LowercaseDocumentFilter.cs

using System.Collections.Generic;
using Swashbuckle.SwaggerGen.Generator;
using Swashbuckle.Swagger.Model;

namespace BoundedContext.Web.Swagger
{
    public class LowercaseDocumentFilter : IDocumentFilter
    {
        public void Apply(SwaggerDocument swaggerDoc, DocumentFilterContext context)
        {
            var paths = new Dictionary<string, PathItem>(swaggerDoc.Paths);
            
            foreach (var path in paths)
            {
                var lowercase_key = path.Key.ToLower();

                if (lowercase_key != path.Key)
                {
                    swaggerDoc.Paths.Add(lowercase_key, path.Value);
                    swaggerDoc.Paths.Remove(path.Key);
                }
            }
        }
    }
}