Convert Blazor Server App to Blazor Web App

This skill helps an agent convert a pre-.NET 8 Blazor Server app into a .NET 8+ Blazor Web App. The old hosting model uses AddServerSideBlazor/MapBlazorHub with a _Host.cshtml Razor Page as the entry point. The new Blazor Web App model uses AddRazorComponents/MapRazorComponents with an App.razor root component, enabling per-component render modes, enhanced navigation, streaming rendering, and other .NET 8+ features. The converted app uses InteractiveServer render mode to preserve existing interactive behavior.

When to Use

• Migrating a Blazor Server app from .NET 6 or .NET 7 to .NET 8+
• App currently uses AddServerSideBlazor() and MapBlazorHub() in Program.cs (or Startup.cs)
• App uses Pages/_Host.cshtml (or _Host.razor) as the host page with Component Tag Helpers
• Want to adopt new Blazor Web App features while keeping interactive server rendering

When Not to Use

• The app already uses AddRazorComponents and MapRazorComponents. It is already a Blazor Web App — no conversion is needed. Stop here and tell the user the app is already using the Blazor Web App model.
• Blazor WebAssembly or hosted Blazor WebAssembly app — these have a different migration path
• The app should stay on the legacy Blazor Server hosting model (just update TFM and packages)
• The app targets .NET Framework — it must be migrated to .NET first

Inputs

Input	Required	Description
Blazor Server project	Yes	The .csproj and source files of the Blazor Server app
Target framework	Yes	.NET 8 or later (e.g., net8.0, net9.0, net10.0)
Program.cs or Startup.cs	Yes	The app's service and middleware configuration
_Host.cshtml location	Recommended	Usually Pages/_Host.cshtml; may be _Host.razor in some projects

Workflow

Commit strategy: Commit after each logical step so the migration is reviewable and bisectable.

Step 1: Update the project file

Update the .csproj file:

1. Change the Target Framework Moniker (TFM) to the target version:

   <TargetFramework>net8.0</TargetFramework>
   

2. Update all Microsoft.AspNetCore.*, Microsoft.EntityFrameworkCore.*, Microsoft.Extensions.*, and System.Net.Http.Json package references to the matching version.

For non-Blazor project file changes (nullable reference types, implicit usings, HTTP/3 support, etc.), see the general ASP.NET Core migration guide.

Step 2: Create Routes.razor from App.razor

The old App.razor contains the <Router> component. This content moves to a new Routes.razor file so that App.razor can become the root HTML document component.

1. Create a new file Routes.razor in the project root.
2. Move the entire content of App.razor into Routes.razor.
3. If the content is wrapped in <CascadingAuthenticationState>, remove that wrapper (it will be replaced by a service in Step 5).
4. Leave App.razor empty for the next step.

The resulting Routes.razor should look similar to:

<Router AppAssembly="@typeof(Program).Assembly">
    <Found Context="routeData">
        <RouteView RouteData="@routeData" DefaultLayout="@typeof(MainLayout)" />
        <FocusOnNavigate RouteData="@routeData" Selector="h1" />
    </Found>
    <NotFound>
        <LayoutView Layout="@typeof(MainLayout)">
            <p>Sorry, there's nothing at this address.</p>
        </LayoutView>
    </NotFound>
</Router>

If the app uses <AuthorizeRouteView> instead of <RouteView>, keep it — it works the same way in Blazor Web Apps.

Step 3: Convert _Host.cshtml to App.razor

Move the HTML shell from Pages/_Host.cshtml into the now-empty App.razor and transform it from a Razor Page into a Razor component:

1. Remove Razor Page directives — delete @page "/", @using Microsoft.AspNetCore.Components.Web, @namespace, and @addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers.

2. Add component injection — if using environment-conditional error UI, add:

   @inject IHostEnvironment Env
   

3. Fix the base tag — replace <base href="~/" /> with <base href="/" />.

4. Replace HeadOutlet Component Tag Helper — replace:

   <component type="typeof(HeadOutlet)" render-mode="ServerPrerendered" />
   

   with:

   <HeadOutlet @rendermode="InteractiveServer" />
   

5. Replace App Component Tag Helper with Routes — replace:

   <component type="typeof(App)" render-mode="ServerPrerendered" />
   

   with:

   <Routes @rendermode="InteractiveServer" />
   

6. Replace Environment Tag Helpers — replace:

   <environment include="Staging,Production">
       An error has occurred. This application may no longer respond until reloaded.
   </environment>
   <environment include="Development">
       An unhandled exception has occurred. See browser dev tools for details.
   </environment>
   

   with:

   @if (Env.IsDevelopment())
   {
       <text>
           An unhandled exception has occurred. See browser dev tools for details.
       </text>
   }
   else
   {
       <text>
           An error has occurred. This app may no longer respond until reloaded.
       </text>
   }
   

7. Update the Blazor script — replace:

   <script src="_framework/blazor.server.js"></script>
   

   with:

   <script src="_framework/blazor.web.js"></script>
   

8. Add render mode import — add to _Imports.razor:

   @using static Microsoft.AspNetCore.Components.Web.RenderMode
   

9. Delete Pages/_Host.cshtml (and Pages/_Host.cshtml.cs if it exists).

Prerendering note: If the original app used render-mode="Server" (not "ServerPrerendered"), prerendering was disabled. Preserve this by using new InteractiveServerRenderMode(prerender: false) instead of InteractiveServer for both HeadOutlet and Routes.

Step 4: Update Program.cs

Make the following changes to Program.cs (or Startup.cs if the app uses the older hosting pattern):

1. Replace Blazor Server services — replace:

   builder.Services.AddServerSideBlazor();
   

   with:

   builder.Services.AddRazorComponents()
       .AddInteractiveServerComponents();
   

   If AddServerSideBlazor had options configured (e.g., circuit options, hub options, detailed errors), migrate them to AddInteractiveServerComponents:

   // Old:
   builder.Services.AddServerSideBlazor(options =>
   {
       options.DetailedErrors = true;
       options.DisconnectedCircuitRetentionPeriod = TimeSpan.FromMinutes(10);
   });
   
   // New:
   builder.Services.AddRazorComponents()
       .AddInteractiveServerComponents(options =>
       {
           options.DetailedErrors = true;
           options.DisconnectedCircuitRetentionPeriod = TimeSpan.FromMinutes(10);
       });
   

2. Replace Blazor endpoint mapping — replace:

   app.MapBlazorHub();
   

   with:

   app.MapRazorComponents<App>()
       .AddInteractiveServerRenderMode();
   

   Ensure there is a using statement for the project's root namespace so that App resolves to the App.razor component.

3. Remove the fallback route — delete:

   app.MapFallbackToPage("/_Host");
   

4. Remove explicit routing middleware — delete if present:

   app.UseRouting();
   

   Endpoint routing is the default and explicit UseRouting() is no longer needed.

5. Add antiforgery middleware — add after UseAuthentication/UseAuthorization if present:

   app.UseAntiforgery();
   

   AddRazorComponents registers antiforgery services automatically, but the middleware must be explicitly added to the pipeline. Without it, form POST requests fail with 400 errors.

Step 5: Migrate CascadingAuthenticationState (if present)

If the app used <CascadingAuthenticationState> to wrap the router:

1. Remove the <CascadingAuthenticationState> component wrapper (already done in Step 2 if following this workflow).
2. Add the cascading authentication state service in Program.cs:

   builder.Services.AddCascadingAuthenticationState();
   

The component wrapper approach does not work across render mode boundaries in Blazor Web Apps. The service-based approach provides Task<AuthenticationState> as a cascading value to all components regardless of render mode.

Step 6: Recommended improvements (optional)

These are optional modernization improvements — not required for the conversion to work. If you suggest any of these, state explicitly that they are optional.

• Replace UseStaticFiles with MapStaticAssets (.NET 9+): app.MapStaticAssets() provides optimized static file serving with fingerprinting, pre-compression, and content-based ETags. See MapStaticAssets documentation.
• Add @attribute [StreamRendering] to pages with async data loading (OnInitializedAsync) for improved perceived performance. The page renders its initial synchronous content immediately and re-renders when async data arrives.
• Update CSS isolation bundle reference if the <link> tag referenced a _Host assembly name; ensure it matches the project's actual assembly name: <link href="{AssemblyName}.styles.css" rel="stylesheet" />.
• For other non-Blazor improvements (minimal hosting, HTTP/3, output caching, etc.), see the general ASP.NET Core migration guide.

Step 7: Verify the migration

1. Build the project targeting the new framework. Confirm no compile errors.
2. Search for remaining references to removed APIs:
   • AddServerSideBlazor
   • MapBlazorHub
   • MapFallbackToPage
   • blazor.server.js
   • _Host.cshtml
3. Run the app and verify:
   • Pages load and render correctly
   • Interactive features work (forms, event handlers, SignalR circuits)
   • Navigation between pages works
   • Authentication and authorization flows work if present
4. Run existing tests.

Validation

• No references to AddServerSideBlazor remain
• No references to MapBlazorHub remain
• No references to MapFallbackToPage("/_Host") remain
• No references to blazor.server.js remain
• Pages/_Host.cshtml has been deleted
• App.razor serves as the root component with a full HTML document structure
• Routes.razor contains the <Router> configuration
• Program.cs uses AddRazorComponents().AddInteractiveServerComponents()
• Program.cs uses MapRazorComponents<App>().AddInteractiveServerRenderMode()
• app.UseAntiforgery() is present in the middleware pipeline
• If the app used <CascadingAuthenticationState>, it has been replaced with AddCascadingAuthenticationState() service registration
• App builds and runs successfully on the target framework

Common Pitfalls

Pitfall	Solution
Missing UseAntiforgery() middleware	AddRazorComponents registers antiforgery services, but the middleware must be explicitly added. Place app.UseAntiforgery() after UseAuthentication/UseAuthorization. Without it, form POST requests fail with 400 errors.
Forgetting to replace blazor.server.js with blazor.web.js	The old script does not work with the Blazor Web App model. Replace all references to _framework/blazor.server.js with _framework/blazor.web.js.
Not removing <CascadingAuthenticationState> wrapper	The component wrapper does not work across render mode boundaries in Blazor Web Apps. Use builder.Services.AddCascadingAuthenticationState() instead.
Leaving app.UseRouting() in the pipeline	Explicit UseRouting() is no longer needed and can interfere with endpoint routing. Remove it unless other middleware specifically requires it.
Using InteractiveServer when prerendering was disabled	If the original app used render-mode="Server" (not "ServerPrerendered"), use new InteractiveServerRenderMode(prerender: false) to preserve the same behavior. Using InteractiveServer enables prerendering which can cause unexpected issues with components that depend on JS interop during initialization.
Not migrating AddServerSideBlazor circuit options	If circuit options, hub options, or detailed error settings were configured, migrate them to AddInteractiveServerComponents(options => { ... }). Otherwise those settings are silently lost.
UseAntiforgery() placed before authentication middleware	The antiforgery middleware must be placed after UseAuthentication and UseAuthorization. Placing it before causes antiforgery validation to run before the user identity is established.
CSS isolation bundle link has wrong assembly name	If the <link href="{Name}.styles.css"> tag referenced the old project name, update it to match the current assembly name.

More Info

• Convert a Blazor Server app into a Blazor Web App — the official step-by-step migration guide
• ASP.NET Core Blazor render modes — understanding InteractiveServer, InteractiveWebAssembly, and InteractiveAuto
• Migrate CascadingAuthenticationState to services — replacing the component wrapper with a service
• MapStaticAssets — optimized static file serving in .NET 9+
• Migrate from ASP.NET Core 7.0 to 8.0 — general migration guide for all ASP.NET Core changes
• Stream rendering with Blazor — @attribute [StreamRendering] for async data loading
• Cascading values and render mode boundaries — why cascading parameters do not cross render mode boundaries