diff --git a/spinifex/handlers/elbv2/service_impl.go b/spinifex/handlers/elbv2/service_impl.go
index c21f30dfd..3eee3bfb1 100644
--- a/spinifex/handlers/elbv2/service_impl.go
+++ b/spinifex/handlers/elbv2/service_impl.go
@@ -895,12 +895,26 @@ func (s *ELBv2ServiceImpl) LBAgentHeartbeat(input *LBAgentHeartbeatInput, accoun
 	}
 
 	now := time.Now().UTC()
+	heartbeatStale := now.Sub(lb.LastHeartbeat) >= heartbeatPersistInterval
+	lb.LastHeartbeat = now
 
-	// Only persist to KV on state transitions or when the stored heartbeat
-	// timestamp is stale. This avoids writing the full record (including
-	// ConfigText) every 5 seconds when nothing changed.
-	if stateChanged || now.Sub(lb.LastHeartbeat) >= heartbeatPersistInterval {
-		lb.LastHeartbeat = now
+	switch {
+	case stateChanged:
+		// Activation is the LB's readiness signal: InstanceID is set and the
+		// agent is alive to consume config. The reactive updateStoredConfig
+		// calls during the create burst no-op while the VM is still launching
+		// (InstanceID empty), so build the data-plane config now from the
+		// listeners/targets created during provisioning. updateStoredConfig
+		// persists the record (state, config, hash, health targets) in one
+		// write, so the ConfigHash returned below is fresh.
+		if err := s.updateStoredConfig(lb); err != nil {
+			slog.Error("LBAgentHeartbeat: failed to build config on activation", "lbId", lbID, "err", err)
+			return nil, errors.New(awserrors.ErrorServerInternal)
+		}
+	case heartbeatStale:
+		// Steady state: refresh the heartbeat timestamp only. Avoids writing
+		// the full record (including ConfigText) every 5 seconds when nothing
+		// changed.
 		if err := s.store.PutLoadBalancer(lb); err != nil {
 			slog.Error("LBAgentHeartbeat: failed to persist LB", "lbId", lbID, "err", err)
 			return nil, errors.New(awserrors.ErrorServerInternal)
diff --git a/spinifex/handlers/elbv2/service_impl_test.go b/spinifex/handlers/elbv2/service_impl_test.go
index 0696c6f48..c53dc1e0a 100644
--- a/spinifex/handlers/elbv2/service_impl_test.go
+++ b/spinifex/handlers/elbv2/service_impl_test.go
@@ -2008,6 +2008,71 @@ func TestLBAgentHeartbeat_ProcessesHealthReport(t *testing.T) {
 	assert.Equal(t, TargetHealthHealthy, stored.Targets[0].HealthState)
 }
 
+// TestLBAgentHeartbeat_BuildsConfigOnActivation covers the create-burst race:
+// the reactive updateStoredConfig calls during CreateListener/RegisterTargets
+// no-op while the LB VM is still launching (InstanceID empty). The agent's
+// first heartbeat (provisioning→active) is the readiness signal that must build
+// the data-plane config from the listeners/targets created during provisioning,
+// so the agent receives a non-empty ConfigHash + backend list on its first poll.
+func TestLBAgentHeartbeat_BuildsConfigOnActivation(t *testing.T) {
+	svc := setupTestService(t)
+
+	lb := &LoadBalancerRecord{
+		LoadBalancerArn: "arn:aws:elasticloadbalancing:us-east-1:123456789012:loadbalancer/net/act-lb/lb-act1",
+		LoadBalancerID:  "lb-act1",
+		Name:            "act-lb",
+		Type:            LoadBalancerTypeNetwork,
+		Scheme:          SchemeInternal,
+		State:           StateProvisioning,
+		InstanceID:      "i-sys-act1",
+		VPCIP:           "10.0.1.100",
+		AccountID:       testAccountID,
+	}
+	require.NoError(t, svc.store.PutLoadBalancer(lb))
+
+	tg := &TargetGroupRecord{
+		TargetGroupArn: "arn:aws:elasticloadbalancing:us-east-1:123456789012:targetgroup/act-tg/tg-act1",
+		TargetGroupID:  "tg-act1",
+		Protocol:       ProtocolTCP,
+		Port:           80,
+		HealthCheck:    DefaultHealthCheck(),
+		Targets: []Target{
+			{Id: "i-target1", Port: 80, HealthState: TargetHealthInitial, PrivateIP: "10.0.1.20"},
+		},
+		AccountID: testAccountID,
+	}
+	require.NoError(t, svc.store.PutTargetGroup(tg))
+
+	require.NoError(t, svc.store.PutListener(&ListenerRecord{
+		ListenerArn:     lb.LoadBalancerArn + "/listener-1",
+		ListenerID:      "lst-act1",
+		LoadBalancerArn: lb.LoadBalancerArn,
+		Protocol:        ProtocolTCP,
+		Port:            80,
+		DefaultActions:  []ListenerAction{{Type: ActionTypeForward, TargetGroupArn: tg.TargetGroupArn}},
+		AccountID:       testAccountID,
+	}))
+
+	// Pre-condition: config was never built during provisioning.
+	pre, err := svc.store.GetLoadBalancer("lb-act1")
+	require.NoError(t, err)
+	require.Empty(t, pre.ConfigHash, "config must be empty before activation")
+
+	out, err := svc.LBAgentHeartbeat(&LBAgentHeartbeatInput{
+		LBID: aws.String("lb-act1"),
+	}, testAccountID)
+	require.NoError(t, err)
+	assert.Equal(t, StateActive, *out.Status)
+	assert.NotEmpty(t, aws.StringValue(out.ConfigHash), "first heartbeat must return a built ConfigHash")
+
+	stored, err := svc.store.GetLoadBalancer("lb-act1")
+	require.NoError(t, err)
+	assert.NotEmpty(t, stored.ConfigText, "data-plane config must be built on activation")
+	assert.NotEmpty(t, stored.ConfigHash)
+	require.Len(t, stored.HealthTargets, 1, "NLB health target must be populated for the registered backend")
+	assert.Equal(t, "10.0.1.20:80", stored.HealthTargets[0].Address)
+}
+
 func TestLBAgentHeartbeat_MissingLBID(t *testing.T) {
 	svc := setupTestService(t)