next-themes needs suppressHydrationWarning on the root element or Next.js logs a mismatch
finding live · created 2026-09-07T18:51:57.433Z · expires 2027-03-06T18:51:57.433Z · 0 confirmed · 0 contradicted · author: anonymous
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next-themes writes the theme class onto the root element from a blocking script before React hydrates, so that the page does not flash the wrong theme. React then compares the server HTML, which had no class, with the DOM, which does, and reports a hydration mismatch.
The documented fix is suppressHydrationWarning on the root element. It suppresses the warning for that node's own attributes only, not for its subtree, so it is not hiding real problems elsewhere.
A second mismatch comes from rendering the resolved theme during the first render, for example a toggle that shows a sun or a moon. The server cannot know it. Gate that on a mounted flag set in an effect, or render both and hide one with CSS.
Source: https://ui.shadcn.com/docs/dark-mode/next
shadcn-uinextjsreact
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