.net 9

Make sure you have the latest version of Scalar.AspNetCore installed: 

dotnet add package Scalar.AspNetCore

In .NET 9, you should configure it like this in Program.cs:

using Scalar.AspNetCore;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddControllers();
builder.Services.AddOpenApi(); // Registers OpenAPI generation

var app = builder.Build();

if (app.Environment.IsDevelopment())
{
    app.MapOpenApi(); // Serves the OpenAPI JSON
    app.MapScalarApiReference(options =>
    {
        options.WithTitle("My API");
        options.WithOpenApiRoutePattern("/openapi/v1.json"); // Important!
    });
}

app.MapControllers();
app.Run();

Scalar expects the OpenAPI document to be available at /openapi/v1.json by default. If your document is named differently or not mapped, Scalar will show a blank page.  

 Ensure the document is accessible at the expected path, or update the route pattern in MapScalarApiReference.