Plug.Conn: response back from post

I have an Elixir server and I “post” some data to an endpoint in order to broadcast this data with SSE. For the moment, I haven’t connected the front-end to check, only testing with curlin the terminal. I am surprised to get back the message I am posting and can’t just set a response like “ok”.

I do:

curl -X POST -H "content-type: application/json" -d '{"test": "hello"}' https://localhost:4000/post
# -> response:
event: message
data: {"test":  "hello"}

The following Plug server is started by the app supervisor ({Plug.Cowboy, scheme: :http, plug: My.Router, options: plug_options}):

defmodule My.Router do
  import Plug.Conn
  use Plug.Router

  plug(:match)
  plug(Plug.Parsers, parsers: [:json], pass: ["text/*", "application/json"], json_decoder: Jason)
  plug(:dispatch)

  post "/post" do
    data = Jason.encode!(conn.params)
    msg = "event: message\ndata: #{data}\n\n"

    conn =
      conn
      |> put_resp_header("connection", "keep-alive")
      |> put_resp_header("content-type", "text/event-stream")
      |> send_chunked(200)

    {:ok, conn} = chunk(conn, msg)
    conn
  end

It is useless to get the message back. Furthermore, I can’t do something like send_resp(conn, 200, :ok) as I get an error message saying that the response was already sent.
Any experience?