Watch: build and check a schema
No sound. Each step is captioned on screen and also written out in the guide below.
AI Tools
AI JSON Schema Builder
Paste an example of the JSON you want back. Get a schema that OpenAI, Claude, or Gemini accepts for structured output, then check a model's reply against it.
Build and check schemas locally
Runs locally in your browser. Plain code, no AI model. Your JSON is not uploaded, saved, or shared.
Builds as you type. A whole AI reply or almost-JSON works too. Give arrays several items to show which fields are optional.
Your schema will appear here.
Paste sample JSON to build its schema.
Notes
| Where | Note |
|---|
Paste any schema, or use the one you built. It's also checked against each provider's rules.
Paste a schema and a reply to check them.
Problems in the reply
| Path | Problem |
|---|
Will each provider accept this schema?
A working schema in three steps
- 01Paste an example
Use JSON shaped like the answer you want. In arrays, a field missing from some items becomes optional.
- 02Pick your provider
The schema follows that provider's documented rules, and anything it would reject is flagged.
- 03Check real replies
Switch to Validate and paste what the model returned to see every field that doesn't fit.
Optional fields, per provider
[{"id": 1, "note": "gift"}, {"id": 2}]
OpenAI: "note": {"type": ["string", "null"]}, required: ["id", "note"]
Claude: "note": {"type": "string"}, required: ["id"]OpenAI strict mode needs every field listed as required, so optional fields accept null instead. Claude and Gemini leave them out of required.
Frequently asked questions
What rules does each provider have?
As of 30 September 2026, from each provider's documentation:
- OpenAI strict mode needs an object at the top level,
"additionalProperties": falseon every object, and every property inrequired. It allows up to 5,000 properties and 10 levels of nesting, and doesn't supportallOf,not, orif/then/else. (OpenAI docs) - Claude needs
"additionalProperties": false, and doesn't support recursion, number or length bounds, orminItemsabove 1. One request may have at most 24 optional fields and 16 fields with several types. (Claude docs) - Gemini documents a smaller set of keywords and only the date-time, date, and time formats. (Gemini docs)
Why are my optional fields required for OpenAI?
OpenAI strict mode requires every property to be listed in required. To keep a field optional, its type also allows null, and the model sends null when it has no value.
Does it guess limits like enums or string lengths?
No. One sample can't show the real limits, and several providers reject those keywords. Add enums and descriptions by hand where they matter; descriptions help the model most.
What does Validate check?
Types, required and extra properties, enums, formats, number and length bounds, patterns, arrays, anyOf/oneOf/allOf, and local $refs. Anything it doesn't check is listed, so a pass never hides a skipped rule.
Is my data sent anywhere?
No. Samples are often real customer data, so everything runs in your browser and nothing is uploaded, saved, or put in share links.