Fixing permission denied error when starting tmux under on NixOS/WSL

I got this error when trying to run tmux from my WSL NixOS install:

error creating /mnt/wslg/runtime-dir/tmux-1001 (Permission denied)

Everything is fine under NixOS in other environments so it seems WSL specific.

I found a similar issue on unix.stackexchange suggesting that tmux’s temp directory is not correctly set. Simply setting TMUX_TMPDIR to /tmp in Home Manager does the trick.

  home.sessionVariables = {
    TMUX_TMPDIR = "/tmp";
  };