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To test how a Playwright UI handles a backend failure, intercept the relevant request before it is sent and decide exactly how it fails: return a controlled HTTP error, abort the request, or take the browser offline. Then assert the visible error state. If the product is supposed to recover, remove or replace the failure and use the same retry path a user would before asserting that content returns.
Choose a failure that matches the user scenario
Playwright can track, modify, and handle browser requests, including fetch and XHR requests, using its network-routing APIs. A controlled HTTP response and a failed network request are different conditions, so choose deliberately rather than treating every failure as a generic “server error.”
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| Scenario | Playwright mechanism | What the page receives |
|---|---|---|
| Backend returns an application-level error | route.fulfill() |
A response with the status and body you choose, such as a 500 or 503. |
| Request fails before an HTTP response arrives | route.abort() |
A failed request, not an HTTP response body. |
| Connectivity is broadly unavailable | Set the browser offline | A broad network failure affecting requests, useful for testing offline behavior. |
| Repeat requests from recorded traffic | HAR replay | Recorded responses, matched strictly by URL and HTTP method. |
| Use the real API response but alter what the page sees | route.fetch(), then route.fulfill() |
The live response with controlled changes. |
| Feature depends on a real-time socket | WebSocket routing or mocking | Controlled socket behavior rather than an HTTP API response. |
These mechanisms are documented in Playwright’s network guide, API mocking guide, and network-mocking guide.
Install the route before the request happens
A route only controls requests made after its handler is registered. Add it before navigation if the page requests data on load, or before the user action that triggers the request. Use page.route() for one page; use browserContext.route() when the handler should apply across a context. If both match, page-level routing takes precedence over context-level routing.
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Match the narrowest URL pattern that represents the dependency under test. Playwright glob patterns match the entire URL, so use a regular expression or predicate if that expresses the intended endpoint more clearly. Every matching handler must resolve the request by continuing, fulfilling, or aborting it.
Example: return a controlled server error
import { test, expect } from '@playwright/test';
test('shows an error when the orders API fails', async ({ page }) => {
await page.route('**/api/orders', route =>
route.fulfill({
status: 503,
contentType: 'application/json',
body: JSON.stringify({ message: 'Service unavailable' }),
})
);
await page.goto('/orders');
await expect(page.getByRole('alert')).toContainText('could not load');
await expect(page.getByRole('button', { name: 'Retry' })).toBeVisible();
});This example verifies an HTTP-level failure and the user-facing result. Adjust the endpoint and assertions to match the application; the test should check the actual error copy, empty state, degraded feature, or retry control that matters to users.
Example: simulate a failed request
await page.route('**/api/orders', route => route.abort());
await page.goto('/orders');
await expect(page.getByRole('alert')).toBeVisible();Aborting tests how the UI handles a request that fails at the network layer. Use offline mode instead when the requirement is a broader loss of connectivity rather than a single endpoint failing. Playwright demonstrates offline network state in its network-mocking documentation.
Assert the state the user experiences
Use web-first assertions such as toBeVisible() or toContainText() for asynchronous UI changes. These assertions retry until the condition is met or the assertion timeout expires. Playwright documents a five-second default assertion timeout, but project configuration and per-assertion settings can change actual timing.
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A fixed sleep only waits for a duration; it does not establish that the intended state appeared. Assert the result that represents the requirement: an error message, a retry action, an empty state, or a feature-specific degraded state. The Playwright assertions reference explains web-first assertion behavior.
Test recovery through the user’s retry path
A failure-state test proves only that the UI responds to failure. When recovery is part of the requirement, also test that the page returns to its normal state after the cause is removed and the user retries.
test('recovers when retry succeeds', async ({ page }) => {
let shouldFail = true;
await page.route('**/api/orders', route => {
if (shouldFail) {
return route.fulfill({
status: 503,
contentType: 'application/json',
body: JSON.stringify({ message: 'Service unavailable' }),
});
}
return route.fulfill({
status: 200,
contentType: 'application/json',
body: JSON.stringify([{ id: 'order-1', name: 'Recent order' }]),
});
});
await page.goto('/orders');
await expect(page.getByRole('alert')).toBeVisible();
shouldFail = false;
await page.getByRole('button', { name: 'Retry' }).click();
await expect(page.getByText('Recent order')).toBeVisible();
});The important part is the sequence: induce the failure, verify the error state, change the route’s behavior, activate the application’s retry control, and verify successful content. Playwright’s network-mocking workflow and CLI network-routing guide demonstrate this failure-and-recovery pattern.
Pick fixtures and routing scope for repeatability
Static responses for known cases
Use route.fulfill() when the test needs a predictable status and body. It keeps the API out of the test path unless the handler first fetches a live response, making the failure fixture easier to control.
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HAR replay for recorded traffic
HAR replay can reuse recorded request and response data. Its matching is strict about URL and HTTP method, so ensure the recording corresponds to the request the test actually makes. See Playwright’s API mocking guide.
Live response with a controlled patch
Use route.fetch() and then route.fulfill() when the test needs to call the real backend but alter the response seen by the page. This retains a live dependency, so it is less isolated than a static fixture.
Page or context routing
Page routing keeps a handler focused on one page. Context routing covers pages in that browser context and is useful when the dependency should be intercepted more broadly. Playwright’s BrowserContext reference documents context routing and its behavior.
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Keep routing tests isolated and diagnose missed requests
Playwright creates isolated browser contexts for tests, separating browser state. Keep each test’s route setup and expected result scoped to the scenario so one failure fixture does not unintentionally affect another. The browser-context guide explains test 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. Playwright documents that its native routing does not intercept requests handled by service workers; when interception is required, the routing reference recommends blocking service workers. Routing also disables the HTTP cache, which can make routed traffic differ from ordinary browser traffic.
Separate application resilience from test retries
A retrying assertion waits for a UI condition; a test-runner retry reruns a failed test. Neither, on its own, proves that the application can recover from a backend failure. Recovery requires an explicit test of the application’s retry or fallback behavior.
For intermittent failures, capture a trace to inspect what happened. Playwright’s test configuration documentation shows trace: 'on-first-retry' as an available setting. Avoid using transport retry settings as a substitute for deliberate failure scenarios: the Route reference says maxRetries, added in v1.46, retries only ECONNRESET, not HTTP response codes. The current reference also documents newer options, so check the API documentation for the installed Playwright version before using version-sensitive settings.
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