A (hopefully) clear and probably oversimplified example:
Query -> Person Lookup -> Result-> Structured Output `{ firstName: "", lastName: "" }`
When result doesn't have relevant information, structured output will basically always output a name, whether it found the correct person or not, because it wants to output something, even if the fields are optional. With this example, prompting can help turn the names into "Unknown", but the prompt usually ends up being excessive and/or time consuming to get correct and fix edge cases for. Weaker models might struggle more on details or relevance with this prompt-only approach.
`{ found: boolean, missingFields: [], missingReason: "", firstName, lastName }`
Including one or more of these additional text output properties has an almost magical affect sometimes, reducing the required prompting and hallucinated/incorrect outputs.
