Blazor Server .NET 9 Preview: CS1061 Error for AddAuthenticationStateSerialization on IServiceCollection

Issue:

I'm developing a Blazor Server application targeting .NET 9. I'm using standard Cookie Authentication and trying to configure authentication state handling for prerendering.

My goal is to ensure the user's identity is available during the initial render after a forced reload post-login. I attempted to use the .NET 8+ recommended service AddAuthenticationStateSerialization.

However, when I add the following line to my Program.cs:

// Correctly called directly on builder.Servicesbuilder.Services.AddAuthenticationStateSerialization();

I get the following compile error:

Error CS1061: 'IServiceCollection' does not contain a definition for 'AddAuthenticationStateSerialization' and no accessible extension method 'AddAuthenticationStateSerialization' accepting a first argument of type 'IServiceCollection' could be found (are you missing a using directive or an assembly reference?)

Program.cs Relevant Section:

Here's the relevant part of my Program.cs configuration:

// Program.cs// Necessary using directive IS present at the top:using Microsoft.AspNetCore.Components.Authorization;// Other standard using directives...var builder = WebApplication.CreateBuilder(args);// ... other services (HttpClient, DbContextFactory, Antiforgery, HttpContextAccessor etc.)// Blazor / interactive components + Auth State Handlingbuilder.Services    .AddRazorComponents()    .AddInteractiveServerComponents();     // Adds interactive server services// Trying to add state serialization here:builder.Services.AddAuthenticationStateSerialization(); // <--- ERROR CS1061 HERE// Keep cascading statebuilder.Services.AddCascadingAuthenticationState();// Conflicting/redundant lines are commented out:// //builder.Services.AddServerSideBlazor();// //builder.Services.AddScoped<AuthenticationStateProvider, ServerAuthenticationStateProvider>();// Cookie authentication setupbuilder.Services.AddAuthentication(CookieAuthenticationDefaults.AuthenticationScheme)    .AddCookie(o =>    {        o.LoginPath = "/login";        // ... other cookie options ...    });builder.Services.AddAuthorization(o => { /* ... policies ... */ });// ... rest of service registrations ...var app = builder.Build();// ... middleware pipeline (UseAuthentication, UseAuthorization, UseAntiforgery etc.) ...// Map componentsapp.MapRazorComponents<App>()   .AddInteractiveServerRenderMode(); // Server endpoints enabled// ... rest of Program.cs ...app.Run();

What I've Tried:

1. Confirmed the using Microsoft.AspNetCore.Components.Authorization;directive is present.
2. Double-checked the spelling of AddAuthenticationStateSerialization.
3. Performed multiple Clean Solution / Rebuild Solution cycles in Visual Studio.
4. Manually deleted the bin and obj folders and rebuilt.

My Question:

Why is the compiler unable to find the AddAuthenticationStateSerialization extension method on IServiceCollection in this .NET 9 Preview environment? Is this method located in a different namespace in .NET 9 previews, is it potentially removed/changed, or could there be an issue with my SDK installation or project references causing this type resolution failure?

(Optional Context): My underlying goal was to fix a common Blazor Server issue where, after a successful cookie login and a NavigationManager.NavigateTo(..., forceLoad: true), the target page with [Authorize] would redirect back to login because the prerendered component didn't recognize the auth state immediately. AddAuthenticationStateSerialization was the recommended fix I was trying to implement when I hit this compile error.