They don't have to agree on anything, just use readline. It's already used by, or optionally used by, tons of software.

Here's a non-exhaustive list that immediately come to mind:

  bash
  rc
  psql
  bc
  ftp
  GDB
  GPG
  Guile
  Lua
  NetworkManager
  Parted
  Python
  Ruby
  SQLite
  wpa_supplicant