You are running a Debian in a KVM VM. You installed spice-vdagent, started and enabled the service, yet nothing works. No screen resizing, nothing.
Note: This problem still exists in Debian 12.2

Here’s the fix:

Copy spice-vdagent.desktop to User’s Autostart Directory:

cp /etc/xdg/autostart/spice-vdagent.desktop ~/.config/autostart/

Edit the Copied File

Open ~/.config/autostart/spice-vdagent.desktop with a text editor.

• Find and comment out the line containing X-GNOME-Autostart-Phase.
• Save the changes.