2.1.3. Structured Outputs and Function Calling
💡 First Principle: Two different problems wear similar clothing. Structured output forces the response shape to match a JSON schema (so your code can parse it). Function (tool) calling lets the model request an action by emitting a structured call your app then executes. Both produce JSON; only one results in something happening.
Structured output (JSON mode / JSON schema) is for when you need machine-readable results — extracting fields, returning a classification with a fixed set of labels, building a form-filler. You hand the model a schema and it returns conforming JSON. Function calling is for when the model needs information or actions it can't produce itself: you describe available functions (name, parameters as JSON schema), the model decides whether to call one and returns the call, your application runs it, and you feed the result back for the next turn.
⚠️ Exam Trap: "Function calling means the model executes the code." It does not — the model only emits a request naming the function and arguments. Your application is responsible for running it and returning the result. A scenario implying the model directly runs your database query or API call is describing the wrong mental model.
⚠️ Exam Trap (second): Structured output guarantees the JSON is well-formed and schema-valid, not that the values are factually correct. A schema-valid response can still contain a hallucinated field. Validate values; don't trust them because they parsed.
Reflection Question: You need the model to (a) return a product category from a fixed list and (b) look up that product's live price. Which mechanism handles (a), which handles (b), and why is (b) impossible with structured output alone?
