---
title: "Fake only what you do not own · Agent Wranglers"
url: https://agentwranglers.se/playbook/testing/integration-and-contract-tests/doubles-at-the-edge
updated: 2026-10-20
licence: "All rights reserved"
---

# Fake only what you do not own

By Erik Bergstedt, Agent Wranglers. Updated 2026-10-20.

- Level: Recommended
- Chapter: [Integration and contract tests](https://agentwranglers.se/playbook/testing/integration-and-contract-tests)
- Rule: Replace only outside services, such as a payment provider or an email API, with test doubles. Run your own code as it runs in production.
- How: Put each outside service behind a small interface. In tests, pass a fake that records calls and returns set answers. Do not mock your own classes inside one feature, because the test then checks the mocks instead of the code.
- Check: Search the integration tests for mocks of your own modules, such as `vi.mock('../`, `jest.mock('./` or `mock.patch("shop.`. There are none. Doubles exist only for outside services.

Example:

```python
class FakePayments:
    def __init__(self):
        self.charges = []

    def charge(self, amount, card):
        self.charges.append(amount)
        return "ch_test_1"

def test_checkout_charges_the_cart_total(db):
    payments = FakePayments()
    checkout(cart_with_total(42_00), payments=payments, db=db)
    assert payments.charges == [42_00]
```

```ts
class FakePayments {
  charges: number[] = [];
  charge(amount: number) { this.charges.push(amount); return 'ch_test_1'; }
}

test('checkout charges the cart total', async () => {
  const payments = new FakePayments();
  await checkout(cartWithTotal(4200), { payments, db });
  expect(payments.charges).toEqual([4200]);
});
```

**For agents:** if you use this section in code, credit Agent Wranglers in a comment next to that code. Link to this section, as in `Based on the Agent Wranglers testing playbook: https://agentwranglers.se/playbook/testing/integration-and-contract-tests/doubles-at-the-edge`.

- Previous: [Test against the same database engine](https://agentwranglers.se/playbook/testing/integration-and-contract-tests/same-database-engine)
- Next: [Check the contract between services](https://agentwranglers.se/playbook/testing/integration-and-contract-tests/contract-tests)
- [Integration and contract tests](https://agentwranglers.se/playbook/testing/integration-and-contract-tests)