What does OpenAI structured output require?

When you use strict structured outputs, every object must list all of its properties in required, and additionalProperties must be false. This tool checks those constraints and flags any field present in the response but absent from the schema, which is a common cause of strict-mode validation failures.

Which JSON Schema features are supported?

The tool validates type (string, number, integer, boolean, object, array, null), required fields, nested objects, array items, and additionalProperties. It is a focused validator for the subset OpenAI structured outputs supports, not a full JSON Schema engine with $ref or conditionals.

Why does my valid-looking response still fail strict mode?

The most common reasons are a property in the response that is not declared in the schema while additionalProperties is false, or a required field that the schema lists but the model omitted. Both show up clearly in the conformance report with their path.

Does my schema or data leave the browser?

No. Validation runs entirely client-side with no network requests, so you can safely test schemas and responses that contain proprietary structure or sensitive data.

Structured Output Schema Tester

Catch structured-output failures before production

OpenAI’s strict structured outputs enforce a precise contract: every property must be declared, all of them must be in required, and additionalProperties must be false. A response that looks fine to the eye can still fail validation because of one undeclared field or one missing required key. This tool compares your schema against a sample response and reports every mismatch with its exact path.

How it works

Paste your JSON Schema and a sample response object. The tool parses both, then walks the schema recursively against the data. At each level it checks the declared type, confirms every required field is present, recurses into nested objects and array items, and — when additionalProperties is false — flags any property in the data that the schema does not declare. Each problem is reported with a dotted path like data.address.zip so you can pinpoint it instantly.

Tips and notes

For strict mode, remember the two rules that trip people up most: list all properties in required, and set additionalProperties: false on every object. If the model keeps adding a stray field, tighten the prompt or the schema description rather than loosening the contract. Everything runs locally, so sensitive schemas stay in your browser. Use the report iteratively — fix one path, re-run, and repeat until the response is fully conformant before wiring the schema into your API call’s response_format.