Page loaded at: {new Date().toLocaleTimeString()}
; // Server renders "10:00:00 AM", client renders "10:00:01 AM" — mismatch } // WRONG — conditional on window export function ThemeIcon() { const isDark = typeof window !== "undefined" && window.matchMedia("(prefers-color-scheme: dark)").matches; return {isDark ? "🌙" : "☀️"}; // Server: always renders "☀️"; client: may render "🌙" — mismatch } ``` The browser console shows: ``` Error: Hydration failed because the initial UI does not match what was rendered on the server. ``` ## Why it happens The agent writes components that seem correct in isolation. It doesn't simulate two separate render passes (SSR + client hydration) and doesn't know that any value that differs between the two will break hydration. ## How to spot it - `Date.now()`, `new Date()`, `Math.random()` called directly in JSX or during render without being inside a `useEffect` or `useState` initializer. - `typeof window !== "undefined"` guards inside the render return — the server always takes the `false` branch. - `localStorage`, `sessionStorage`, or `navigator` read during render. - Mismatched HTML tags or attribute values between server and client output (visible in the React DevTools hydration warning). ## How to fix it Defer browser-only values to after mount, or generate stable values server-side and pass them as props. ```tsx // CORRECT — defer client-only value to after mount "use client"; import { useState, useEffect } from "react"; export function Timestamp() { const [time, setTime] = useStateLoading time…
; // matches server output returnPage loaded at: {time}
; } ``` ```tsx // CORRECT — theme icon: use CSS / data attribute, avoid JS render branch // Set data-theme on in a blocking inline script (not React) // Then use CSS: [data-theme="dark"] .icon { content: "🌙" } // Or use Next.js next-themes which handles SSR safely: import { useTheme } from "next-themes"; import { useEffect, useState } from "react"; export function ThemeIcon() { const { resolvedTheme } = useTheme(); const [mounted, setMounted] = useState(false); useEffect(() => setMounted(true), []); if (!mounted) return ; // stable placeholder return {resolvedTheme === "dark" ? "🌙" : "☀️"}; } ``` ```txt [ ] No Date.now() / Math.random() / new Date() called directly during render [ ] No typeof window / localStorage / navigator accessed during render [ ] Browser-only state initializes to null/undefined, set in useEffect [ ] Render a stable placeholder (same as server output) until after mount [ ] Use suppressHydrationWarning only on elements where the mismatch is intentional (e.g. timestamp) [ ] Run "next build && next start" and check browser console for hydration errors ``` ## Fix Prompt ```txt title="Fix Prompt" This component causes a React hydration mismatch because it reads browser-only values (Date, window, localStorage, Math.random) during render. Fix it: move all browser-only reads into a useEffect that sets state after mount, render a stable placeholder on first render that matches what the server produces, and never conditionally return different JSX trees based on typeof window. Explain why each change prevents the mismatch. ``` ## Test ```bash # Build and start, then check for hydration errors with curl diff next build 2>&1 | grep -i "hydrat\|mismatch" && echo "FAIL: hydration error in build" || echo "Build OK" # Runtime: open browser console after next start and look for hydration warnings next start & sleep 3 curl -s http://localhost:3000 | grep -i "data-nextjs-scroll-focus-boundary" > /dev/null && echo "Server rendered OK" ```