JSON Schema Validation Before API Calls

JSON Schema Validation Before API Calls

JSON Schema is a contract that defines the structure, types, and constraints of a JSON document. Validate payloads before sending them to an API, and you catch errors client-side instead of debugging cryptic 400 responses.

What JSON Schema Looks Like

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "type": "object",
  "properties": {
    "email": {
      "type": "string",
      "format": "email"
    },
    "age": {
      "type": "integer",
      "minimum": 0,
      "maximum": 150
    },
    "role": {
      "type": "string",
      "enum": ["admin", "user", "viewer"]
    }
  },
  "required": ["email", "role"]
}

This schema enforces: email must be a valid email string, age must be an integer 0-150, role must be one of three values, and both email and role are required.

Why Validate Before Sending

Validation in JavaScript (ajv)

import Ajv from 'ajv';
import addFormats from 'ajv-formats';

const ajv = new Ajv();
addFormats(ajv); // adds "email", "uri", "date-time", etc.

const schema = {
  type: 'object',
  properties: {
    email: { type: 'string', format: 'email' },
    role:  { type: 'string', enum: ['admin', 'user', 'viewer'] }
  },
  required: ['email', 'role']
};

const validate = ajv.compile(schema);

const payload = { email: 'not-an-email', role: 'superadmin' };
if (!validate(payload)) {
  console.error(validate.errors);
  // [{keyword: 'format', message: 'must match format "email"'}, ...]
}

Validation in Python (jsonschema)

from jsonschema import validate, ValidationError

schema = {
    "type": "object",
    "properties": {
        "email": {"type": "string", "format": "email"},
        "role":  {"type": "string", "enum": ["admin", "user", "viewer"]}
    },
    "required": ["email", "role"]
}

try:
    validate(instance={"email": "[email protected]"}, schema=schema)
except ValidationError as e:
    print(e.message)  # 'role' is a required property

Workflow Tip

Before testing any API call, paste your JSON payload into the JSON Formatter to validate syntax and prettify the structure. A single trailing comma or misquoted key will fail silently in many HTTP clients -- catch it visually first.

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