diff --git a/drill/doctor.sh b/drill/doctor.sh
index 34769cc..464ef59 100755
--- a/drill/doctor.sh
+++ b/drill/doctor.sh
@@ -78,8 +78,28 @@ else
   inf "claudenet does not exist (a fresh host — setup-host.sh will create it)"
 fi
 
+head_ "Firewall — the box-to-box drop"
+if nft list table bridge claudebox >/dev/null 2>&1 || sudo -n nft list table bridge claudebox >/dev/null 2>&1; then
+  ok "nft bridge table 'claudebox' is present — boxes cannot reach each other"
+else
+  no "the box-to-box drop is MISSING — boxes can reach each other"
+  inf "an L3 ACL never sees frames switched between two ports of one bridge;"
+  inf "the drop is an nft BRIDGE-family rule, and without it siblings are wide open."
+  inf "fix:  sudo /usr/local/sbin/claudebox-firewall"
+  inf "      (or: sudo systemctl restart claudebox-firewall.service)"
+fi
+
 head_ "Profile — claude-dev (the NIC is the isolation contract)"
 if incus profile show claude-dev >/dev/null 2>&1; then
+  iso="$(incus profile device get claude-dev eth0 security.port_isolation 2>/dev/null)"
+  if [ "$iso" = "true" ]; then
+    ok "security.port_isolation = true — boxes cannot reach each other at L2"
+  else
+    no "security.port_isolation is NOT set — BOXES CAN REACH EACH OTHER"
+    inf "an L3 ACL cannot do this: two boxes on one bridge are on the same L2"
+    inf "segment, so their frames are switched, never routed past the ACL."
+    inf "fix:  re-run  ~/.local/share/claudebox/host/setup-host.sh"
+  fi
   for k in security.mac_filtering security.ipv4_filtering; do
     v="$(incus profile device get claude-dev eth0 "$k" 2>/dev/null)"
     if [ -z "$v" ]; then
diff --git a/drill/drill.sh b/drill/drill.sh
index 7a0e423..49f14e9 100755
--- a/drill/drill.sh
+++ b/drill/drill.sh
@@ -204,18 +204,16 @@ KEEP="${KEEP:-0}"
 # clean-ish slate and setup-host is the no-op it should be.
 # A host still carrying a previous run's phase-D mutations mints boxes with no
 # DNS, and then reports the resulting breakage as a finding. Refuse to run.
+# NOTE: dns.mode=none is now part of the SHIPPED stack (it closes the sibling
+# DNS-enumeration leak), so it is no longer "dirt" from a rehearsal — do not
+# revert it. Only the vetoed NIC filtering counts as leftover.
 dirty=""
-[ -n "$(incus network get claudenet dns.mode 2>/dev/null)" ] && dirty="dns.mode"
 [ -n "$(incus profile device get claude-dev eth0 security.ipv4_filtering 2>/dev/null)" ] && dirty="$dirty ipv4_filtering"
 [ -n "$(incus profile device get claude-dev eth0 security.mac_filtering 2>/dev/null)" ] && dirty="$dirty mac_filtering"
 if [ -n "$dirty" ]; then
-  note "this host still carries a previous run's phase-D mutations:$dirty — reverting them now"
-  incus network unset claudenet dns.mode >/dev/null 2>&1
+  note "this host carries the VETOED NIC filtering from an old rehearsal:$dirty — reverting"
   incus profile device unset claude-dev eth0 security.mac_filtering >/dev/null 2>&1
   incus profile device unset claude-dev eth0 security.ipv4_filtering >/dev/null 2>&1
-  incus network acl rule remove claude-isolate egress action=drop destination=@internal >/dev/null 2>&1
-  still="$(incus network get claudenet dns.mode 2>/dev/null)"
-  [ -n "$still" ] && { echo "drill: could not revert dns.mode ('$still'). run: bash drill/doctor.sh --fix" >&2; exit 1; }
 fi
 
 inf "clearing anything a previous run left behind…"
diff --git a/host/setup-host.sh b/host/setup-host.sh
index 4b26ccf..12a1721 100755
--- a/host/setup-host.sh
+++ b/host/setup-host.sh
@@ -70,7 +70,14 @@ fi
 sudo install -m 755 "$here/host/claudebox-firewall.sh" /usr/local/sbin/claudebox-firewall
 sudo install -m 644 "$here/host/claudebox-firewall.service" /etc/systemd/system/
 sudo systemctl daemon-reload
-sudo systemctl enable --now claudebox-firewall.service
+sudo systemctl enable claudebox-firewall.service
+# RESTART, not 'enable --now'. The unit is RemainAfterExit, so once it has run
+# it stays "active" forever — and 'enable --now' does nothing to an active unit.
+# Re-running setup-host after upgrading claudebox therefore installed the new
+# rules to /usr/local/sbin and never applied them: the host kept the old
+# firewall, silently, and the box→box hole stayed open through a release that
+# claimed to close it. Restart re-runs the script, which is idempotent by design.
+sudo systemctl restart claudebox-firewall.service
 
 # Profile
 if ! incus profile show claude-dev >/dev/null 2>&1; then
@@ -78,4 +85,13 @@ if ! incus profile show claude-dev >/dev/null 2>&1; then
 fi
 incus profile edit claude-dev < "$here/profiles/claude-dev.yaml"
 
+# The sibling drop is the one rule whose absence is invisible: everything keeps
+# working, and boxes can simply reach each other. Assert it landed.
+if nft list table bridge claudebox >/dev/null 2>&1; then
+  echo "Isolation: box-to-box drop is live (nft bridge table 'claudebox')."
+else
+  echo "WARNING: the box-to-box drop is NOT active — boxes can reach each other." >&2
+  echo "         check: sudo /usr/local/sbin/claudebox-firewall ; sudo nft list table bridge claudebox" >&2
+fi
+
 echo "Host ready. Launch with: claudebox new --name <box>"
diff --git a/profiles/claude-dev.yaml b/profiles/claude-dev.yaml
index 4b4850d..1273fdd 100644
--- a/profiles/claude-dev.yaml
+++ b/profiles/claude-dev.yaml
@@ -8,6 +8,17 @@ devices:
     type: nic
     network: claudenet
     name: eth0
+    # Boxes must not reach each other. This is the mechanism that actually does
+    # it: the kernel bridge's port-isolation flag, which stops two isolated
+    # ports exchanging frames at L2.
+    #
+    # It is not an ACL rule, and it cannot be. Incus ACLs are L3/L4, and two
+    # boxes on one bridge are on the same L2 segment — their frames are switched
+    # between ports and never traverse the netfilter path an ACL lives on. That
+    # is why the ACL's drop on 10.0.0.0/8 (which contains claudenet) and its
+    # default ingress drop BOTH looked airtight while box→box was wide open: a
+    # live probe found box A's SYN arriving at box B and B answering with a RST.
+    security.port_isolation: "true"
   root:
     type: disk
     pool: default