Context:

I am using Tesla + Plug to ‘post’ some data to an image processing API.

This is what I am sending green=yes&blue=no&shades[]=300&shades[]=199

Notice how I have the same parameter shades with two values 300 and 199

Unfortunately the image processing API returns 'shades' is missing

The parameter is shades not shades[] so I should be sending green=yes&blue=no&shades=300&shades=199 without the square brackets.

Spotting The Problem:

plug Tesla.Middleware.FormUrlencoded,
    encode: &Plug.Conn.Query.encode/1,
    decode: &Plug.Conn.Query.decode/1

The issue is coming from plug Here

# covers non-keyword lists
  defp encode_pair(parent_field, list, encoder) when is_list(list) do
    mapper = fn
      value when is_map(value) and map_size(value) != 1 ->
        raise ArgumentError,
              "cannot encode maps inside lists when the map has 0 or more than 1 element, " <>
                "got: #{inspect(value)}"

      value ->
        # The Issue Is Here, You Can See The Square Brackets 
        [?&, encode_pair(parent_field <> "[]", value, encoder)]
    end

My Questions:

1. How do I pass my own :encode option to Tesla.Middleware.FormUrlencoded

2. Why the plug repository maintainers do not have an option for this?