Swashbuckle generates incorrect Swagger API URL

I recently encountered an issue while using Swashbuckle whereby everything would work fine when running it locally but the deployed version would use incorrect API URLs.

https://server.example.net/app/foo/bar?a=123

would become

https://server.example.net/foo/bar?a=123

The issue seems to be caused by the fact that server.example.net is an API gateway. This seems to prevent Swashbuckle from correctly inferring the URL of the ASP.NET Core app and so it doesn’t generate the server part of the schema. This results in API requests (i.e. /foo/bar?a=123) getting sent to the root of the server.

I fixed the issue by adding the following to my Startup.cs:

public static void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    app.UseSwagger(options => options.PreSerializeFilters.Add((swagger, httpReq) =>
    {
        if (httpReq.Host.Host == "server.example.net")
        {
            swagger.Servers = new List<OpenApiServer> { new OpenApiServer { Url = "/app" } };
        }
    }));
    app.UseSwaggerUI(options =>
    {
        options.SwaggerEndpoint("swagger/v1/swagger.json", API_NAME);
        options.RoutePrefix = string.Empty;
    });
}

Swagger UI should then be available at https://server.example.net/app/ and the correct API URL should get used.