diff --git a/contrib/testing/README.md b/contrib/testing/README.md
index 5088ece..97f8b07 100644
--- a/contrib/testing/README.md
+++ b/contrib/testing/README.md
@@ -94,19 +94,19 @@ reserve was subtracted without 32-bit truncation:
 ./contrib/testing/queue-disk-space-check.sh
 ```
 
-The Queue failure-safety check retains one isolated message while it first
-raises the operative reserve above the filesystem's available bytes and then
-makes the spool unreadable. It verifies rejection/defer responses, restores
-the configuration and permissions through traps, and compares the retained
-message byte-for-byte after both recoveries:
+The Queue failure-safety check retains one isolated message in an 8 MiB tmpfs
+fixture while it first fills that filesystem below the operative reserve and
+then overlays the spool with an unreadable directory. It verifies
+rejection/defer responses, restores all mounts through traps, and compares the
+retained message byte-for-byte after both recoveries:
 
 ```sh
 ./contrib/testing/queue-failure-safety-check.sh
 ```
 
-It requires passwordless permission for `bongo-admin __config-read` and
-`__config-replace`, Bongo service restarts, and `stat`/`chmod` on the
-dedicated test spool. Run it only on a disposable test installation.
+It requires passwordless permission for Bongo service start/stop/restart,
+`mount`, `umount`, and `stat` on the dedicated test spool. Run it only on a
+disposable test installation.
 
 The Debian source probes and package-build commands used by BLD-14/BLD-15 are
 consolidated in `contrib/debian/build-trixie-bundle.sh`; they are not maintained
diff --git a/contrib/testing/queue-failure-safety-check.sh b/contrib/testing/queue-failure-safety-check.sh
index 6d049c1..efdd4f0 100755
--- a/contrib/testing/queue-failure-safety-check.sh
+++ b/contrib/testing/queue-failure-safety-check.sh
@@ -6,7 +6,6 @@
 set -eu
 
 queue_tool=${BONGO_QUEUE_TOOL:-/usr/bin/bongo-queuetool}
-admin_tool=${BONGO_ADMIN_TOOL:-/usr/bin/bongo-admin}
 service=${BONGO_SERVICE:-bongo.service}
 spool=${BONGO_QUEUE_SPOOL:-/var/lib/bongo/spool}
 run_as=${BONGO_TEST_RUN_AS:-bongo}
@@ -20,24 +19,30 @@ fi
 
 work=$(mktemp -d "${TMPDIR:-/tmp}/bongo-stq04.XXXXXX")
 chmod 0755 "$work"
+tmpfs="$work/tmpfs"
+blocked="$work/blocked"
+mkdir "$tmpfs" "$blocked"
+chmod 0777 "$tmpfs"
+chmod 000 "$blocked"
 queue_id=
-spool_mode=
-config_changed=0
+tmpfs_mounted=0
+spool_mounted=0
+blocked_mounted=0
 
 restore()
 {
     status=$?
-    if [ -n "$spool_mode" ]; then
-        $as_root /usr/bin/chmod "$spool_mode" "$spool" >/dev/null 2>&1 || true
+    $as_root /usr/bin/systemctl stop "$service" >/dev/null 2>&1 || true
+    if [ "$blocked_mounted" -eq 1 ]; then
+        $as_root /usr/bin/umount "$spool" >/dev/null 2>&1 || true
     fi
-    if [ "$config_changed" -eq 1 ]; then
-        $as_root "$admin_tool" __config-replace queue \
-            <"$work/queue.original" >/dev/null 2>&1 || true
-        $as_root /usr/bin/systemctl restart "$service" >/dev/null 2>&1 || true
+    if [ "$spool_mounted" -eq 1 ]; then
+        $as_root /usr/bin/umount "$spool" >/dev/null 2>&1 || true
     fi
-    if [ -n "$queue_id" ]; then
-        $as_bongo "$queue_tool" delete "$queue_id" >/dev/null 2>&1 || true
+    if [ "$tmpfs_mounted" -eq 1 ]; then
+        $as_root /usr/bin/umount "$tmpfs" >/dev/null 2>&1 || true
     fi
+    $as_root /usr/bin/systemctl start "$service" >/dev/null 2>&1 || true
     rm -rf "$work"
     exit "$status"
 }
@@ -57,6 +62,15 @@ wait_queue()
     return 1
 }
 
+$as_root /usr/bin/systemctl stop "$service"
+$as_root /usr/bin/mount -t tmpfs \
+    -o size=8m,mode=0777 bongo-stq04 "$tmpfs"
+tmpfs_mounted=1
+$as_root /usr/bin/mount --bind "$tmpfs" "$spool"
+spool_mounted=1
+$as_root /usr/bin/systemctl start "$service"
+wait_queue
+
 printf '%s\r\n' \
     'From: STQ-04 <stq04@bongo.test>' \
     'To: stqqueue@bongo.test' \
@@ -68,32 +82,16 @@ printf '%s\r\n' \
     >"$work/message.eml"
 chmod 0644 "$work/message.eml"
 
-$as_root "$admin_tool" __config-read queue >"$work/queue.original"
 queue_id=$($as_bongo "$queue_tool" hold-local "$work/message.eml")
 $as_bongo "$queue_tool" message "$queue_id" >"$work/before.eml"
 before_hash=$(sha256sum "$work/before.eml" | awk '{ print $1 }')
 
 set -- $($as_bongo "$queue_tool" space details)
-free_before=$1
-free_after=$2
+reported_before=$4
 block=$5
-python3 - "$work/queue.original" "$work/queue.full" \
-    "$free_before" "$free_after" "$block" <<'PY'
-import json
-import sys
-
-source, destination = sys.argv[1:3]
-free_before, free_after, block = map(int, sys.argv[3:])
-with open(source, "r", encoding="utf-8") as stream:
-    config = json.load(stream)
-config["minimumfreespace"] = max(free_before, free_after) + block + 1
-with open(destination, "w", encoding="utf-8") as stream:
-    json.dump(config, stream, indent=2, sort_keys=True)
-    stream.write("\n")
-PY
-
-$as_root "$admin_tool" __config-replace queue <"$work/queue.full"
-config_changed=1
+fill_blocks=$((reported_before / block + 1))
+/usr/bin/dd if=/dev/zero of="$tmpfs/fill" bs="$block" \
+    count="$fill_blocks" status=none
 $as_root /usr/bin/systemctl restart "$service"
 wait_queue
 
@@ -114,13 +112,11 @@ if ! grep -F 'Free disk space too low' "$work/full.err" >/dev/null; then
 fi
 $as_bongo "$queue_tool" message "$queue_id" >"$work/after-full.eml"
 
-$as_root "$admin_tool" __config-replace queue <"$work/queue.original"
-config_changed=0
-$as_root /usr/bin/systemctl restart "$service"
+$as_root /usr/bin/systemctl stop "$service"
+$as_root /usr/bin/mount --bind "$blocked" "$spool"
+blocked_mounted=1
+$as_root /usr/bin/systemctl start "$service"
 wait_queue
-
-spool_mode=$($as_root /usr/bin/stat -c %a "$spool")
-$as_root /usr/bin/chmod 000 "$spool"
 if $as_bongo "$queue_tool" message "$queue_id" \
         >"$work/unreadable.out" 2>"$work/unreadable.err"; then
     echo "STQ-04: Queue read a deliberately unreadable spool" >&2
@@ -131,8 +127,11 @@ if ! grep -F "Can't read queue data" "$work/unreadable.err" >/dev/null; then
     sed -n '1,20p' "$work/unreadable.err" >&2
     exit 1
 fi
-$as_root /usr/bin/chmod "$spool_mode" "$spool"
-spool_mode=
+$as_root /usr/bin/systemctl stop "$service"
+$as_root /usr/bin/umount "$spool"
+blocked_mounted=0
+$as_root /usr/bin/systemctl start "$service"
+wait_queue
 $as_bongo "$queue_tool" message "$queue_id" >"$work/after-unreadable.eml"
 
 if ! cmp -s "$work/before.eml" "$work/after-full.eml" ||
@@ -150,6 +149,6 @@ if [ "$($as_root /usr/bin/systemctl is-active "$service")" != active ]; then
     exit 1
 fi
 
-printf 'STQ-04 PASS queue=%s sha256=%s full-reported=%s spool-mode=%s\n' \
+printf 'STQ-04 PASS queue=%s sha256=%s full-reported=%s tmpfs-bytes=%s\n' \
     "$queue_id" "$after_hash" "$reported" \
-    "$($as_root /usr/bin/stat -c %a "$spool")"
+    "$($as_root /usr/bin/stat -f -c %s "$spool")"