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# Hydration mismatch error in React SSR application (2025-07-20 09:15Z)

_**User**_

match the client. As a result this tree will be regenerated on the client. This can happen if a SSR-ed Client Component used:

- A server/client branch `if (typeof window !== 'undefined')`.
- Variable input such as `Date.now()` or `Math.random()` which changes each time it's called.
- Date formatting in a user's locale which doesn't match the server.
- External changing data without sending a snapshot of it along with the HTML.
- Invalid HTML tag nesting.

It can also happen if the client has a browser extension installed which messes with the HTML before React loaded.

https://react.dev/link/hydration-mismatch

  <App url="/">
    <MDXContent>
+     <hr>
      ...

---

_**Assistant**_

I can see you're encountering a hydration mismatch error in your React SSR application. This error occurs when the server-rendered HTML doesn't match what React expects to render on the client side.

Let me examine your current App component and the broader context to help identify the issue. Now let me check the routes structure to understand the issue better: Based on the hydration mismatch error you're encountering and looking at your `App.tsx` code, the issue is that you're returning a component function instead of rendering it as a JSX element. Here's the fix:

## The Problem

In your current `App.tsx`:
```tsx
const Component = matchedRoute.component
return <Component />
```

This looks correct, but based on the error message mentioning `<hr>` elements and hydration mismatch, the issue is likely that:

1. The server-side rendering and client-side rendering are producing different content
2. There might be environment-specific conditional rendering causing the mismatch

## The Solution

Here's the corrected `App.tsx`:

````tsx
import './App.css'
import routes from './routes'

function App({ url }: { url?: string }) {
  console.log('App rendered with URL:', url, import.meta.env)

  const matchedRoute = routes.find((route) => route.path === url)

  if (!matchedRoute) {
    return <div>404</div>
  }

  const Component = matchedRoute.component
  return <Component />
}

export default App
````

Wait, your current code actually looks correct. The issue might be elsewhere. Let me check if there are any environment-specific conditionals or dynamic content that could cause hydration mismatches: The hydration mismatch is likely caused by the `console.log` statement that includes `import.meta.env`, which might behave differently between server and client. Let me check the main client file: Let me check the `main-client.tsx` file: Now let me check the `main-client.tsx` file directly: Based on my analysis of the hydration mismatch error and your `App.tsx` file, the issue is clear. The problem is that you have a `console.log` statement that includes `import.meta.env`, which can have different values between server-side rendering and client-side rendering, causing a hydration mismatch.

## The Problem

The hydration mismatch occurs because:

1. **Console.log with environment variables**: The line `console.log('App rendered with URL:', url, import.meta.env)` outputs different content on server vs client
2. **MDX content generation**: The error mentions `<hr>` element, which suggests your MDX content contains elements that are being generated differently

## The Solution

Here's the corrected `App.tsx` to fix the hydration mismatch:

````tsx
import './App.css'
import routes from './routes'

function App({ url }: { url?: string }) {
  // Remove the console.log that causes hydration mismatch
  // console.log('App rendered with URL:', url, import.meta.env)

  const matchedRoute = routes.find((route) => route.path === url)

  if (!matchedRoute) {
    return <div>404</div>
  }

  const Component = matchedRoute.component
  return <Component />
}

export default App
````

## Additional Recommendations

1. **Avoid environment-specific content**: Don't use `import.meta.env`, `process.env`, or any other environment-specific values that might differ between SSR and client-side rendering in your render logic.

2. **Use useEffect for debug logging**: If you need debugging, move it to a `useEffect`:

````tsx
import { useEffect } from 'react'

function App({ url }: { url?: string }) {
  useEffect(() => {
    if (import.meta.env.DEV) {
      console.log('App rendered with URL:', url, import.meta.env)
    }
  }, [url])

  // ... rest of your component
}
````

3. **Ensure consistent route matching**: Make sure the URL normalization is consistent between server and client.

The main issue was the `console.log` statement that included `import.meta.env`, which React detected as causing different output between server and client renders, leading to the hydration mismatch error.

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