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What a hydration mismatch means in the App Router
Hydration is React’s process for attaching event handlers to server-rendered HTML so the page becomes interactive. For an initial load, the server output and the browser’s first React tree must agree. If they do not, React may report errors such as “Text content does not match server-rendered HTML” or “hydration failed because the initial UI does not match.” The warning identifies a difference, not necessarily its cause. Next.js documents the common causes and troubleshooting options.
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In the App Router, pages and layouts are Server Components by default. A Client Component is still eligible to be prerendered into HTML during an initial load; its client boundary enables client-side features, and hydration makes that UI interactive. On later navigations, Client Components render on the client without server-rendered HTML for that navigation. Next.js explains the initial-load and navigation flow.
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Trace the first difference before changing code
- Reproduce it on the affected route. Hard reload or open the URL directly. Record the exact path, query string, and whether the route is subject to a rewrite or Proxy. Compare development and production behavior when possible.
- Compare what the server sent with what the browser parsed. Inspect the document response and the live DOM. Look for the earliest text or structural difference; the visible warning may be downstream of the first divergence.
- Rule out browser and delivery changes. Retry in a clean profile with extensions disabled, and check whether the response changes between the origin and deployed page. Extensions or an edge/CDN transformation can modify markup before React hydrates it.
- Then inspect the component and data behind that difference. Look for invalid nesting, browser-only values, time-dependent output, pathname differences, or styling setup that changes rendered markup.
Fix invalid or browser-repaired HTML
React builds a tree from your JSX, but the browser parses the HTML before hydration. Invalid nesting can cause the parsed DOM to differ from the tree React expects. Correct the structure rather than trying to silence the resulting warning.
- Do not nest a paragraph inside another paragraph, or place a
divor list inside a paragraph. - Do not nest an anchor or button inside another anchor or button.
- Check list and other container elements for invalid child structures.
These markup errors can be easy to miss when JSX is split across components, so inspect the rendered DOM as well as the component that seems to own the text.
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Make the first render deterministic
A mismatch often comes from code that produces different output on the server and in the browser. Common examples include checking window during render, reading localStorage immediately, or using a value that changes with the current time. If that value changes the first UI, the server and browser can render different trees.
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'use client'
import { useEffect, useState } from 'react'
export function SavedPreference() {
const [preference, setPreference] = useState(null)
useEffect(() => {
setPreference(window.localStorage.getItem('preference'))
}, [])
return <p>{preference ?? 'Loading preference…'}</p>
}The initial render uses the same fallback on the server and in the browser. The effect runs in the browser after hydration and then updates the preference. Keep the fallback limited to the UI that actually depends on the browser value.
Handle rewrites and pathname-dependent output
usePathname can create a mismatch when a statically prerendered route’s path differs from the URL visible in the browser because of a rewrite or Proxy. If pathname-dependent text is the divergent area, isolate it: render a stable server value and update it after mount rather than letting it change the page’s initial output. See the Next.js usePathname guidance for the relevant behavior.
Choose a deliberate strategy for time-dependent UI
Decide whether the page needs a time value in its server-visible output. If it does, use an approach that supplies a stable fallback for prerendering; Next.js documents a Suspense fallback approach for current-time access in a Client Component. Its relative-time example uses warning suppression only for the intentionally different text. Confirm the technique against the rendering behavior and versions in your application. The current-time guidance describes the fallback requirement.
If time is not needed in the initial HTML, render a stable placeholder and calculate the value after mount. That avoids asking React to hydrate text that differs by design.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Disable prerendering only for a component that needs it
For a component that fundamentally depends on browser globals, or a library that cannot render on the server, Next.js supports disabling prerendering for that specific component with dynamic(..., { ssr: false }). The component then contributes no prerendered UI. Use this as a targeted exception, not as a way to hide an unexplained mismatch or remove server rendering from a larger region.
Check browser, styling, and deployment transformations
- Extensions: Test with extensions disabled or in a clean browser profile; an extension may alter the page before hydration.
- iOS link detection: Safari may automatically turn phone numbers, dates, email addresses, or addresses into links. Next.js documents a
format-detectionmeta tag option to disable this behavior when it causes a mismatch. See the Next.js hydration error guidance. - CSS-in-JS: Verify that the library’s integration follows the official setup for the framework. A styling configuration that emits different markup or styles can affect the rendered result.
- CDN or edge processing: Check whether HTML minification or another transformation changes the response between server rendering and the browser.
Use suppressHydrationWarning only for a known exception
suppressHydrationWarning is an escape hatch for a narrow, intentional difference, such as text that is expected to vary. It applies only one level deep, and React does not patch mismatched text in that case. It silences a warning; it does not make the server and browser output equal or fix a structural mismatch. Next.js documents its limits.
Choose the smallest fix that preserves useful server rendering
| Cause or need | Preferred response | Effect on initial UI |
|---|---|---|
| Invalid nesting or duplicated interactive elements | Correct the markup. | Preserves server rendering and fixes the underlying structure. |
| Browser-only state changes rendered output | Render a stable fallback, then update the dependent UI in an effect. | Preserves server rendering; the dependent area may briefly show a fallback. |
| A component cannot render without browser APIs | Disable prerendering for that component with dynamic(..., { ssr: false }). |
That component has no prerendered UI. |
| A known, intentional text difference | Use suppressHydrationWarning narrowly. |
Silences the warning but does not patch the mismatched text. |
Check examples against the Next.js and React versions installed in your project: framework APIs and behavior can change over time.
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