Trying to use scalar with .net9, getting the error Request finished HTTP/2 GET https://localhost:7160/scalar/v1/scalar.aspnetcore.js - 404

I guess you‘re calling /scalar/v1/ in your browser. Please try it again without the trailing slash /scalar/v1.

Doing the basics by using OpenApi. When i run it I see the UI but i dont see any of my methods. In the output window i see Request finished HTTP/2 GET https://localhost:7160/scalar/v1/scalar.aspnetcore.js - 404 0 - 38.9624ms

The issue might relate the request URL.

By default, when using Scalar for interactive API documentation and to view the Scalar UI, the request URL is https://localhost:<port>/scalar/v1: (you can find the port from Properties\launchSettings.json file)

To configure Scalar, we need to install the Scalar.AspNetCore package, and the Program.cs file should like this: we need to add app.MapScalarApiReference();:

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.

builder.Services.AddControllers();
// Learn more about configuring OpenAPI at https://aka.ms/aspnet/openapi
builder.Services.AddOpenApi();

builder.Services.AddEndpointsApiExplorer();

builder.Services.AddSwaggerGen();

var app = builder.Build();

// Configure the HTTP request pipeline.
if (app.Environment.IsDevelopment())
{
    app.MapOpenApi();
    app.MapScalarApiReference();
}

if (app.Environment.IsDevelopment())
{
    app.UseSwagger();
    app.UseSwaggerUI();
};

app.UseHttpsRedirection();

app.UseAuthorization();

app.MapControllers();

app.MapProductEndpoints();

app.Run();

Besides, please make sure you add the API controller or endpoints:

controller:

[Route("api/[controller]")]
[ApiController]
public class ValuesController : ControllerBase
{
    // GET: api/<ValuesController>
    [HttpGet]
    public IEnumerable<string> Get()
    {
        return new string[] { "value1", "value2" };
    }

    // GET api/<ValuesController>/5
    [HttpGet("{id}")]
    public string Get(int id)
    {
        return "value";
    }

endpoints:

public class Product
{
    public int ID { get; set; }
    public string Name { get; set; }
}


public static class ProductEndpoints
{
    public static void MapProductEndpoints (this IEndpointRouteBuilder routes)
    {
        var group = routes.MapGroup("/api/Product").WithTags(nameof(Product));

        group.MapGet("/", () =>
        {
            return new [] { new Product() };
        })
        .WithName("GetAllProducts")
        .WithOpenApi();

        group.MapGet("/{id}", (int id) =>
        {
            //return new Product { ID = id };
        })
        .WithName("GetProductById")
        .WithOpenApi();

Then the Scalar view as below: