SKILL.md

$27

// Assert NO router requests are made (data served from cache).

// Prefer this whenever possible — it's the strongest assertion.

await act(async () => { ... }, 'no-requests')

// Expect at least one response containing this substring

await act(async () => { ... }, { includes: 'Page content' })

// Expect multiple responses (checked in order)

await act(async () => { ... }, [

  { includes: 'First response' },

  { includes: 'Second response' },

])

// Assert the same content appears in two separate responses

await act(async () => { ... }, [

  { includes: 'Repeated content' },

  { includes: 'Repeated content' },

])

// Expect at least one request, don't assert on content

await act(async () => { ... })

How includes Matching Works

The includes substring is matched against the HTTP response body. Use text content that appears literally in the rendered output (e.g. 'Dynamic content (stale time 60s)').

Extra responses that don't match any includes assertion are silently ignored — you only need to assert on the responses you care about. This keeps tests decoupled from the exact number of requests the router makes.

Each includes expectation claims exactly one response. If the same substring appears in N separate responses, provide N separate { includes: '...' } entries.

What act Does Internally

act intercepts all router requests — prefetches, navigations, and Server Actions — made during the scope:

Installs a Playwright route handler to intercept router requests

Runs your scope function

Waits for a requestIdleCallback (captures IntersectionObserver-triggered prefetches)

Fulfills buffered responses to the browser

Repeats steps 3-4 until no more requests arrive

Asserts on the responses based on the config

Responses are buffered and only forwarded to the browser after the scope function returns. This means you cannot navigate to a new page and wait for it to render within the same scope — that would deadlock. Trigger the navigation (click the link) and let act handle the rest. Read destination page content after act returns:

await act(

  async () => {

    /* toggle accordion, click link */

  },

  { includes: 'Page content' }

)

// Read content after act returns, not inside the scope

expect(await browser.elementById('my-content').text()).toBe('Page content')

LinkAccordion Pattern

Why LinkAccordion Exists

LinkAccordion controls when <Link> components enter the DOM. A Next.js <Link> triggers a prefetch when it enters the viewport (via IntersectionObserver). By hiding the Link behind a checkbox toggle, you control exactly when prefetches happen — only when you explicitly toggle the accordion inside an act scope.

// components/link-accordion.tsx

'use client'

import Link from 'next/link'

import { useState } from 'react'

export function LinkAccordion({ href, children, prefetch }) {

  const [isVisible, setIsVisible] = useState(false)

  return (

    <>

      <input

        type="checkbox"

        checked={isVisible}

        onChange={() => setIsVisible(!isVisible)}

        data-link-accordion={href}

      />

      {isVisible ? (

        <Link href={href} prefetch={prefetch}>

          {children}

        </Link>

      ) : (

        `${children} (link is hidden)`

      )}

    </>

  )

}

Standard Navigation Pattern

Always toggle the accordion and click the link inside the same act scope:

await act(

  async () => {

    // 1. Toggle accordion — Link enters DOM, triggers prefetch

    const toggle = await browser.elementByCss(

      'input[data-link-accordion="/target-page"]'

    )

    await toggle.click()

    // 2. Click the now-visible link — triggers navigation

    const link = await browser.elementByCss('a[href="/target-page"]')

    await link.click()

  },

  { includes: 'Expected page content' }

)

Common Sources of Flakiness

Using browser.back() with open accordions

Do not use browser.back() to return to a page where accordions were previously opened. BFCache restores the full React state including useState values, so previously-opened Links are immediately visible. This triggers IntersectionObserver callbacks outside any act scope — if the cached data is stale, uncontrolled re-prefetches fire and break subsequent no-requests assertions.

The only safe use of browser.back()/browser.forward() is when testing BFCache behavior specifically.

Fix: navigate forward to a fresh hub page instead. See [Hub Pages](#hub-pages).

Using visible

components outside act scopes

Any <Link> visible in the viewport can trigger a prefetch at any time via IntersectionObserver. If this happens outside an act scope, the request is uncontrolled and can interfere with subsequent assertions. Always hide links behind LinkAccordion and only toggle them inside act.

Using retry/polling timers to wait for network activity

retry(), setTimeout, or any polling pattern to wait for prefetches or navigations to settle is inherently flaky. act deterministically waits for all router requests to complete before returning.

Navigating and waiting for render in the same act scope

Responses are buffered until the scope exits. Clicking a link then reading destination content in the same scope deadlocks. Read page content after act returns instead.

Hub Pages

When you need to navigate away from a page and come back to test staleness, use "hub" pages instead of browser.back(). Each hub is a fresh page with its own LinkAccordion components that start closed.

Hub pages use connection() to ensure they are dynamically rendered. This guarantees that navigating to a hub always produces a router request, which lets act properly manage the navigation and wait for the page to fully render before continuing.

Hub page pattern:

// app/my-test/hub-a/page.tsx

import { Suspense } from 'react'

import { connection } from 'next/server'

import { LinkAccordion } from '../../components/link-accordion'

async function Content() {

  await connection()

  return <div id="hub-a-content">Hub a</div>

}

export default function Page() {

  return (

    <>

      <Suspense fallback="Loading...">

        <Content />

      </Suspense>

      <ul>

        <li>

          <LinkAccordion href="/my-test/target-page">Target page</LinkAccordion>

        </li>

      </ul>

    </>

  )

}

Target pages link to hubs via LinkAccordion too:

// On target pages, add LinkAccordion links to hub pages

<LinkAccordion href="/my-test/hub-a">Hub A</LinkAccordion>

Test flow:

// 1. Navigate to target (first visit)

await act(

  async () => {

    /* toggle accordion, click link */

  },

  { includes: 'Target content' }

)

// 2. Navigate to hub-a (fresh page, all accordions closed)

await act(

  async () => {

    const toggle = await browser.elementByCss(

      'input[data-link-accordion="/my-test/hub-a"]'

    )

    await toggle.click()

    const link = await browser.elementByCss('a[href="/my-test/hub-a"]')

    await link.click()

  },

  { includes: 'Hub a' }

)

// 3. Advance time

await page.clock.setFixedTime(startDate + 60 * 1000)

// 4. Navigate back to target from hub (controlled prefetch)

await act(async () => {

  const toggle = await browser.elementByCss(

    'input[data-link-accordion="/my-test/target-page"]'

  )

  await toggle.click()

  const link = await browser.elementByCss('a[href="/my-test/target-page"]')

  await link.click()

}, 'no-requests') // or { includes: '...' } if data is stale

Fake Clock Setup

Segment cache staleness tests use Playwright's clock API to control Date.now():

async function startBrowserWithFakeClock(url: string) {

  let page!: Playwright.Page

  const startDate = Date.now()

  const browser = await next.browser(url, {

    async beforePageLoad(p: Playwright.Page) {

      page = p

      await page.clock.install()

      await page.clock.setFixedTime(startDate)

    },

  })

  const act = createRouterAct(page)

  return { browser, page, act, startDate }

}

setFixedTime changes Date.now() return value but timers still run in real time

The segment cache uses Date.now() for staleness checks

Advancing the clock doesn't trigger IntersectionObserver — only viewport changes do

setFixedTime does NOT fire pending setTimeout/setInterval callbacks

Reference

createRouterAct: test/lib/router-act.ts

LinkAccordion: test/e2e/app-dir/segment-cache/staleness/components/link-accordion.tsx

Example tests: test/e2e/app-dir/segment-cache/staleness/

router-act