I write the integration tests in ExUnit which has the benefit of the tests being ran as part of the normal test suite. I abstract away any Phoenix helpers since they are taking some shortcuts. I follow the approach of Hex: we should be testing behaviours, not the implementation, but the unit of behaviour here is an API endpoint - it’s the public contract. I try to avoid shared setups as much as possible - this way the tests are self-contained. I invest into building helpers to combat code duplication. UserUI is a module with helpers that mimic user actions in the web app (creating account, sending a message, etc.). MobileApi is a module with helpers that basically do HTTP requests but are tailored to this specific API (API-key based auth with JSON). Example test with test helpers:

defmodule MobileApiTest do
  use MyApp.Case, async: true

  alias MyApp.TestHelpers.UserUI
  alias MyApp.TestHelpers.MobileApi

  describe "GET /api/mobile/messages" do
    test "lists messages sent to the person" do
      # Setup...
      account = UserUI.create_account!()
      message1 = UserUI.create_message!(account)
      # More setup...

      assert MobileApi.get("/api/mobile/messages", api_key) == %{
               status: 200,
               body: %{
                 "messages" => [
                   %{
                     "id" => message2.id,
                     "subject" => message2.subject,
                     "sent_at" => DateTime.to_iso8601(message2.inserted_at)
                   },
                   %{
                     "id" => message1.id,
                     "subject" => message1.subject,
                     "sent_at" => DateTime.to_iso8601(message1.inserted_at)
                   }
                 ]
               }
             }
    end

    test "returns 401 on expired API key" do
      # Setup...

      assert %{
               status: 401,
               body: %{"error" => "expired_api_key"}
             } = MobileApi.get("/api/mobile/messages", api_key)
    end
  end
end

defmodule MyApp.TestHelpers.MobileApi do
  @moduledoc """
  Test helpers for interacting with the mobile app API.
  """

  alias Plug.Conn
  alias Phoenix.ConnTest
  require Phoenix.ConnTest

  # The default endpoint for testing.
  @endpoint MyApp.Endpoint

  def get(url, api_key \\ nil) do
    %{status: status, resp_body: body} =
      api_key
      |> build_conn()
      |> ConnTest.get(url)

    %{status: status, body: if(body != "", do: Jason.decode!(body), else: "")}
  end

  defp build_conn(api_key) do
    conn =
      ConnTest.build_conn()
      |> Conn.put_req_header("accept", "application/json")

    if api_key do
      conn |> Conn.put_req_header("authorization", "Bearer #{api_key}")
    else
      conn
    end
  end
end