Hi Loïc, Compiling lastest Cowboy from github with R15B01 and running Dialyzer, I got this: cowboy $ make dialyze Checking whether the PLT .cowboy.plt is up-to-date... yes Proceeding with analysis... cowboy_http_protocol.erl:36: Callback info about the cowboy_protocol behaviour is not available Did you noticed that? Regards, Zabrane