# HP4001: The page was modified before React hydrated
> HP4001 (pre-hydration-mutation) means a script, extension or third-party tag changed the server HTML before React hydrated it. Find the script and move it.
Source: https://hydration.jscrate.dev/docs/issues/hp4001
Last updated: 2026-09-18
HP4001 (`pre-hydration-mutation`) means something other than React changed the
server-rendered DOM after the browser parsed it and before React hydrated it: a
script, a browser extension or a third-party tag. React then hydrates markup it
did not render. Find the script and run it after hydration, or keep it out of
React's tree.
| | |
| --- | --- |
| Code | `HP4001` |
| Name | `pre-hydration-mutation` |
| Default severity | Error |
| Group | Changes made outside React |
| What it means | A script, browser extension or third-party tag changed the server-rendered DOM before hydration. |
## What the HP4001 pre-hydration mutation finding means
hydration-proof compares the tree the browser parsed from the server HTML with
the DOM right before React's first hydration commit. React's own streaming
moves (Suspense content revealed from the stream) are undone first, so what is
left was done by other code. The finding shows the value on each side:
```text
Text changed before hydration: "Pro" became "Pro (recommended)".
```
The severity depends on where the change happened:
| Where | Severity |
| ------------------------------------------------ | -------------------------------- |
| On an element React renders, inside a React root | error |
| On ``, `` or `` | warning |
| Outside every React root | info (it cannot break hydration) |
Some changes are not reported: ``, `