JSON Schema Generator

What the generator infers

The generator walks your JSON sample recursively and infers a schema for each value. Here is exactly what each JSON value becomes:

The generated schema is a starting point

Auto-inference from a single sample is fast but not complete. A sample with one object cannot tell you which fields are truly optional, what the minimum/maximum values should be, or what the allowed enum values are. After generating, review and tighten the schema:

// Generated from: { "status": "active", "age": 28 }
{
  "$schema": "https://json-schema.org/draft-07/schema#",
  "type": "object",
  "properties": {
    "status": { "type": "string" },   // ← tighten: add "enum": ["active","inactive","pending"]
    "age":    { "type": "integer" }   // ← tighten: add "minimum": 0, "maximum": 120
  },
  "required": ["status", "age"]       // ← review: is "age" truly required?
}

Once you have reviewed the schema, paste both the JSON and the schema into the JSON Schema Validator to confirm the validation works as expected.

Using the schema with Kafka Avro

JSON Schema and Avro schema are different formats — this tool generates JSON Schema, not Avro. However, the output is useful as a structure reference when writing Avro schemas by hand. The type mapping is:

Read the companion guide: Kafka Avro Schema: Backward Compatibility Explained.

Understanding the Generator Modes

The generator has two modes to handle null values in your JSON sample:

Simple Mode (default)

Outputs strict JSON Schema draft-07 that matches jsonlint and most other tools. When a field is null, it becomes {"type":"null"} — technically valid but rarely useful for APIs.

// Input: { "deletedAt": null }
// Simple Mode output:
"deletedAt": { "type": "null" }

// Problem: ✗ Rejects any non-null value, even if the API sends a timestamp string

⚡ Smart Mode (production-ready)

Outputs nullable union types — the pattern real APIs use. Null fields get all possible types plus a description telling you to refine once you have real data.

// Input: { "deletedAt": null }
// Smart Mode output:
"deletedAt": {
  "type": ["null","string","number","boolean","object","array"],
  "description": "Type inferred as null only — refine after seeing non-null samples"
}

// Advantage: ✓ Accepts any type while you're building the schema
// Next step: Once you see { "deletedAt": "2026-05-23T14:00:00Z" }, refine to:
//   "type": ["string","null"],
//   "format": "date-time"

Smart Mode is unique to JSON Dev Tools — it solves the null-type trap that every other generator falls into. For a detailed explanation of why type: null is problematic and when to use each mode, see JSON Schema type: null — Why It's Wrong for Real APIs.