"Failed to fetch http://localhost:57773/api/swagger.json" when running from Azure Functions with Aspire

[schuettecarsten]

Description

I've created a new Azure Functions project with new Aspire in Visual Studio 2022 using net9.0. Then, I've added NuGet depencency Microsoft.Azure.Functions.Worker.Extensions.OpenApi in version 1.5.1. When starting this, the default Function1 and Swagger UI is listed in console output of the App as expected:

Functions:
 
	Function1: [GET,POST] http://localhost:7227/api/Function1
 
	RenderOAuth2Redirect: [GET] http://localhost:7227/api/oauth2-redirect.html
 
	RenderOpenApiDocument: [GET] http://localhost:7227/api/openapi/{version}.{extension}
 
	RenderSwaggerDocument: [GET] http://localhost:7227/api/swagger.{extension}
 
	RenderSwaggerUI: [GET] http://localhost:7227/api/swagger/ui

When I now open http://localhost:7227/api/swagger/ui, I get an error Failed to load API definition. and Fetch error, Failed to fetch http://localhost:57773/api/swagger.json. It looks like the Aspire port 7227 is not picked up by Swagger UI so it tries to load definitions from a wrong address.

Steps to reproduce

See description.

[Puharesource]

I just hit this issue myself and fixed it by pointing my OpenApiServer instance to a relative path like this in the Program.cs file:

builder.Services.AddSingleton<IOpenApiConfigurationOptions>(_ => new OpenApiConfigurationOptions
{
    Info = new()
    {
        Version = "1.0.0",
        Title = "My API",
        Description = "My API description",
    },
    Servers =
    [
        new()
        {
            Url = "/"
        }
    ],
    OpenApiVersion = OpenApiVersionType.V3,
    IncludeRequestingHostName = false,
    ForceHttps = false,
    ForceHttp = false
});