Introduction

What it looks like

config.yaml

apiKey: "test-key"
port: 3000
responses:
  - id: "greeting"
    matcher:
      type: "contains"
      pattern: "hello"
    response:
      content: "Hello! How can I help you today?"

// Your test code
const openai = new OpenAI({
  baseURL: 'http://localhost:3000/v1',
  apiKey: 'test-key'
});

const response = await openai.chat.completions.create({
  model: 'gpt-4',
  messages: [{ role: 'user', content: 'Say hello!' }]
});
// Returns: "Hello! How can I help you today!"

Why use it

Testing LLM applications is hard because:

• Real API responses vary each time
• API calls cost money and have rate limits
• Network issues can break tests
• You can’t test specific edge cases reliably

This mock server solves these issues by giving you complete control over responses during testing.