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Use Playwright routing to make a backend failure predictable: register a route before the page sends the request, then fulfill it with an HTTP error, abort it to simulate a failed request, or switch the browser offline. Assert the resulting user-visible state with a web-first assertion. If the application is meant to recover, remove or replace the failure and exercise the same retry path a user would.
Choose the failure that matches the user scenario
Playwright can track, modify, and handle browser requests, including XHR and fetch requests, according to its Network documentation. The important distinction is whether the application receives an HTTP response or the request fails before one arrives.
| Test need | Mechanism | What the page receives |
|---|---|---|
| API returns a server error | route.fulfill() |
A controlled HTTP response, such as status 500 or 503, with a body you choose. |
| Request delivery fails | route.abort() |
A failed request rather than an HTTP response body. |
| Connectivity is broadly unavailable | Set the browser context offline | Requests fail across the page, useful for testing offline behavior. |
| Replay a repeatable API fixture | HAR replay | Recorded responses, matched strictly by URL and HTTP method. |
| Use the real response but control what the page sees | route.fetch(), then route.fulfill() |
The real response, optionally modified before delivery to the page. |
| Test a socket-dependent feature | WebSocket routing or mocking | Controlled WebSocket traffic rather than ordinary HTTP request handling. |
Playwright documents these mocking approaches in its Mock APIs guide, Network guide, and Network & Mocking guide. For a specific server-side error, fulfilling a 5xx response is usually easier to reason about than simulating a transport failure: it tests how the application handles a response with an error status. Aborting is the better fit when the request itself cannot complete.
Install a narrow route before the request happens
Register the handler before navigation or before the action that triggers the endpoint. Match only the dependency under test so unrelated calls continue normally. Playwright glob patterns match the entire URL; use a regular expression or predicate when that makes the intended match clearer.
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Use page.route() for a single page, or browserContext.route() when the rule should cover pages in that context. If both page and context handlers match, the page-level route takes precedence. Every matching handler must resolve the request by continuing, fulfilling, or aborting it. See the Network guide and BrowserContext API reference.
import { test, expect } from '@playwright/test';
test('shows an API error when the report service fails', async ({ page }) => {
await page.route('**/api/reports', route =>
route.fulfill({
status: 503,
contentType: 'application/json',
body: JSON.stringify({ message: 'Service unavailable' }),
})
);
await page.goto('/reports');
await expect(page.getByRole('alert')).toContainText('could not be loaded');
});The URL pattern and UI text in this example are illustrative; substitute the endpoint and accessible user-facing state in your application. A fulfilled response is a controlled fixture: unless the handler fetches the real response first, the backend is not called.
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Assert the state users actually encounter
A resilient test checks the visible consequence of failure, not merely that a route callback ran. Depending on the product requirement, that may be an error message, retry button, empty state, or a feature that remains usable in degraded form.
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- Use Playwright web-first assertions, which retry until the condition is met or the assertion timeout expires. The documented default assertion timeout is five seconds, but project configuration and per-assertion settings can change the actual wait. See PlaywrightAssertions.
- Avoid fixed sleeps for asynchronously rendered error states; they wait an arbitrary duration rather than checking whether the expected state appeared.
Test recovery through the real retry path
If recovery is part of the requirement, make the failure temporary, then use the same retry control a user would. Remove or replace the route only after the first failed state is established; trigger the retry and assert that normal content returns.
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test('recovers when the report service becomes available', async ({ page }) => {
let shouldFail = true;
await page.route('**/api/reports', route => {
if (shouldFail) {
return route.fulfill({ status: 503, body: 'Unavailable' });
}
return route.fulfill({
status: 200,
contentType: 'application/json',
body: JSON.stringify([{ title: 'Quarterly report' }]),
});
});
await page.goto('/reports');
await expect(page.getByRole('alert')).toBeVisible();
shouldFail = false;
await page.getByRole('button', { name: 'Retry' }).click();
await expect(page.getByText('Quarterly report')).toBeVisible();
});This pattern keeps one matching route in place and changes its behavior between attempts. Playwright’s documented network-mocking workflows also demonstrate inducing an API failure, checking the error UI, removing the mock, retrying, and checking for restored content: Network & Mocking and Network & Mocking: Playwright CLI.
Keep routed tests isolated and diagnose missing interceptions
Playwright creates isolated browser contexts for tests, helping keep browser state separate. Use a fresh test context and keep route patterns focused so one test’s failure setup does not unintentionally shape another test’s behavior. The Isolation guide explains context isolation.
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If a request does not reach a native page or context route handler, check whether a service worker is handling it. The BrowserContext API reference states that native routing does not intercept requests already handled by a service worker and recommends blocking service workers when interception is required.
Routing also disables the HTTP cache, as documented by the BrowserContext routing reference. This can make routed tests behave differently from ordinary browser traffic, so account for cache behavior when interpreting results.
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Separate application resilience from test retries
A web-first assertion waiting for an error banner is not the same as the test runner retrying a failed test. The assertion waits for a UI condition within its timeout; a test retry reruns the test. Neither, by itself, proves that the application recovers from an outage. Recovery needs an explicit test path that changes the backend condition, retries through the UI, and checks the restored state.
For intermittent failures, capture a Playwright trace to inspect what happened. The Test Configuration documentation shows trace: 'on-first-retry' as an available setting.
Version-sensitive routing behavior
Playwright’s official API pages are rolling documentation. The Route API reference identifies maxRetries as added in v1.46 and says it retries only ECONNRESET, not HTTP response codes; the current reference also shows newer options. Do not treat transport retry options as a substitute for deliberately testing an application’s response to a 5xx or failed request. For most UI failure scenarios, an explicit fulfilled error or aborted route is more direct.
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