OpenAPI Mock Data Generator

What is the OpenAPI Mock Data Generator?

The OpenAPI Mock Data Generator parses your OpenAPI 3.x or Swagger 2.0 specification and produces realistic JSON payloads for every endpoint. It reads your schemas — including string formats, enums, nested objects, and arrays — and generates data you can use immediately for testing, prototyping, or documentation. All processing happens in your browser — your API specification never leaves your device.

How to Use

1. Paste your OpenAPI or Swagger specification in JSON or YAML format
2. Optionally set a seed number for reproducible output
3. Click "Generate Mock Data" or wait for automatic processing
4. Review generated request and response payloads per endpoint
5. Copy results to use in tests, documentation, or API consumers

Example: Generating Mock Responses

Given this schema definition in your OpenAPI spec:

components:
  schemas:
    User:
      type: object
      properties:
        id:
          type: integer
        name:
          type: string
        email:
          type: string
          format: email
        createdAt:
          type: string
          format: date-time

The generator produces realistic mock data like:

{
  "id": 472,
  "name": "A sample description",
  "email": "alice@example.com",
  "createdAt": "2024-01-15T10:30:00Z"
}

Email fields get realistic addresses, date-time fields get ISO timestamps, and integers respect any min/max constraints defined in the schema.

Reproducible Output with Seeds

The seed option controls the random number generator. Using the same seed with the same specification always produces identical mock data. This is useful for:

• Writing deterministic integration tests that depend on specific values
• Generating consistent documentation examples across builds
• Comparing output before and after schema changes
• Sharing reproducible test scenarios with teammates

Supported Schema Features

• Primitive types — string, number, integer, boolean, null
• String formats — email, uuid, date, date-time, uri, ipv4, ipv6, hostname, password
• Enums — picks a random valid value from the enum list
• Arrays — generates items respecting minItems/maxItems constraints
• Nested objects — recursively generates properties up to 10 levels deep
• $ref references — resolves both #/components/schemas/ and #/definitions/
• Composition — supports allOf (merge), oneOf/anyOf (pick first)
• Constraints — honors minimum, maximum, minLength, maxLength
• Examples and defaults — uses schema example or default values when provided

Use Cases

• API testing — Generate request payloads to feed into Postman, curl, or test suites
• Frontend prototyping — Create mock responses for UI development before the backend is ready
• Documentation — Produce example payloads for API documentation
• Contract testing — Validate that consumers handle all response shapes correctly
• Data seeding — Generate initial data for development databases

Supported Formats

Both JSON and YAML input formats are supported. The tool accepts OpenAPI 3.x and Swagger 2.0 specifications. Swagger 2.0 specs using the definitions keyword are handled the same as OpenAPI 3.x components/schemas.

Privacy and Security

All mock data generation happens entirely in your browser using JavaScript. Your API specifications — which may contain internal endpoints, authentication details, and business logic — never leave your device. No data is stored, logged, or transmitted.