Based on your description, I would say that the 'something inside' is either redirecting stderr to stdout, or it writes the prompt on stdout to begin with.

Standard shells write the prompt to stderr, which you can prove for yourself by running:

You will see a prompt, plus command line input, but no command generates any visible output, because it goes into the file.

You will need to determine what is generating the prompt, and see if you can fix it to write to stderr.