# Source: https://www.better-auth.com/llms.txt/docs/reference/errors/state_mismatch.md

# state_mismatch

The state parameter in the request doesn't match the state parameter in the cookie.



## What is it?

When an OAuth flow begins, a unique `state` value is generated and stored in a cookie.
After the user returns from the OAuth provider, this `state` is compared with the one provided in the callback.
If they don't match, the request is rejected to prevent unauthorized access.

This check exists to prevent CSRF (Cross-Site Request Forgery) and replay attacks during the OAuth flow -
basically, to make sure the callback that hits your `/api/auth/callback` endpoint really belongs to the
same browser session that started it.

## Common Causes

* The cookie wasn't set or readable during callback (common with `.vercel.app` preview domains or cross-domain issues).
* The cookie domain/path doesn't match between your app and callback route.
* The browser blocked third-party cookies (especially on Safari / iOS).
* You started the OAuth flow in one tab but finished it in another (different cookie context).
* The preview vs production domain mismatch (e.g., `preview.myapp.com` vs `myapp.com`).

## How to resolve

### Use a constant domain

* The best fix is to use a constant domain for your app and callback route.
* Avoid `.vercel.app` subdomains - browsers treat them as public suffixes, so cookies can't be shared across subdomains.

### Verify cookie configurations

* It's possible that you've configured custom cookie attributes in your auth config that can cause this issue.
* Check that cookies are not blocked by browser settings or privacy modes.
* Ensure you're starting and ending the OAuth flow in the same browser session.

### Skip state cookie check

If you know what you are doing, you can skip the state cookie check by
setting the `account.skipStateCookieCheck` option to `true` in your auth config.

<Callout type="warn">
  Please note that this is a security risk and should only be enabled if you know what you are doing.
</Callout>

### Production Debug

Head to your production site, and use your browser's DevTools â Application â Cookies to confirm:

* The state cookie is being set before redirect.
* It still exists when the OAuth provider redirects back.