Structured Outputs and JSON Mode
Programs need values, not essays. Structured output makes an LLM fill a form instead of writing free text — and grammar-constrained decoding is the only method that guarantees the output matches your schema, by blocking every token that would break it. This topic covers how that works, what "strict mode" really enforces, and the semantics-versus-syntax gap retries cannot close.
01.The Problem: Your Program Needs a Number, the Model Wrote an Essay
You build an agent that extracts ticket details from support emails:
"The crash started on Sev-2 and we need an owner by Tuesday, ref TKT-4417."
Your code needs three values: severity = 2, due = Tuesday, id = TKT-4417.
A helpful model answers: "This sounds like a serious incident — you should assign it to the on-call team quickly!"
Pretty words. Zero values. Your parser chokes.
So the question becomes
How do you make a language model produce output a program can consume — every single time?
The general shape: you define a schema — a form describing exactly which fields you need and what shape each may have — and you make the model fill the form. That is "structured output". The methods to enforce it range from polite asking to mathematical guarantee, and the difference between them is this whole topic.
Carry one analogy through: a government form vs a blank letter.
- Free text = a letter. The clerk (your code) has to read it and guess where the answers are.
- JSON mode = a letter with a note: "please write your answers at the bottom". Better odds. Still guesses.
- A schema-constrained form = boxes and dropdowns that physically reject anything outside them. You can still write a legible but WRONG number in a box — forms guarantee shape, not truth.
Both failure classes need fixes, and confusing them is how outages happen.
Three Ways to Get JSON, Only One Is a Guarantee 🧱
Three Ways to Get JSON, Only One Is a Guarantee 🧱
Prompting and fine-tuning raise the probability of valid output; constrained decoding makes syntactic validity a property of the sampler. Semantics still need their own checks.
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