# JSON Schema generator: run it free, or from your agent

A draft 2020-12 JSON Schema from sample JSON, plus Anthropic and OpenAI tool wrappers.

## Prompt

Use looot to give my agent a typed tool from this sample JSON: [paste it]. 1. Set up https://looot.ai/skill.md. 2. Write a JSON Schema draft 2020-12 for the sample, every key required, additionalProperties false. 3. Wrap it as a tool definition for the API my agent uses. 4. Run one catalog endpoint whose output matches the schema and check the answer validates.

## What it needs

A JSON sample: an object, an array or a value

## What comes back

- JSON Schema draft 2020-12
- Anthropic tool with input_schema
- OpenAI function with parameters
- required keys and string formats

## Endpoints and providers

None. The tool on the web page runs in the browser and calls no endpoint.

## What it costs

Nothing. The tool on the web page runs in the visitor's browser: no account, no API call and no limit per hour. The prompt above runs the same job from your own looot account with current data, billed per call.

## Tool calling

Turn a sample request body into the input schema of a tool your model can call.

## Structured output

Pin the shape you want back from a model, then validate every answer against it.

## Validate API data

Check webhook payloads or API answers against the schema in tests.

## Skip the hand typing

Get nested objects, arrays and nullable fields right without writing the schema by hand.

## How do I generate a JSON Schema from JSON?

Paste a sample. The generator reads each value's type, builds properties for every object, merges the items of each array into one shape and marks keys as required. The schema updates as you type.

## Which JSON Schema version does it write?

Draft 2020-12. The $schema line says so. The tool wrappers leave $schema out, as the model APIs expect.

## How are arrays with different items handled?

Every item is read and the shapes are merged. A key missing from some items is not required unless Every field required is on. A value that is null in one item and a string in another becomes a nullable string.

## What is the difference from the LLM tool schema generator?

That tool builds a schema from fields you list by hand. This one infers it from real JSON, which is faster when you already have a sample payload.

## Does it work with OpenAI strict mode?

Turn on Every field required and Strict objects and the OpenAI output gets strict: true. Strict mode needs every key required and additionalProperties false on each object.

## Which string formats does it detect?

date-time, date, email, uri and uuid, by pattern. Turn String formats off if a field only looks like one by chance.

## Is my JSON sent anywhere?

No. Everything runs in your browser. There is no account and no limit.

## Related tools

- /free-tools/llm-tool-schema-generator
- /free-tools/json-formatter
- /free-tools/mock-data-generator

## Last reviewed

2026-10-14.