githubmirror/next.js

errors/blocking-prerender-random-client.mdx
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Aurora Scharff
[PP] Surface URL data during prefetching as an Instant insight with rule page (#95365)
08 июл 2026, 05:04
08 июл 2026, 05:0476628b4
---
title: Next.js encountered the unstable value Math.random() in a Client Component
kind: insight
---

<div
  style={{
    padding: '1.25rem 1.5rem',
    border: '1px solid var(--ds-gray-400)',
    borderRadius: '12px',
    background: 'var(--ds-background-200)',
    margin: '1.5rem 0 2rem',
    fontSize: '0.95rem',
    lineHeight: '1.6',
  }}
>
  This Insight is part of the [Instant
  Navigations](https://nextjs.org/blog/next-16-3-instant-navigations) feature
  introduced in Next.js 16.3. If you're new to it, start with the [Ensuring
  instant
  navigations](https://preview.nextjs.org/docs/app/guides/instant-navigation)
  guide for an overview of what instant navigations are and how Next.js
  validates them, then come back here for the specific fix.
</div>

A [Client Component](/docs/app/getting-started/server-and-client-components#using-client-components) called [`Math.random()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/random) inline during render, and the surrounding tree had no [`<Suspense>`](https://react.dev/reference/react/Suspense) boundary. Client Components are server-side rendered on first load, so Next.js can't bake an unpredictable value into the prerendered HTML. The SSR value won't match the value the client computes on hydration, so you need to choose: defer the value behind a [`<Suspense>`](https://react.dev/reference/react/Suspense) boundary so SSR can stream it, or move the call into [`useEffect`](https://react.dev/reference/react/useEffect) (or an event handler) so it only runs on the client.

The Server Component case is handled at [`Math.random()` during prerendering](/docs/messages/blocking-prerender-random). Other unpredictable client-side APIs ([`Date.now()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/now), [`crypto.randomUUID()`](https://developer.mozilla.org/en-US/docs/Web/API/Crypto/randomUUID)) have parallel error pages: [`Date.now()` in a Client Component](/docs/messages/blocking-prerender-current-time-client) and [Crypto APIs in a Client Component](/docs/messages/blocking-prerender-crypto-client).

## Ways to fix this

<FixCardGrid>
  <FixCard
    group="stream"
    title="Wrap in or move into Suspense"
    href="#wrap-in-or-move-into-suspense"
    snippets={[
      { text: '<Suspense fallback={…}>', highlight: true },
      { text: '  <RandomId />' },
      { text: '</Suspense>', highlight: true },
    ]}
  />
  <FixCard
    group="defer"
    title="Move into effect or event handler"
    href="#move-into-effect-or-event-handler"
    snippets={[
      { text: '<button onClick={() => {', highlight: true },
      { text: '  setId(Math.random())' },
      { text: '}} />' },
    ]}
  />
</FixCardGrid>

## Wrap in or move into Suspense

Choose this fix when the random value is part of the rendered output and a brief fallback during SSR is acceptable. Wrap the consuming Client Component in [`<Suspense>`](https://react.dev/reference/react/Suspense) from its parent. The fallback ships in the prerendered HTML, and Next.js fills in the real component when the browser hydrates.

### Patterns

#### Wrap from a Server Component parent

Place the [`<Suspense>`](https://react.dev/reference/react/Suspense) boundary in the Server Component that renders the Client Component. The fallback prerenders, the inner Client Component runs in the browser, and you only handle the random value once.

```jsx filename="app/page.js"
import { Suspense } from 'react'
import { Avatar } from './avatar'

export default function Page() {
  return (
    <Profile>
      <Suspense fallback={<div className="avatar-skeleton" />}>
        <Avatar />
      </Suspense>
    </Profile>
  )
}
```

```jsx filename="app/avatar.js"
'use client'

export function Avatar() {
  const color = `#${Math.random().toString(16).slice(2, 8)}`
  return <div style={{ background: color }} />
}
```

Learn more: [Streaming](https://preview.nextjs.org/docs/app/guides/streaming).

### Trade-off

The component shows the fallback during SSR and the first paint. For above-the-fold UI this can be visible. Use [`loading.js`](https://preview.nextjs.org/docs/app/api-reference/file-conventions/loading) for full-segment fallback or pick a fallback that matches the final layout so it doesn't cause a layout shift when the component hydrates. See [minimizing layout shift](https://preview.nextjs.org/docs/app/guides/streaming#cls-cumulative-layout-shift).

### Gotchas

- Any UI rendered as part of the prerender shell must be deterministic, including [`<Suspense>`](https://react.dev/reference/react/Suspense) fallbacks, [`loading.js`](https://preview.nextjs.org/docs/app/api-reference/file-conventions/loading), [`error.js`](https://preview.nextjs.org/docs/app/api-reference/file-conventions/error), [`not-found.js`](/docs/app/api-reference/file-conventions/not-found), and [`global-error.js`](https://preview.nextjs.org/docs/app/api-reference/file-conventions/error#global-error). Calling [`Math.random()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/random) in any of them raises this same error. Use stable placeholder content.
- The inner Client Component still runs during SSR, behind the boundary. If you need to guarantee the random value only runs in the browser, use [Move into effect or event handler](#move-into-effect-or-event-handler) instead.
- A [`<Suspense>`](https://react.dev/reference/react/Suspense) boundary only fixes the prerender/hydration mismatch, not client re-renders. If the component using [`Math.random()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/random) re-renders on the client (a parent state change, a context update), it produces a new value each time. To stabilize the value across re-renders, call [`Math.random()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/random) once in a [`useState`](https://react.dev/reference/react/useState) initializer or [`useRef`](https://react.dev/reference/react/useRef), or compute it on the server and pass it down as a prop.

## Move into effect or event handler

Choose this fix when the random value isn't needed for the first paint. Move the [`Math.random()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/random) call into [`useEffect`](https://react.dev/reference/react/useEffect) (for first-paint-after-mount values) or an event handler (for interaction values). The initial render uses a deterministic placeholder, so SSR and hydration agree.

### Patterns

#### Use `useEffect` for an initial value after mount

For values that should appear shortly after the page loads. Initialize state to `null` (or another deterministic stand-in) and assign the random value inside [`useEffect`](https://react.dev/reference/react/useEffect).

```jsx filename="app/avatar.js"
'use client'

import { startTransition, useEffect, useState } from 'react'

export function Avatar() {
  const [color, setColor] = useState('#eaeaea')
  useEffect(() => {
    // Wrap in startTransition so that if any component below suspends
    // during this update, React keeps the existing UI visible instead
    // of flashing the nearest outer <Suspense> fallback.
    startTransition(() => {
      setColor(`#${Math.random().toString(16).slice(2, 8)}`)
    })
  }, [])
  return <div style={{ background: color }} />
}
```

Learn more: [`useEffect`](https://react.dev/reference/react/useEffect).

#### Compute on user interaction

When the random value is in response to a click ("reshuffle", "new card"), compute it in the event handler. No SSR concern at all.

```jsx filename="app/shuffle.js"
'use client'

import { useState } from 'react'

export function Shuffle() {
  const [seed, setSeed] = useState(0)
  return (
    <button onClick={() => setSeed(Math.random())}>Shuffle ({seed})</button>
  )
}
```

#### Lazy-initialize a stable ID with `useRef`

When a component needs a stable ID for the lifetime of its mount (a tracking ID, a correlation key), produce it lazily inside a [`useRef`](https://react.dev/reference/react/useRef) getter. The ref initializer runs after mount, so SSR sees `null` and the browser fills in the value. Subsequent renders read the same ref so the ID stays stable.

```jsx filename="app/workflow.js"
'use client'

import { useRef } from 'react'

function getOrCreateId(ref) {
  if (!ref.current) {
    ref.current = Math.random().toString(36).slice(2)
  }
  return ref.current
}

export function Workflow({ onNext }) {
  const idRef = useRef(null)
  return (
    <button
      onClick={() => {
        trackEvent(getOrCreateId(idRef), 'forward')
        onNext()
      }}
    >
      Next
    </button>
  )
}
```

Learn more: [`useRef`](https://react.dev/reference/react/useRef).

### Trade-off

The user sees the placeholder briefly before the real value. For interactions the wait is invisible, but for `useEffect`-based values there's a flash of the initial state. See [Preventing flash before hydration](https://preview.nextjs.org/docs/app/guides/preventing-flash-before-hydration) for techniques that eliminate the flash.

### Gotchas

- Don't compute the random value inline during render even with `useState((/* ... */) => Math.random())`. The lazy initializer still runs during SSR and triggers the error.
- Calling [`Math.random()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/random) inline during render in a server-rendered Client Component also causes a hydration mismatch (the SSR HTML uses one value, the browser uses another). The `useEffect` and event handler patterns above avoid both the error and the mismatch.
- If the value needs to be hydration-stable (the SSR HTML and the hydrated render must match exactly), use [Wrap in or move into Suspense](#wrap-in-or-move-into-suspense) instead.
- When you call `setState` from inside [`useEffect`](https://react.dev/reference/react/useEffect), wrap it in [`startTransition`](https://react.dev/reference/react/startTransition). Cascading state updates during hydration can cause an outer [`<Suspense>`](https://react.dev/reference/react/Suspense) boundary's fallback to briefly flash. `startTransition` marks the update as non-blocking so React keeps the existing UI in place while the new value resolves.

## Other options

### Suspend with `use(io())`

When the read genuinely needs to happen per visit and you can't move it to an effect or event, call [`io()`](https://preview.nextjs.org/docs/app/api-reference/functions/io) from `next/cache` before the read with React's [`use`](https://react.dev/reference/react/use) hook. Client Components prerender on the server during SSR, where the read would otherwise be included in the static shell. `use(io())` suspends the prerender so the component is excluded from the shell and rendered on every request from the nearest [`<Suspense>`](https://react.dev/reference/react/Suspense) boundary.

```jsx filename="app/components/random-banner.js"
'use client'
import { use } from 'react'
import { io } from 'next/cache'

export function RandomBanner() {
  use(io())
  return <span>Banner #{Math.floor(Math.random() * 100)}</span>
}
```

Wrap the component in `<Suspense>` so the surrounding shell stays prerendered.

```jsx filename="app/page.js"
import { Suspense } from 'react'
import { RandomBanner } from './components/random-banner'

export default function Page() {
  return (
    <Suspense fallback={null}>
      <RandomBanner />
    </Suspense>
  )
}
```

Learn more: [`io`](https://preview.nextjs.org/docs/app/api-reference/functions/io).

## Verifying the fix

After applying a fix, reload the route and confirm the page immediately paints meaningful UI, with any `<Suspense>` fallbacks covering only the regions that stream in. A [`<Suspense>`](https://react.dev/reference/react/Suspense) boundary placed around the whole page body can pass validation with an empty shell, which defeats the point of an instant navigation.

In [`next dev`](https://preview.nextjs.org/docs/app/api-reference/cli/next#next-dev-options), the error overlay points at the failing component with file paths and line numbers. When working from a build instead, the default [`next build`](https://preview.nextjs.org/docs/app/api-reference/cli/next#next-build-options) output is more abbreviated. Run `next build --debug-prerender` for full user-frame stack traces and `next build --debug-build-paths /dashboard /settings` to iterate on specific routes.

## Why `instant = false` doesn't clear this error

This error fires from the prerender, not from instant-navigation validation. `Math.random()` returns a different value on every render, so the prerender can't bake it into a static shell regardless of the segment's `instant` config or [`experimental.instantInsights.validationLevel`](https://preview.nextjs.org/docs/app/api-reference/file-conventions/route-segment-config/instant#configuring-validation-defaults). Use one of the fixes above.

## Related Insights

- [Runtime data during prerendering](/docs/messages/blocking-prerender-runtime)
- [Uncached data during prerendering](/docs/messages/blocking-prerender-dynamic)
- [URL data in a Client Component outside of Suspense](/docs/messages/blocking-prerender-client-hook)
- [Runtime data in `generateMetadata()`](/docs/messages/blocking-prerender-metadata-runtime)
- [Uncached data in `generateMetadata()`](/docs/messages/blocking-prerender-metadata-dynamic)
- [Runtime data in `generateViewport()`](/docs/messages/blocking-prerender-viewport-runtime)
- [Uncached data in `generateViewport()`](/docs/messages/blocking-prerender-viewport-dynamic)
- [`Math.random()` while prerendering](/docs/messages/blocking-prerender-random)
- [`Date.now()` while prerendering](/docs/messages/blocking-prerender-current-time)
- [`Date.now()` in a Client Component](/docs/messages/blocking-prerender-current-time-client)
- [Crypto APIs while prerendering](/docs/messages/blocking-prerender-crypto)
- [Crypto APIs in a Client Component](/docs/messages/blocking-prerender-crypto-client)
- [Dynamic data during prefetching](/docs/messages/instant-link-prefetch-partial)
- [URL data outside of Suspense](/docs/messages/instant-shell-url-data)
- [Unrendered segment](/docs/messages/instant-unrendered-segment)