First step is probably to try running it in “prod mode” locally, see if you can reproduce the issue. But after that it depends on what your production environment looks like. You’re on AWS? So you have an ELB in front of your app? Are you also running a reverse proxy like nginx?

Thanks for the answer, I am able to reproduce it locally, I am not convinced that the aws stuff is related.
Here it is:

** (exit) an exception was raised:
    ** (Protocol.UndefinedError) protocol Phoenix.HTML.Safe not implemented for %{data: nil, errors: nil, meta: %{message: "Something went wrong.", status: 500}}. This protocol is implemented for: Decimal, Integer, Float, Tuple, Phoenix.HTML.Form, Date, BitString, DateTime, NaiveDateTime, List, Time, Atom
        (phoenix_html) deps/phoenix_html/lib/phoenix_html/safe.ex:1: Phoenix.HTML.Safe.impl_for!/1
        (phoenix_html) deps/phoenix_html/lib/phoenix_html/safe.ex:15: Phoenix.HTML.Safe.to_iodata/1
        (phoenix) lib/phoenix/controller.ex:729: Phoenix.Controller.__put_render__/5
        (xd_endpoint) lib/xd_endpoint/endpoint.ex:1: XD.Endpoint.instrument/4
        (phoenix) lib/phoenix/endpoint/render_errors.ex:75: Phoenix.Endpoint.RenderErrors.instrument_render_and_send/5
        (phoenix) lib/phoenix/endpoint/render_errors.ex:62: Phoenix.Endpoint.RenderErrors.__catch__/5
        (plug_cowboy) lib/plug/cowboy/handler.ex:18: Plug.Adapters.Cowboy.Handler.upgrade/4
        (cowboy) /application/xd_umbrella/deps/cowboy/src/cowboy_protocol.erl:442: :cowboy_protocol.execute/4

the same thing happens btw if i don’t change the lenght value in Plug.Parsers

so it seems lit wants to give back an html error and not a json

So it only happens when you run with MIX_ENV=prod? I’d compare the dev and prod configs, maybe take a look at render_errors and debug_errors, off the top of my head.

this is how the assings look
I am trying to upload an file in form data

%{
  conn: %Plug.Conn{
    adapter: {Plug.Adapters.Cowboy.Conn, :...},
    assigns: %{
      kind: :error,
      layout: false,
      reason: %Plug.Parsers.RequestTooLargeError{
        message: "the request is too large. If you are willing to process larger requests, please give a :length to Plug.Parsers",
        plug_status: 413
      },
      stack: [
        {Plug.Parsers, :reduce, 7, [file: 'lib/plug/parsers.ex', line: 279]},
        {DotooEndpoint.Endpoint, :plug_builder_call, 2,
         [file: 'lib/xd_endpoint/endpoint.ex', line: 1]},
        {DotooEndpoint.Endpoint, :call, 2,
         [file: 'lib/xd_endpoint/endpoint.ex', line: 1]},
        {Plug.Adapters.Cowboy.Handler, :upgrade, 4,
         [file: 'lib/plug/cowboy/handler.ex', line: 18]},
        {:cowboy_protocol, :execute, 4,
         [
           file: '/application/xd_umbrella/deps/cowboy/src/cowboy_protocol.erl',
           line: 442
         ]}
      ]
    },
    before_send: [],
    body_params: %Plug.Conn.Unfetched{aspect: :body_params},
    cookies: %Plug.Conn.Unfetched{aspect: :cookies},
    halted: false,
    host: "localhost",
    method: "POST",
    owner: #PID<0.595.0>,
    params: %{},
    path_info: ["topics", "4gvJMZkdbBpN0e65", "post_attachment"],
    path_params: %{},
    port: 4000,
    private: %{
      phoenix_endpoint: XD.Endpoint,
      phoenix_format: "html",
      phoenix_layout: false,
      phoenix_template: "413.html",
      phoenix_view: XD.ErrorView
    },
    query_params: %{},
    query_string: "",
    remote_ip: {0, 0, 0, 0, 0, 65535, 44051, 1},
    req_cookies: %Plug.Conn.Unfetched{aspect: :cookies},
    req_headers: [
      {"authorization",
       "Bearer xxx"},
      {"content-type",
       "multipart/form-data; boundary=--------------------------773415269124190613492413"},
      {"cache-control", "no-cache"},
      {"postman-token", "88c8a61c-ed3e-4002-925e-e0caf67e338f"},
      {"user-agent", "PostmanRuntime/7.6.0"},
      {"accept", "*/*"},
      {"host", "localhost:4000"},
      {"accept-encoding", "gzip, deflate"},
      {"content-length", "31457602"},
      {"connection", "keep-alive"}
    ],
    request_path: "/topics/4gvJMZkdbBpN0e65/post_attachment",
    resp_body: nil,
    resp_cookies: %{},
    resp_headers: [{"cache-control", "max-age=0, private, must-revalidate"}],
    scheme: :http,
    script_name: [],
    secret_key_base: :...,
    state: :unset,
    status: 413
  },
  kind: :error,
  reason: %Plug.Parsers.RequestTooLargeError{
    message: "the request is too large. If you are willing to process larger requests, please give a :length to Plug.Parsers",
    plug_status: 413
  },
  stack: [
    {Plug.Parsers, :reduce, 7, [file: 'lib/plug/parsers.ex', line: 279]},
    {XDEndpoint.Endpoint, :plug_builder_call, 2,
     [file: 'lib/xd_endpoint/endpoint.ex', line: 1]},
    {XDEndpoint.Endpoint, :call, 2,
     [file: 'lib/xd_endpoint/endpoint.ex', line: 1]},
    {Plug.Adapters.Cowboy.Handler, :upgrade, 4,
     [file: 'lib/plug/cowboy/handler.ex', line: 18]},
    {:cowboy_protocol, :execute, 4,
     [
       file: '/application/xd_umbrella/deps/cowboy/src/cowboy_protocol.erl',
       line: 442
     ]}
  ],
  view_module: XDEndpoint.ErrorView,
  view_template: "413.html"
}

yes it only happens in production there it gives back the html error altough i’d prefer the json in both cases

I was able to make it arrive here:

  def render("413.json", _params) do
    IO.puts('here')

    %{
      meta: %{status: 413, message: "Too large file"},
      errors: nil,
      data: nil
    }
  end

but at the end it still put out the empty 500 error

and it logs out this:

 (Protocol.UndefinedError) protocol Phoenix.HTML.Safe not implemented for %{data: nil, errors: nil, meta: %{message: "Too large file", status: 413}}.

the same as with the 500 earlier

You are returning a structure from the render function instead of a binary. If you want it to return json then you need to convert it to json instead of an elixir map.

  def render("413.json", _params) do
    IO.puts('here')

    %{
      meta: %{status: 413, message: "Too large file"},
      errors: nil,
      data: nil
    } |> Jason.encode!()
  end

Other than that you shouldn’t need to return any body for a 413 as the status code itself already told the issue, that’s not the body’s job.

That can’t be right, I’m returning maps from my render functions and I get JSON. I couldn’t tell you why though, is it inferred from the view name?

Also, the post author says it works with MIX_ENV=dev, it just fails with prod, which made me suggest it is config related.

Edit: Googled a bit and the Phoenix View documentation shows rendering JSON from a map in an example. So it should work.

https://hexdocs.pm/phoenix/Phoenix.View.html#module-rendering

Yeah, I ended up doing this, and also the guy who wanted it figured it out that the status code is enough for him too… Thank you for the answer