A Next.js hydration error means the HTML prerendered on the server does not match the page React renders in the browser on its initial render. Find the first differing element or value, make its initial output deterministic, and reserve browser-only rendering or warning suppression for cases that genuinely need it.
What a hydration error means
Next.js sends prerendered HTML to the browser. During hydration, React attaches event handlers to that HTML. The server-rendered tree and the browser’s initial render need to agree; when they do not, React reports a hydration mismatch. See the Next.js hydration error reference.
Adding "use client" does not, by itself, prevent this problem. In the App Router, Client Components are prerendered on an initial visit and then hydrated. On subsequent navigations, Next.js says Client Components are rendered entirely on the client. Pages Router pages are also prerendered by default. The mismatch causes discussed here apply across Next.js, but check your installed version’s documentation for router-specific edge cases.
Find the source of the mismatch
- Read the complete warning. Note the route and the element or text React identifies. Reproduce the route with the same data and browser or device if possible.
- Compare initial values. Check what the server sent against what the component produces on its first browser render. Do not compare only what the page displays after effects have run.
- Check HTML structure first. Look for a paragraph inside another paragraph, a
<div>inside a<p>, or nested interactive elements such as links or buttons. Invalid nesting can cause the browser to parse a DOM that differs from the intended React tree. - Search render logic for environment-dependent values. Inspect uses of
typeof window,window,localStorage, current time such asDate(), andMath.random(). These can produce different output on the server and during the first browser render. - Check for changes outside your component. Browser extensions can alter markup. On iOS, automatic link detection can turn phone numbers, email addresses, dates, or addresses into links. Also check your CSS-in-JS setup against the integration instructions for your installed version and look for CDN features that transform HTML, such as Cloudflare Auto Minify.
Make the initial render deterministic
Move browser-only reads into an effect
If a value is available only in the browser, do not use it to produce different markup during the initial render. Render a stable initial value, then read the browser API in useEffect and update state. For example, read a saved preference from localStorage after mount rather than using it to choose the server-rendered initial markup. An event handler is another suitable place for work that should happen only after user interaction.
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"use client";
import { useEffect, useState } from "react";
export function ThemeLabel() {
const [theme, setTheme] = useState("light");
useEffect(() => {
setTheme(localStorage.getItem("theme") ?? "light");
}, []);
return <p>Theme: {theme}</p>;
}The server and first browser render both show the same initial value; the browser preference is applied after hydration. This may mean a brief display of the default before the effect runs, so choose a stable initial state that is acceptable to show.
Handle current time and randomness deliberately
A timestamp can change between server prerendering and hydration, and Math.random() can return a different value in each environment. If the content can wait, keep the initial output stable and update it in an effect or event handler. Where it fits the component, use a Suspense fallback so the initial output is an agreed fallback rather than an unstable value. Next.js documents these patterns for current-time access and random values.
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When to disable prerendering for a component
If a component cannot render meaningfully without browser-only APIs, isolate that component and selectively disable its prerendering rather than disabling server rendering for a wider part of the page. Next.js describes this as a scoped option for browser-dependent components in its hydration error guidance. Use it only when a deterministic server-rendered initial state or fallback is not a reasonable fit.
Use suppression only for an unavoidable local difference
suppressHydrationWarning is a narrow escape hatch for an unavoidable difference, such as a timestamp in a specific element. The Next.js reference says it works only one level deep, and React will not patch mismatched text when suppression is set. It does not make divergent markup a sound rendering strategy; fix the source when possible.
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Investigate prerender failures during builds
A build-time prerender error is not the same thing as a universal browser-console hydration debugger. Inspect the build output and the page’s returned HTML. For a prerender error, run next build --debug-prerender to get unminified stack traces with source maps; Next.js documents this option in its prerender error guidance.
Account for markup altered by the browser or delivery path
If the component output appears consistent but the mismatch remains, test without extensions that modify the page and inspect whether a CDN is rewriting HTML. For iOS link detection, Next.js documents using a format-detection meta tag to disable automatic conversion when appropriate. If the discrepancy comes from CSS-in-JS, compare the project configuration with the official integration example for the framework and version in use.
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