Every team that ships an LLM feature eventually writes the same function. It takes the model's reply, strips the stray markdown fence, hunts for the first { , calls JSON.parse inside a try , and on failure asks the model again with a sterner prompt. It works most of the time. "Most of the time" is a terrible property for code that sits between a model and your database.

Structured outputs replace that whole function. Instead of asking nicely for JSON, you hand the API a JSON Schema and the model is physically unable to emit tokens that break it. That changes what you need to defend against, but it doesn't remove the need for defense entirely, and the gap between the two is where the bugs live.

Prompting for JSON vs. constraining it

When you prompt for JSON, you are asking the model to follow an instruction. It usually does. Sometimes it adds a friendly sentence first, forgets a required field, or returns "price": "12.99" as a string when you wanted a number.

Structured outputs work differently.…