VS.NET Intelli-sense relies on the Web.config that VS.NET looks for in the root directory of your host projects. As self-hosting projects are Console Applications they instead use App.config instead which is all ServiceStack looks at for configuring Razor.

Unfortunately as VS.NET's Razor intelli-sense is coupled to ASP.NET MVC, it requires a dummy Web.config in your self-hosted projects which just contains a copy of the Razor configuration in your App.config (which was originally populated when adding the ServiceStack.Razor NuGet Package to your project). The Web.config is otherwise benign and has no other effect other than enabling VS.NET's intelli-sense.

The @model T attribute isn't known to VS.NET intelli-sense when self-hosting which means you need to its more verbose alias:

@inherits ViewPage<T>

All ASP.NET Razor VS.NET Templates in ServiceStackVS uses the optimal Web.config template below Web.config for editing Razor pages without designer errors in VS.NET 2015:

<configuration>
    <configSections>
        <sectionGroup name="system.web.webPages.razor" type="System.Web.WebPages.Razor.Configuration.RazorWebSectionGroup, System.Web.WebPages.Razor, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35">
            <section name="host" type="System.Web.WebPages.Razor.Configuration.HostSection, System.Web.WebPages.Razor, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" requirePermission="false"/>
            <section name="pages" type="System.Web.WebPages.Razor.Configuration.RazorPagesSection, System.Web.WebPages.Razor, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" requirePermission="false"/>
        </sectionGroup>
    </configSections>

    <appSettings>
        <add key="webPages:Enabled" value="false" />
    </appSettings>

    <system.web.webPages.razor>
        <host factoryType="System.Web.Mvc.MvcWebRazorHostFactory, System.Web.Mvc, Version=5.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
        <pages pageBaseType="ServiceStack.Razor.ViewPage">
            <namespaces>
                <add namespace="System" />
                <add namespace="System.Linq" />
                <add namespace="ServiceStack" />
                <add namespace="ServiceStack.Html" />
                <add namespace="ServiceStack.Razor" />
                <add namespace="ServiceStack.Text" />
                <add namespace="ServiceStack.OrmLite" />
                <add namespace="ProjectNamespace" />
                <add namespace="ProjectNamespace.ServiceModel" />
            </namespaces>
        </pages>
    </system.web.webPages.razor>
</configuration>

The ServiceStack.Razor NuGet package uses the official Microsoft.AspNet.Razor but to minimize errors in VS.NET's Razor editor, the ServiceStack' Razor templates also reference MVC's Microsoft.AspNet.WebPages NuGet package which is only used to assist Razor intellisense as ServiceStack doesn't use or need itself.

To remove the last designer error Content pages can inherit:

@inherits ViewPage

And Typed View Pages can inherit:

@inherits ViewPage<TResponse>

Which also doesn't affect the pages behavior, but can remove the final design-time warning showing up in VS.NET's error list.

ServiceStack doesn't use the ASP.NET WebPages implementation itself, the configuration is primarily included to enable VS.NET intelli-sense and provide a way to configure the default namespaces added to Razor pages.

This can also be done in code by adding to the Config.RazorNamespaces collection, but adding them to the config section lets VS.NET knows about them so you can get proper intelli-sense.