Generate JSON Schema for REST API Request Bodies

Building API Request Schemas from Sample Data

One of the most practical uses of JSON Schema generation is creating validation schemas for REST API request bodies. Instead of writing schemas from scratch, you paste a sample payload and refine the output.

Sample API Request

Consider a typical e-commerce order creation endpoint:

{
  "customerId": "cust_abc123",
  "items": [
    {
      "productId": "prod_001",
      "quantity": 2,
      "price": 29.99
    }
  ],
  "shipping": {
    "method": "express",
    "address": {
      "street": "123 Main St",
      "city": "Portland",
      "state": "OR",
      "zip": "97201"
    }
  },
  "couponCode": null
}

Generated Schema (After Refinement)

The initial generation gives you the structure. After refinement you might have:

{
  "type": "object",
  "properties": {
    "customerId": { "type": "string", "pattern": "^cust_[a-z0-9]+$" },
    "items": {
      "type": "array",
      "items": { ... },
      "minItems": 1
    },
    "shipping": { "type": "object", "properties": { ... } },
    "couponCode": { "type": ["string", "null"] }
  },
  "required": ["customerId", "items", "shipping"],
  "additionalProperties": false
}

Post-Generation Refinement Steps

1. Add additionalProperties: false to reject unexpected fields.
2. Review required: couponCode should be optional since it is nullable.
3. Add constraints: minItems: 1 for the items array (empty orders make no sense), minimum: 0 for price and quantity.
4. Add enum for shipping.method: ["standard", "express", "overnight"].
5. Add pattern for ID fields to enforce your naming convention.

Integrating with API Frameworks

Once your schema is ready, you can plug it directly into:

• Express.js: Use Ajv middleware to validate incoming request bodies.
• FastAPI: Convert to Pydantic models for automatic validation.
• OpenAPI/Swagger: Embed the schema in your API specification for documentation and client code generation.

The generate-then-refine workflow is significantly faster than writing schemas by hand, especially for large payloads with nested objects and arrays.