diff --git a/spinifex/services/vpcd/topology.go b/spinifex/services/vpcd/topology.go
index 431356294..b83370bd4 100644
--- a/spinifex/services/vpcd/topology.go
+++ b/spinifex/services/vpcd/topology.go
@@ -8,6 +8,7 @@ import (
 	"net"
 	"strconv"
 	"strings"
+	"time"
 
 	"github.com/mulgadc/spinifex/spinifex/services/vpcd/dhcp"
 	"github.com/mulgadc/spinifex/spinifex/services/vpcd/nbdb"
@@ -16,6 +17,39 @@ import (
 	"github.com/nats-io/nats.go"
 )
 
+// waitForFlowsHV shells out to `ovn-nbctl --wait=hv sync`, which bumps
+// NB_Global.nb_cfg and blocks until every connected chassis has acknowledged
+// the new sequence number — i.e. ovn-northd has compiled NB -> SB and
+// ovn-controller has installed the resulting flows. Used after IGW attach
+// so newly-launched VMs aren't unreachable while their gateway chassis is
+// still catching up (mulga-siv-105).
+//
+// Bounded by ovn-nbctl --timeout=30 (seconds). On overrun we log a WARN
+// and return nil — the caller continues. In practice flows converge within
+// seconds; a 30s overrun means something OVN-side is wedged enough that
+// failing VPC create wouldn't improve things.
+//
+// Declared as a var so tests can stub it.
+var waitForFlowsHV = func() error {
+	start := time.Now()
+	cmd := sudoCommand("ovn-nbctl",
+		"--no-leader-only",
+		"--timeout=30",
+		"--wait=hv",
+		"sync",
+	)
+	if out, err := cmd.CombinedOutput(); err != nil {
+		slog.Warn("vpcd: OVN flows-ready barrier overran; continuing without confirmation",
+			"elapsed", time.Since(start),
+			"err", err,
+			"output", strings.TrimSpace(string(out)),
+		)
+		return nil
+	}
+	slog.Debug("vpcd: OVN flows-ready barrier complete", "elapsed", time.Since(start))
+	return nil
+}
+
 // NATS topics for VPC lifecycle events published by the daemon.
 const (
 	TopicVPCCreate        = "vpc.create"
@@ -1174,6 +1208,14 @@ func (h *TopologyHandler) handleIGWAttach(msg *nats.Msg) {
 		"wan_gateway", wanGateway,
 		"chassis_count", len(h.chassisNames),
 	)
+
+	// Block until ovn-northd compiles + the gateway chassis has installed
+	// flows for this VPC. Without this, a subsequent RunInstances ->
+	// vpc.add-nat can complete while the datapath is still dark, and the
+	// new VM is unreachable on its public IP for the duration of the
+	// flow-install latency (typically seconds; observed up to minutes when
+	// the chassis was loaded during CI). mulga-siv-105.
+	_ = waitForFlowsHV()
 	respond(msg, nil)
 }
 
diff --git a/spinifex/services/vpcd/topology_test.go b/spinifex/services/vpcd/topology_test.go
index 587f7838a..81c26c749 100644
--- a/spinifex/services/vpcd/topology_test.go
+++ b/spinifex/services/vpcd/topology_test.go
@@ -877,6 +877,49 @@ func TestTopologyHandler_IGWAttach(t *testing.T) {
 	}
 }
 
+// TestTopologyHandler_IGWAttach_InvokesFlowBarrier asserts handleIGWAttach
+// calls waitForFlowsHV after the OVN writes complete (mulga-siv-105). The
+// barrier itself shells out to ovn-nbctl --wait=hv sync; stub it here so
+// the test doesn't depend on a live OVN.
+func TestTopologyHandler_IGWAttach_InvokesFlowBarrier(t *testing.T) {
+	orig := waitForFlowsHV
+	var called int
+	waitForFlowsHV = func() error { called++; return nil }
+	defer func() { waitForFlowsHV = orig }()
+
+	_, nc := startTestNATS(t)
+	mock := NewMockOVNClient()
+	_ = mock.Connect(context.Background())
+	ctx := context.Background()
+
+	topo := NewTopologyHandler(mock)
+	subs, err := topo.Subscribe(nc)
+	require.NoError(t, err)
+	defer func() {
+		for _, s := range subs {
+			_ = s.Unsubscribe()
+		}
+	}()
+
+	require.NoError(t, mock.CreateLogicalRouter(ctx, &nbdb.LogicalRouter{
+		Name: "vpc-vpc-barrier1",
+		ExternalIDs: map[string]string{
+			"spinifex:vpc_id": "vpc-barrier1",
+			"spinifex:cidr":   "10.0.0.0/16",
+		},
+	}))
+
+	evt := types.IGWEvent{InternetGatewayId: "igw-barrier1", VpcId: "vpc-barrier1"}
+	data, _ := json.Marshal(evt)
+	resp, err := nc.Request(TopicIGWAttach, data, 5_000_000_000)
+	require.NoError(t, err)
+	assertSuccess(t, resp, "attach IGW")
+
+	if called != 1 {
+		t.Errorf("expected waitForFlowsHV to run exactly once after IGW attach, got %d", called)
+	}
+}
+
 func TestTopologyHandler_IGWAttach_WithExternalPool(t *testing.T) {
 	_, nc := startTestNATS(t)
 	mock := NewMockOVNClient()