete, schema-shaped result. OpenAI documents refusal handling in the Structured Outputs guide.
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- The model needs live data or an application action: define a function tool, validate its arguments, and implement the execution flow.
- The model only needs to return a predictable answer: define a response schema and use Structured Outputs where the endpoint and model support it.
- The model must both act and provide structured data: use function calling with a constrained argument schema, then separately shape the final user-facing response if needed.
- You only need valid JSON: JSON mode may produce valid JSON, but it does not promise adherence to your schema; use Structured Outputs when schema matching matters.
API behavior, strict-mode defaults, supported models, and schema support can change. Check the documentation for the specific endpoint and model you plan to use; the official pages cited here were checked on October 7, 2026.
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