diff --git a/.github/workflows/kvm-publishable-selfhosted.yml b/.github/workflows/kvm-publishable-selfhosted.yml
index b2a44c5..a802f22 100644
--- a/.github/workflows/kvm-publishable-selfhosted.yml
+++ b/.github/workflows/kvm-publishable-selfhosted.yml
@@ -57,6 +57,9 @@ jobs:
           if [[ -f /sys/module/kvm_amd/parameters/nested ]]; then
             echo "kvm_amd_nested=$(cat /sys/module/kvm_amd/parameters/nested)"
           fi
+          echo "runner_temp=${RUNNER_TEMP}"
+          df -h / /tmp /var/tmp "$RUNNER_TEMP" || true
+          df -h /nix || true
 
       - name: Run Publishable KVM Suite
         run: |
diff --git a/nix/test-cluster/run-publishable-kvm-suite.sh b/nix/test-cluster/run-publishable-kvm-suite.sh
index 326d79d..0e3b1de 100755
--- a/nix/test-cluster/run-publishable-kvm-suite.sh
+++ b/nix/test-cluster/run-publishable-kvm-suite.sh
@@ -13,6 +13,27 @@ log() {
   printf '[publishable-kvm-suite] %s\n' "$*"
 }
 
+choose_runtime_root() {
+  local candidate avail best="" best_avail=-1
+
+  for candidate in /nix/var/tmp /var/tmp /tmp "${HOME}"; do
+    mkdir -p "${candidate}" 2>/dev/null || continue
+    avail="$(df -Pk "${candidate}" 2>/dev/null | awk 'NR==2 { print $4 }')"
+    [[ -n "${avail}" ]] || continue
+    if (( avail > best_avail )); then
+      best="${candidate}"
+      best_avail="${avail}"
+    fi
+  done
+
+  [[ -n "${best}" ]] || {
+    log "no writable runtime root found"
+    return 1
+  }
+
+  printf '%s\n' "${best}/ultracloud-publishable-kvm-suite"
+}
+
 get_hostname() {
   if command -v hostname >/dev/null 2>&1; then
     hostname
@@ -21,6 +42,16 @@ get_hostname() {
   fi
 }
 
+prepare_runtime_dirs() {
+  local runtime_root
+
+  runtime_root="$(choose_runtime_root)"
+  export ULTRACLOUD_KVM_RUNTIME_ROOT="${runtime_root}"
+  export TMPDIR="${runtime_root}/tmp"
+  export XDG_CACHE_HOME="${runtime_root}/xdg-cache"
+  mkdir -p "${TMPDIR}" "${XDG_CACHE_HOME}"
+}
+
 capture_environment() {
   {
     printf 'started_at=%s\n' "$(date -Is)"
@@ -33,6 +64,9 @@ capture_environment() {
     printf 'branch=%s\n' "$(git -C "${REPO_ROOT}" branch --show-current)"
     printf 'commit=%s\n' "$(git -C "${REPO_ROOT}" rev-parse HEAD)"
     printf 'nix_version=%s\n' "$(nix --version)"
+    printf 'runtime_root=%s\n' "${ULTRACLOUD_KVM_RUNTIME_ROOT:-}"
+    printf 'tmpdir=%s\n' "${TMPDIR:-}"
+    printf 'xdg_cache_home=%s\n' "${XDG_CACHE_HOME:-}"
     printf 'kvm_present=%s\n' "$([[ -e /dev/kvm ]] && echo yes || echo no)"
     if [[ -e /dev/kvm ]]; then
       printf 'kvm_stat=%s\n' "$(stat -c '%A %U %G %t:%T' /dev/kvm)"
@@ -79,6 +113,7 @@ run_case() {
 }
 
 main() {
+  prepare_runtime_dirs
   capture_environment
 
   [[ -e /dev/kvm ]] || {