NotFoundPage does not work in .NET 10

[MarvinKlein1508]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

I was trying to use the new NotFoundPage parameter within .NET 10 which ist described here:
https://learn.microsoft.com/en-us/aspnet/core/release-notes/aspnetcore-10.0?view=aspnetcore-9.0#blazor-router-has-a-notfoundpage-parameter

However it does not work in fresh new BlazorWebApp. Please see this video for a short demonstration:

2025-06-18.22-49-03.mp4

Expected Behavior

I expect the UI from NotFound.razor to show up.

Steps To Reproduce

1. Create new project
2. Open a page via URL which does not exist (for example /test)

Exceptions (if any)

No response

.NET Version

10.0.100-preview.5.25277.114

Anything else?

No response

[javiercn]

@MarvinKlein1508 thanks for contacting us.

@MarvinKlein1508 The UseStatusCodePagesWithReexecute middleware is missing on the preview5 template, I believe.

If you type a url that doesn't exist on the browser, the request doesn't even make it to Blazor. UseStatusCodePagesWithReexecute is meant to handle the situation in that case.

[javiercn]

@ilonatommy can confirm.

[MarvinKlein1508]

@javiercn yes, adding this call fixes this for me:

app.UseStatusCodePagesWithReExecute("/not-found", "?statusCode={0}");

This needs to be added to the program before:

app.UseAntiforgery();

Otherwise you'll get this exception:

InvalidOperationException: Endpoint /not-found (/not-found) contains anti-forgery metadata, but a middleware was not found that supports anti-forgery.
Configure your application startup by adding app.UseAntiforgery() in the application startup code. If there are calls to app.UseRouting() and app.UseEndpoints(...), the call to app.UseAntiforgery() must go between them. Calls to app.UseAntiforgery() must be placed after calls to app.UseAuthentication() and app.UseAuthorization(). 

[ilonatommy]

@MarvinKlein1508, thank you for your report. @javiercn is right, the template change got merged in preview 6: #62067.

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