How to disable X server autostart in Debian Jessie?

The disable didn't work because the Debian /etc/X11/default-display-manager logic is winding up overriding it.

In order to make text boot the default under systemd (regardless of which distro, really):

systemctl set-default

To change back to booting to the GUI,

systemctl set-default

I confirmed those work on my Jessie VM and Slashback confirmed it on Stretch, too.

PS: You don't actually need the X server on your machine to run X clients over ssh. The X server is only needed where the display (monitor) is.

you can also add to the kernel command line, if you can't access the running system.

I don't know much about Debian, but on my Ubuntu 14.04 laptop, here is the standard procedure I use when I want to disable X and boot to command prompt for troubleshooting:

  1. Edit the file /etc/default/grub in superuser mode and set GRUB_CMDLINE_LINUX_DEFAULT="text"

Optionally, also uncomment this line: #GRUB_TERMINAL=console and then do sudo update-grub. The machine will boot in text mode.

  1. To start the graphical interface from there, you then do sudo service lightdm start #could be gdm or startx in some situations