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JSON Schema

JSON Schema is a vocabulary for describing the structure of valid JSON: required fields, types, allowed values, nested-object shape. Use it for API request validation, OpenAPI docs, and structured data validation. Modern alternatives: TypeScript types (for compile time), Zod / Yup / Valibot (for runtime validation in JS).

Updated May 2026 · 4 min read
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Definition

JSON Schema is a vocabulary for describing the structure of valid JSON: required fields, types, allowed values, nested-object shape. Use it for API request validation, OpenAPI docs, and structured data validation. Modern alternatives: TypeScript types (for compile time), Zod / Yup / Valibot (for runtime validation in JS).

What it means

JSON Schema is itself written in JSON. Example: <code>&#123;&quot;type&quot;: &quot;object&quot;, &quot;required&quot;: [&quot;name&quot;], &quot;properties&quot;: &#123;&quot;name&quot;: &#123;&quot;type&quot;: &quot;string&quot;&#125;, &quot;age&quot;: &#123;&quot;type&quot;: &quot;integer&quot;, &quot;minimum&quot;: 0&#125;&#125;&#125;</code> says &ldquo;name is required string, age is non-negative integer.&rdquo; The Draft 2020-12 spec is current; Draft 7 is widely supported. Validators exist in every major language (Ajv for JavaScript, jsonschema for Python, Pydantic for Python with auto-generation). OpenAPI specs use JSON Schema for request/response validation. Modern JS alternatives (Zod, Yup, Valibot) write validation in the language directly with auto-inferred TypeScript types — often more ergonomic than separate JSON Schema files.

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Why it matters

Strict JSON catches syntax errors but not semantic errors. <code>&#123;&quot;userId&quot;: &quot;abc&quot;&#125;</code> is valid JSON but invalid in an API expecting numeric ID. JSON Schema catches that. For REST/GraphQL APIs: documenting expected shape via OpenAPI lets clients verify contracts before deployment. For form validation: schema-based validation centralizes rules in one place. For data-pipeline ETL: schema validation catches malformed data before it propagates.

Example

Schema: <code>&#123;&quot;type&quot;: &quot;object&quot;, &quot;required&quot;: [&quot;email&quot;], &quot;properties&quot;: &#123;&quot;email&quot;: &#123;&quot;type&quot;: &quot;string&quot;, &quot;format&quot;: &quot;email&quot;&#125;&#125;&#125;</code>. Valid: <code>&#123;&quot;email&quot;: &quot;a@b.com&quot;&#125;</code>. Invalid: <code>&#123;&quot;email&quot;: 42&#125;</code> (wrong type), <code>&#123;&#125;</code> (missing required), <code>&#123;&quot;email&quot;: &quot;not-an-email&quot;&#125;</code> (format violation).

Related free tools

Free toolJSON FormatterPaste JSON to format, validate, and minify. Clear error messages with line numbers. Free and runs in your browser.Open tool →Free toolJSON Schema GeneratorPaste a JSON sample and get a draft-07 JSON Schema with inferred types, required fields, and nested structure.Open tool →Free toolJSON Schema to TypeScriptPaste a JSON sample — get a clean TypeScript interface inferred automatically.Open tool →

Frequently asked questions

JSON Schema vs Zod vs Pydantic?

JSON Schema: language-neutral, written in JSON. Zod (JS/TS), Pydantic (Python), Joi (older JS): language-native, more ergonomic for that language but not portable. Modern apps often use Zod for runtime validation + auto-generate JSON Schema for OpenAPI docs.

What versions exist?

Drafts 4, 6, 7, 2019-09, 2020-12 (current). Most validators support Draft 7+. Differences are minor for most use cases.

Can I auto-generate schemas from data?

Yes — tools like quicktype.io infer schema from sample JSON. Useful starting point but typically needs manual refinement (sample data may not capture edge cases or required fields).

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