What does structured output guarantee—and what must your code still check?

Shared system · deterministic provider fixtures

Harbor Support Drafting API. One bounded request moves through typed items, tools, state, budgets, and evidence without live provider calls.

01 · Smallest useful mechanism

Separate syntax, schema, semantic rules, and authority.

Structured-output modes constrain generated shapes. Applications still validate provider-supported schema subsets, domain invariants, source evidence, permissions, and safe handling of refusals or incomplete generations.

A valid object is a candidate for business logic, not proof that the business claim is correct.

02 · Experiment

Test the prediction

Deterministic protocol fixture

This workbench uses inspectable provider-event fixtures and injected failures. It does not make live calls or execute external effects.

Protocol workbench · harbor-schema-v1

Choose the validation pipeline that can safely display a return decision.

Injected failure

The candidate is schema-valid but references another customer and an expired policy.

Choose the implementation

Trust successful deserializationRender any object accepted by the generated TypeScript type.Ask the model to verify itselfSend the same candidate back for a confidence score.Apply layered validationCheck schema, business invariants, tenant ownership, evidence, and current policy.

The trace has not run.

Commit to an implementation, then inject the failure and inspect the actual state transitions.

03 · Implementation brief

Build it, break it, prove the outcome.

Implement: Define a versioned output schema and a separate semantic validator.
Break it on purpose: Supply impossible dates, a cross-tenant ID, an unknown evidence ID, and a refusal.
Completion evidence: Each branch produces a stable error category and no invalid candidate reaches downstream code.

Your learning artifact

Write the argument you would defend

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Your response

04 · Check your understanding

Which guarantee can strict structured output provide?

Primary sources · verified 2026-08-02

What the fixture does not prove

Teams often remove defensive validation after enabling a strict mode, then execute plausible but unauthorized or internally inconsistent values.

Common mistake: Strict structured output makes hallucinations impossible. It can guarantee supported structural constraints, not truth, freshness, authorization, or correspondence to external evidence.

Transfer exercise: Generate a schema-valid return decision whose deadline predates the purchase and whose customer ID belongs to another account.

OpenAI — Function callingOfficial tool-call lifecycle and strict JSON Schema requirements for application-executed functions.Google — Structured outputsOfficial JSON Schema guidance and the important distinction between structured final output and tool execution.JSON Schema — 2020-12 specificationNormative model for JSON instance validation and schema vocabularies; providers may intentionally support a subset.

Optional referenceDeep dive / reference chapterOpen the complete essay, diagrams, mathematics, exercises, glossary, and sources when you want more depth.on demand

Next: Tool calling uses the same structured boundary, then adds the possibility of a real external effect.