How to change context variables based on tags provided by test

I want to test a endpoint that makes external api calls. What I want to do is to only do this external api call once (generate a payload) and only generate new ones when the tag ;value is provided. Here is my tring but on each test it generates a new payload anyway

defmodule TestesPayTest.BusinessRules do
  use ExUnit.Case, async: true
  use Plug.Test
  require Logger
  @create_qr_url "https://sandbox.asaas.com/api/v3/pix/qrCodes/static"
  @pix_key "ab09d852-14bd-43ac-91eb-d38b717e6aea"
 

  setup_all do
    access_token = System.get_env("ACCESS_TOKEN")
    base_fee = 0.0765
    basic_ref_fee = 0.0165
  
    payload =
    case create_payload(100) do
      {:ok, payload} -> payload
      {:error, _reason} -> nil
    end

    :ok = Ecto.Adapters.SQL.Sandbox.checkout(MyApp.Repo)
    Ecto.Adapters.SQL.Sandbox.mode(MyApp.Repo, {:shared, self()})
  
    {:ok, %{access_token: access_token, base_fee: base_fee, basic_ref_fee: basic_ref_fee}}
  end

  setup context do
    value = context[:value]
    if value do
      new_payload = 
      case create_payload(value) do
        {:ok, new_payload} ->
          Map.put(context, :payload, new_payload)

        {:error, reason} ->
          Logger.error("Falha ao criar novo payload: #{inspect(reason)}")
          nil
      end
    end

  end

tests

describe "chain invalida" do
    @tag value: 630

    test "POST /api/create_contract", %{payload: payload} do
      Logger.debug("deve ser padrao #{inspect(payload)}")
          body = Jason.encode!(%{
       "chain" => "qlqr",
       "coin" => "ETH",
       "payload" => payload
     })
        conn = conn(:post, "/api/create_contract", body)
    |> put_req_header("content-type", "application/json")
    |> TestesPay.MyRouter.call(body)
    IO.inspect(conn.resp_body)
    expected_error = %{
      "field" => "chain",
      "msg" => "rede invalida"
    }
    assert conn.status == 400
    assert Jason.decode!(conn.resp_body) == expected_error
  end
end


describe "coin invalida" do
  test "POST /api/create_contract", %{payload: payload} do
    Logger.debug("deve ser padrao #{inspect(payload)}")
        body = Jason.encode!(%{
     "chain" => "eth",
     "coin" => "xereb",
     "payload" => payload
   })
      conn = conn(:post, "/api/create_contract", body)
  |> put_req_header("content-type", "application/json")
  |> TestesPay.MyRouter.call(body)
  IO.inspect(conn.resp_body)
  expected_error = %{
    "field" => "coin",
    "msg" => "moeda invalida"
  }

  assert conn.status == 400
  assert Jason.decode!(conn.resp_body) == expected_error
end
end