How do I stub third-party API calls in my tests?

To stub third-party API calls in tests, the usual goal is to replace the real network call with a predictable fake so your tests are fast, reliable, and don’t depend on an external service.

Common approaches

1) Mock the HTTP client used by your code

If your code uses something like fetch, axios, requests, httpx, etc., stub at that layer.

Example idea:

• Your function calls fetch("https://api.example.com/user")
• In tests, replace fetch with a stub that returns a fixed response

This is usually the simplest and most maintainable approach.

2) Use a library designed for HTTP mocking

Depending on your language/framework, there are tools like:

• JavaScript/TypeScript: MSW, nock, jest mocks
• Python: responses, respx, unittest.mock
• Ruby: WebMock, VCR
• Java: WireMock, Mockito
• Go: httptest, gock

These libraries intercept outgoing HTTP requests and return mocked responses.

3) Dependency injection

Instead of calling the API directly inside your business logic, pass in a client object or function.

Example pattern:

• Production: pass the real API client
• Test: pass a fake client returning known data

This makes tests easier to isolate and often avoids heavy mocking.

4) Record/replay with cassettes

Tools like VCR record real API responses once, then replay them in tests later.

Useful when:

• You want realistic responses
• The API is stable
• You don’t want to handcraft every mock

Downsides:

• Can get brittle if API responses change
• May accidentally capture sensitive data unless sanitized

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Best practices

• Stub at the network boundary, not deep inside business logic.
• Return realistic responses that match what your code expects.
• Cover error cases too: timeouts, 500s, invalid JSON, rate limits.
• Don’t hit real external services in unit tests if you can avoid it.
• Keep integration tests separate if you do want to test the real API.

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Tiny examples

JavaScript with Jest

global.fetch = jest.fn(() =>
  Promise.resolve({
    json: () => Promise.resolve({ name: "Alice" })
  })
);

Python with unittest.mock

from unittest.mock import patch, Mock

@patch("my_module.requests.get")
def test_user(mock_get):
    mock_get.return_value = Mock(status_code=200, json=lambda: {"name": "Alice"})

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If you tell me your language/framework and which third-party API library you’re using, I can show you the exact way to stub it.