Assert receive gen_cast

It is not its own client, it is in another GenServer (I am working on someone else’s project, why it is structured like this is another question)

A little tangent to your question, but I’d consider a simple refactoring. Make the something handle_cast function call send_data directly instead of calling the client write function. It’s unconventional for a genserver to call its own client functions. If you really need your genserver to send itself a message, then I’d prefer to see that more explicit so that the handle_cast for something calls GenServer.cast(self(), {:write_value, "test"}

I think :sys.get_state can help You in test by ensuring cast are executed…

Something like that…

      MatchEngine.accept_reply request.uuid, @mock_sender, @mock_receiver
      :sys.get_state ReplyServer
      assert length(MatchEngine.list_of_replies) == 0

You can look at Why is my test failing when I use cast?