diff --git a/drill/doctor.sh b/drill/doctor.sh
index 9f8ba33..464ef59 100755
--- a/drill/doctor.sh
+++ b/drill/doctor.sh
@@ -91,6 +91,15 @@ 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/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