Structured outputs versus JSON mode: valid syntax still needs field checks
Distinguish valid JSON, schema-conforming output and factual field accuracy, and explain how programs should choose constraints and handle exceptional model responses.
Choose an approach
When downstream code needs fixed fields and types, first check whether the interface supports structured output with the required schema. JSON mode may suffice when only valid JSON is needed. Neither guarantees that fields come from the source. Define required fields and missing-value handling, then evaluate parsing, structure and facts separately.
Valid JSON and field constraints are different guarantees
Valid JSON can be a string or an object missing the desired fields. OpenAI distinguishes JSON validity from schema adherence, and Anthropic also provides structural constraints. Supported schema features differ across interfaces; do not assume the full JSON Schema specification is supported.
Even strictly conforming output can contain an incorrect amount or invented name. Field constraints restrict types but cannot establish supporting evidence. Defining an explicit unknown value preserves incomplete information more faithfully than requiring every field to be filled.
Compare complete and incomplete source materials
Extract a name and date from the same registration form, then remove the date and retry. Check JSON-mode parsing and structured-output field constraints, but verify in both cases that the missing date was not invented. Identical source versions make it possible to identify which errors the constraints address.
Also cover refusals, interrupted output and service errors. Check request status before parsing. With streaming, wait for a complete structure before validation; an incomplete interim chunk is not automatically a final format failure.
Update downstream consumers when the schema changes
Old programs may still expect previous field names or enum values. Save the schema version and use earlier samples to test downstream compatibility. Do not silently discard unknown fields merely to make parsing pass; follow business rules and preserve comparison with source material for review.
Check model support and interface limitations before choosing constraints, and use actual request documentation for charges. Keep format and factual errors separate so you can decide whether to change the schema or source processing. This article checks official guidance without model calls or a claim that errors are eliminated.
Checked sources
These sources support the concepts and methods described here. Examples and test scope are labeled separately; check the version of the tools you use.