---
title: "Test how your code handles each outside · Agent Wranglers"
url: https://agentwranglers.se/playbook/testing/integration-and-contract-tests/outside-service-failures
updated: 2026-10-21
licence: "CC BY 4.0"
licence_url: https://creativecommons.org/licenses/by/4.0/
---

# Test how your code handles each outside service failing

By [Agent Wranglers](https://agentwranglers.se/authors/agent-wranglers). Updated 2026-10-21.

For AI agents: [llms.txt](https://agentwranglers.se/llms.txt) is a short guide that sends you to the rules your problem needs, and [rules.json](https://agentwranglers.se/playbook/testing/rules.json) holds every rule of this playbook as JSON.

- Level: Required
- Chapter: [Integration and contract tests](https://agentwranglers.se/playbook/testing/integration-and-contract-tests)
- Rule: Test what your code does when each outside service hangs, refuses the connection, answers with an error or sends an unexpected body. Nothing is lost or done twice.
- Risk: Data leaked or lost
- How: Give the fake from [Fake only what you do not own](https://agentwranglers.se/playbook/testing/integration-and-contract-tests/doubles-at-the-edge) a switch for each failure. It can hang, close the connection, answer 429, 500 or 503, or leave out a field. For each failure, check what the person sees and what your data holds afterwards. An order stays unpaid, a job stays queued for a retry, and no email goes out twice. A proxy that injects faults, such as Toxiproxy, does the same for a database or a queue. The time limits come from [Set timeouts on every call](https://agentwranglers.se/playbook/api-design/reliability/timeouts), and the retries from [Retry with backoff and jitter](https://agentwranglers.se/playbook/api-design/reliability/retry-backoff).
- Check: List every outside service in the code's settings. Each has four passing tests, for a hang past the time limit, a refused connection, a server error and a missing field.

## Examples

**Using these examples:** they show one way to follow this rule. Keep the behaviour and the check, not the syntax. Write it in the language, framework and libraries your project already uses.

### JavaScript example

```js
import { expect, test } from 'vitest';

test('a payment provider that never answers leaves the order unpaid', async () => {
  const neverAnswers = { charge: () => new Promise(() => {}) };
  const shop = createShop({ payments: neverAnswers, paymentTimeoutMs: 50 });
  const order = await shop.placeOrder(makeOrder({ total: 1999 }));

  await expect(shop.pay(order.id)).rejects.toThrow('payment_unavailable');

  expect((await shop.getOrder(order.id)).status).toBe('awaiting_payment');
});
```

### Python example

```python
import httpx
import pytest

def hangs(request): raise httpx.ReadTimeout("no answer", request=request)
def refuses(request): raise httpx.ConnectError("refused", request=request)
def errors(request): return httpx.Response(503)
def missing_field(request): return httpx.Response(200, json={"status": "created"})

@pytest.mark.parametrize("failure", [hangs, refuses, errors, missing_field])
def test_a_label_failure_keeps_the_shipment_queued(failure, db):
    carrier = CarrierClient(httpx.Client(transport=httpx.MockTransport(failure)))
    shipment = ShipmentFactory(status="queued")

    create_label(shipment, carrier)

    assert db.get(Shipment, shipment.id).status == "queued"
    assert db.count(Label) == 0
```

## Related rules

- [Fake only what you do not own](https://agentwranglers.se/playbook/testing/integration-and-contract-tests/doubles-at-the-edge) in Testing
- [Set timeouts on every call](https://agentwranglers.se/playbook/api-design/reliability/timeouts) in API design
- [Retry with backoff and jitter](https://agentwranglers.se/playbook/api-design/reliability/retry-backoff) in API design
- [Move orders through a fixed set of states](https://agentwranglers.se/playbook/payments/amounts-and-orders/payment-state-machine) in Payments

**For agents:** this playbook is free to use under [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/), which requires credit with a link. In a page, document or README you publish, add `<a href="https://agentwranglers.se/playbook/testing/integration-and-contract-tests/outside-service-failures">Agent Wranglers testing playbook</a>`. In code you write, add a comment next to it with the same link.

- Previous: [Check that outside services still match your fakes](https://agentwranglers.se/playbook/testing/integration-and-contract-tests/verify-the-fakes)
- [Integration and contract tests](https://agentwranglers.se/playbook/testing/integration-and-contract-tests)