Program.cs

using Asp.Versioning;
using Scalar.AspNetCore;
using System.Reflection;

[assembly: Microsoft.AspNetCore.Mvc.ApiController]
[assembly: AssemblyDescription( "An example API" )]

var builder = WebApplication.CreateBuilder( args );

builder.Services.AddControllers();
builder.Services.AddProblemDetails();
builder.Services.AddApiVersioning(
                    options =>
                    {
                        // reporting api versions will return the headers
                        // "api-supported-versions" and "api-deprecated-versions"
                        options.ReportApiVersions = true;

                        options.Policies.Deprecate( 0.9 )
                                        .Effective( DateTimeOffset.Now )
                                        .Link( "policy.html" )
                                            .Title( "Version Deprecation Policy" )
                                            .Type( "text/html" );

                        options.Policies.Sunset( 0.9 )
                                        .Effective( DateTimeOffset.Now.AddDays( 60 ) )
                                        .Link( "policy.html" )
                                            .Title( "Version Sunset Policy" )
                                            .Type( "text/html" );
                    } )
                .AddMvc()
                .AddApiExplorer(
                    options =>
                    {
                        // add the versioned api explorer, which also adds IApiVersionDescriptionProvider service
                        // note: the specified format code will format the version as "'v'major[.minor][-status]"
                        options.GroupNameFormat = "'v'VVV";

                        // note: this option is only necessary when versioning by url segment. the SubstitutionFormat
                        // can also be used to control the format of the API version in route templates
                        options.SubstituteApiVersionInUrl = true;
                    } )
                .AddOpenApi( options => options.Document.AddScalarTransformers() );

var app = builder.Build();

if ( app.Environment.IsDevelopment() )
{
    app.MapOpenApi().WithDocumentPerVersion();
    app.MapScalarApiReference(
        options =>
        {
            var descriptions = app.DescribeApiVersions();

            for ( var i = 0; i < descriptions.Count; i++ )
            {
                var description = descriptions[i];
                var isDefault = i == descriptions.Count - 1;

                options.AddDocument( description.GroupName, description.GroupName, isDefault: isDefault );
            }
        } );
}

app.UseHttpsRedirection();
app.UseAuthorization();
app.MapControllers();
app.Run();