Why does React throw Error 418 in Next.js? I broke down the 5 most common hydration culprits and built a free in-browser diagnostic tool to fix them.
If you have spent any time building with Next.js App Router or React 18/19 SSR, you know this exact sinking feeling:
You run npm run dev, open your browser, and the console explodes in red text:
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 <p>.
Uncaught Error: Minified React error #418; visit https://react.dev/errors/418
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