Can copy from Debian 8 host and paste into Win 8.1 guest, but the reverse does not hold true.  When trying to copy from the guest to the host, sometimes the clipboard will be blank, sometimes it will contain the previous data on the clipboard from the Debian 8 host.

C:\Windows\Temp\vdagent.log does not show anything when copying from the guest.

However, then copying from the host, it shows these lines:
2480::INFO::2016-05-16 16:21:55,774::VDAgent::handle_clipboard_release::Received clipboard release from client while clipboard is not owned by client
2480::INFO::2016-05-16 16:21:55,775::VDAgent::handle_clipboard_grab::grab type 1
2480::INFO::2016-05-16 16:22:02,154::VDAgent::handle_control_event::Control command 3

Note, I am using virt-manager.  I have seen this same behavior on Ubuntu 15.10 and 16.04.  Also, using spicec (0.12.5), I experience the same behavior.

The windows guest utilities are version 0.100 (the latest available on the website).

Below is the libvirt/qemu process:
libvirt+  6271     1 85 15:36 ?        00:46:55 qemu-system-x86_64 -enable-kvm -name win81coe -S -machine pc-i440fx-2.1,accel=kvm,usb=off -cpu Haswell,+abm,+pdpe1gb,+rdrand,+f16c,+osxsave,+pdcm,+xtpr,+tm2,+est,+smx,+vmx,+ds_cpl,+monitor,+dtes64,+pbe,+tm,+ht,+ss,+acpi,+ds,+vme -m 4096 -realtime mlock=off -smp 2,sockets=2,cores=1,threads=1 -uuid 6c2d7ef0-082f-40bf-bdf9-c3d8a674b15d -no-user-config -nodefaults -chardev socket,id=charmonitor,path=/var/lib/libvirt/qemu/win81coe.monitor,server,nowait -mon chardev=charmonitor,id=monitor,mode=control -rtc base=localtime,driftfix=slew -global kvm-pit.lost_tick_policy=discard -no-hpet -no-shutdown -global PIIX4_PM.disable_s3=1 -global PIIX4_PM.disable_s4=1 -boot menu=on,strict=on -device ich9-usb-ehci1,id=usb,bus=pci.0,addr=0x5.0x7 -device ich9-usb-uhci1,masterbus=usb.0,firstport=0,bus=pci.0,multifunction=on,addr=0x5 -device ich9-usb-uhci2,masterbus=usb.0,firstport=2,bus=pci.0,addr=0x5.0x1 -device ich9-usb-uhci3,masterbus=usb.0,firstport=4,bus=pci.0,addr=0x5.0x2 -device virtio-serial-pci,id=virtio-serial0,bus=pci.0,addr=0x6 -drive file=/VM/virt/win81coe_disk1.qcow2,if=none,id=drive-ide0-0-0,format=qcow2 -device ide-hd,bus=ide.0,unit=0,drive=drive-ide0-0-0,id=ide0-0-0,bootindex=1 -drive if=none,id=drive-ide0-0-1,readonly=on,format=raw -device ide-cd,bus=ide.0,unit=1,drive=drive-ide0-0-1,id=ide0-0-1 -drive file=/VM/virt/win81coe_disk2.qcow2,if=none,id=drive-ide0-1-0,format=qcow2 -device ide-hd,bus=ide.1,unit=0,drive=drive-ide0-1-0,id=ide0-1-0 -netdev tap,fd=24,id=hostnet0 -device rtl8139,netdev=hostnet0,id=net0,mac=52:54:00:8c:ad:f3,bus=pci.0,addr=0x3 -chardev pty,id=charserial0 -device isa-serial,chardev=charserial0,id=serial0 -chardev spicevmc,id=charchannel0,name=vdagent -device virtserialport,bus=virtio-serial0.0,nr=1,chardev=charchannel0,id=channel0,name=com.redhat.spice.0 -device usb-tablet,id=input0 -spice port=5900,addr=127.0.0.1,disable-ticketing,seamless-migration=on -device qxl-vga,id=video0,ram_size=67108864,vram_size=67108864,bus=pci.0,addr=0x2 -device intel-hda,id=sound0,bus=pci.0,addr=0x4 -device hda-duplex,id=sound0-codec0,bus=sound0.0,cad=0 -chardev spicevmc,id=charredir0,name=usbredir -device usb-redir,chardev=charredir0,id=redir0 -chardev spicevmc,id=charredir1,name=usbredir -device usb-redir,chardev=charredir1,id=redir1 -chardev spicevmc,id=charredir2,name=usbredir -device usb-redir,chardev=charredir2,id=redir2 -chardev spicevmc,id=charredir3,name=usbredir -device usb-redir,chardev=charredir3,id=redir3 -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x7 -msg timestamp=on