Free JSON Schema Validator
JSON Schema Validator checks JSON documents, JSON Lines files and array items against a schema in draft 4, 7, 2019-09 or 2020-12, listing every error with its JSON Pointer, keyword and schema location; click one to jump there. It also checks MCP, OpenAI and Anthropic tool definitions, including OpenAI strict mode, and validates example arguments. Remote $ref schemas are never downloaded.
- Drafts:
- 4, 7, 2019-09, 2020-12 (6 checked as 7)
- Inputs:
- JSON, JSON Lines, array items, up to 20 MB
- Tool checks:
- MCP, OpenAI (incl. strict), Anthropic
- Price:
- Free
- Runs:
- Locally, in your browser
- Uploads:
- None: files never leave your device
- Account:
- Not needed
Quick answers
Are my schema and data uploaded?
No. Validation runs in a background worker in your browser and nothing is sent anywhere. A $ref is never downloaded either: paste the schemas it points to under Referenced schemas.
Which JSON Schema drafts are supported?
Draft 4, draft 7 (draft 6 is checked with the draft 7 rules), 2019-09 and 2020-12. The draft is read from $schema, otherwise 2020-12 is used, and you can pick one by hand. $dynamicRef from 2020-12 is not supported: it is ignored and the page warns you.
Processed on your device: your files are never uploaded. How it works
Network activity in this tab
What this page has requested since you opened the site in this tab. An upload is any request that sends data: POST, PUT, PATCH or a request with a body.
No uploads: nothing has been sent from this page.
No network requests yet.
To see every request, open your browser's developer tools (Network tab).
How to use the JSON Schema Validator
- Paste a JSON Schema on the left or open a .json file: the draft comes from $schema, or pick it in the Draft menu.
- Paste the data on the right or open a .json, .jsonl or .ndjson file, and choose One JSON document, JSON Lines or Array items.
- Read the results: every invalid instance lists its errors with the JSON Pointer, the message and the schema keyword. Click an error to jump to it in the data, or press Schema to see the keyword.
- For LLM tools, switch to Check tool definitions, paste MCP, OpenAI or Anthropic tool definitions and, if you like, example arguments or a result. Copy or download the report as JSON.
Credits & open source
Our own code plus open-source libraries. Thank you to the people who make these projects.
- @cfworker/json-schemaJSON Schema validator for drafts 4, 7, 2019-09 and 2020-12, without code generation (no eval). © Jeremy Danyow · MIT
- jsonc-parserFault-tolerant JSON scanner that reports error positions. © Microsoft Corporation · MIT
- CodeMirror 6Code editor with syntax highlighting. © Marijn Haverbeke and others · MIT
- @uiw/react-codemirrorReact wrapper for CodeMirror 6. © Kenny Wong (uiwjs) · MIT
Web platform APIs used: Web Workers, File API, Clipboard API.
Full license texts: third-party notices · all credits
Frequently asked questions
Does it check format, such as email or date-time?
Yes, by default and in every draft, for 17 formats including email, date-time, date, time, uri, hostname, ipv4, ipv6 and uuid. Any other format name always passes. Turn off Check format to treat format as an annotation only, the default of the 2019-09 and 2020-12 specifications.
How do I validate a JSONL or NDJSON file?
Choose JSON Lines, or open a .jsonl or .ndjson file and the mode switches by itself. Each non-empty line is validated on its own and the results show the line numbers, up to 100,000 lines and 20 MB.
Can it check MCP tool schemas and OpenAI strict mode?
Yes. Switch to Check tool definitions and paste MCP tools, a tools/list result, OpenAI function tools or Anthropic tools. Each check links to the documentation it comes from: MCP type object and naming rules, OpenAI and Anthropic name patterns, OpenAI strict mode, Anthropic input_examples. Example arguments and results are validated too.
Is the schema itself validated against the meta-schema?
Not exactly. The validator library ships no meta-schemas, so the page runs its own structural check: keyword types and ranges for the chosen draft, invalid regular expressions, typos, keywords of other drafts and contradictions. It catches the common mistakes, but it is not a full meta-schema validation.