Basic structured output gets you a reliable shape. This deep dive is for when the shape needs to be airtight: nested objects, optional and conditional fields, enums that must be respected, and outputs that feed directly into code or another system with no human in between.

Function calling as an interface

Function calling is structured output pointed at action: you describe functions the model can "call" (with typed parameters), and the model returns a structured request to call one, which your code then executes. This is the backbone of tool use later. The key discipline is writing parameter schemas and descriptions precise enough that the model calls the right function with the right arguments.

Design principles

Constrain aggressively (use enums and required fields to eliminate invalid outputs), describe each field as if to a literal junior (ambiguous descriptions produce ambiguous outputs), and validate even structured results (constrained does not always mean correct in meaning).