React 19 / Next.js 15: Hydration failed because initial UI does not match server HTML
Error: Hydration failed because the initial UI does not match what was rendered on the server. Warning: Expected server HTML to contain a matching <div> in <div>.
Immediate Remediation
tsx
// Option 1: Disable SSR for dynamic / client-only components
import dynamic from 'next/dynamic';
export const UserTimestamp = dynamic(() => import('./UserTimestamp'), { ssr: false });
// Option 2: Add suppressHydrationWarning to mismatched text element
<time suppressHydrationWarning>{new Date(date).toLocaleDateString()}</time>Root Cause Analysis
The initial server-rendered HTML differed from the DOM rendered on the client. Common triggers include browser extensions, client-side dates, or reading localStorage during render.
Verification & Guardrails
- Never read window, localStorage, or dynamic timezone dates during the initial synchronous render pass.
- Use a useEffect hook or mounted state flag before showing browser-specific values.
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