Stick this in your shell configuration (e.g. ~/.bashrc or ~/.zshrc) and run ssh-reagent whenever you have an agent session running and a terminal that can't see it.

ssh-reagent

ssh-reagent () {
  for agent in /tmp/ssh-*/agent.*; do
    export SSH_AUTH_SOCK=$agent
    if ssh-add -l 2>&1 > /dev/null; then
      echo Found working SSH Agent:
      ssh-add -l
      return
    fi
  done
  echo Cannot find ssh agent - maybe you should reconnect and forward it?
}