diff --git a/cmd/ateapi/internal/controlapi/metrics.go b/cmd/ateapi/internal/controlapi/metrics.go
index be46f3b80b..e8e539515b 100644
--- a/cmd/ateapi/internal/controlapi/metrics.go
+++ b/cmd/ateapi/internal/controlapi/metrics.go
@@ -102,7 +102,12 @@ func RegisterWorkerCount(meter metric.Meter, workers func() ([]*ateapipb.Worker,
 			if w.GetStatus().GetAllocated().GetActors() > 0 {
 				state = ateattr.WorkerStateAssigned
 			}
-			tally[key{w.GetWorkerNamespace(), w.GetWorkerPool(), state, w.GetSandboxClass()}]++
+			// CreateWorker does not validate the class, thus a worker can have an
+			// empty one. Report it as unknown, not as the pool's class: the
+			// scheduler puts no actor on a worker that offers no class, thus the
+			// pool's series must not count it as free capacity.
+			class := ateattr.NormalizeSandboxClass(w.GetSandboxClass())
+			tally[key{w.GetWorkerNamespace(), w.GetWorkerPool(), state, class}]++
 		}
 		for k, n := range tally {
 			o.ObserveInt64(counter, n, metric.WithAttributes(
@@ -191,7 +196,7 @@ func lifecycleOpAttrs(actor *ateapipb.Actor, template *ateapipb.ActorTemplate, s
 	ass := actor.GetStatus().GetWorkerAssignment()
 	attrs = append(attrs, ateattr.WorkerPoolAttributes(ass.GetWorkerNamespace(), ass.GetWorkerPool())...)
 	if template != nil {
-		attrs = append(attrs, ateattr.SandboxClassKey.String(sandboxClassString(template.GetSandboxConfig().GetSandboxClass())))
+		attrs = append(attrs, ateattr.SandboxClassAttribute(sandboxClassString(template.GetSandboxConfig().GetSandboxClass())))
 	}
 	if snapshotKind != "" {
 		attrs = append(attrs, ateattr.SnapshotKindKey.String(snapshotKind))
diff --git a/cmd/ateapi/internal/controlapi/metrics_test.go b/cmd/ateapi/internal/controlapi/metrics_test.go
index 58df824131..51d812c207 100644
--- a/cmd/ateapi/internal/controlapi/metrics_test.go
+++ b/cmd/ateapi/internal/controlapi/metrics_test.go
@@ -400,3 +400,75 @@ func TestWorkerCountSeedsZeroForKnownPools(t *testing.T) {
 		}
 	}
 }
+
+// TestWorkerCountEmptyClassWorker covers the workers that CreateWorker accepted
+// with no sandbox class. They report as unknown, not as an empty-string class,
+// and not as capacity of the pool they name: the scheduler puts no actor on
+// them, thus the pool's own series stays at 0 and an idle==0 alert on it still
+// fires.
+func TestWorkerCountEmptyClassWorker(t *testing.T) {
+	const (
+		gvisor  = string(atev1alpha1.SandboxClassGvisor)
+		microvm = string(atev1alpha1.SandboxClassMicroVM)
+	)
+	tests := []struct {
+		name    string
+		pools   []*atev1alpha1.WorkerPool
+		workers []*ateapipb.Worker
+		want    map[series]int64
+	}{
+		{
+			name:  "gvisor pool",
+			pools: []*atev1alpha1.WorkerPool{workerPool("ns-1", "pool-empty", "")},
+			workers: []*ateapipb.Worker{
+				worker("ns-1", "pool-empty", "", false),
+				worker("ns-1", "pool-empty", "", false),
+				worker("ns-1", "pool-empty", "", false),
+			},
+			want: map[series]int64{
+				{"ns-1", "pool-empty", ateattr.WorkerStateIdle, gvisor}:                      0,
+				{"ns-1", "pool-empty", ateattr.WorkerStateAssigned, gvisor}:                  0,
+				{"ns-1", "pool-empty", ateattr.WorkerStateIdle, ateattr.SandboxClassUnknown}: 3,
+			},
+		},
+		{
+			// No gvisor series shows for a pool that runs no gvisor.
+			name:    "microvm pool",
+			pools:   []*atev1alpha1.WorkerPool{workerPool("ns-1", "pool-micro", atev1alpha1.SandboxClassMicroVM)},
+			workers: []*ateapipb.Worker{worker("ns-1", "pool-micro", "", false)},
+			want: map[series]int64{
+				{"ns-1", "pool-micro", ateattr.WorkerStateIdle, microvm}:                     0,
+				{"ns-1", "pool-micro", ateattr.WorkerStateAssigned, microvm}:                 0,
+				{"ns-1", "pool-micro", ateattr.WorkerStateIdle, ateattr.SandboxClassUnknown}: 1,
+			},
+		},
+		{
+			// A worker that matches no pool has no class to fall back to.
+			name:    "orphan worker",
+			pools:   nil,
+			workers: []*ateapipb.Worker{worker("ns-1", "pool-orphan", "", false)},
+			want: map[series]int64{
+				{"ns-1", "pool-orphan", ateattr.WorkerStateIdle, ateattr.SandboxClassUnknown}: 1,
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			pools := func(labels.Selector) ([]*atev1alpha1.WorkerPool, error) { return tt.pools, nil }
+			workers := func() ([]*ateapipb.Worker, error) { return tt.workers, nil }
+			reader := newWorkerCountReader(t, workers, pools)
+
+			sum := mustMetric(t, reader, workerpoolWorkersMetric).Data.(metricdata.Sum[int64])
+			got := seriesCounts(sum)
+			if len(got) != len(tt.want) {
+				t.Fatalf("got %d series, want %d: %v", len(got), len(tt.want), got)
+			}
+			for k, v := range tt.want {
+				if gv, ok := got[k]; !ok || gv != v {
+					t.Errorf("series %v = %d (present=%v), want %d", k, gv, ok, v)
+				}
+			}
+		})
+	}
+}
diff --git a/cmd/ateapi/internal/scheduling/metrics.go b/cmd/ateapi/internal/scheduling/metrics.go
index 93f1c2a00b..06ee8da2a4 100644
--- a/cmd/ateapi/internal/scheduling/metrics.go
+++ b/cmd/ateapi/internal/scheduling/metrics.go
@@ -92,7 +92,7 @@ func (s *scheduler) recordEligibleWorkers(ctx context.Context, matching []*ateap
 		s.eligibleWorkers.Record(ctx, count, metric.WithAttributes(
 			ateattr.WorkerPoolNamespaceKey.String(k.namespace),
 			ateattr.WorkerPoolNameKey.String(k.pool),
-			ateattr.SandboxClassKey.String(constraints.SandboxClass),
+			ateattr.SandboxClassAttribute(constraints.SandboxClass),
 			ateattr.SchedulingConstraintKey.String(constraintStr),
 		))
 	}
diff --git a/cmd/ateapi/internal/scheduling/scheduling_test.go b/cmd/ateapi/internal/scheduling/scheduling_test.go
index 9365b21e0f..19dfa4fbdb 100644
--- a/cmd/ateapi/internal/scheduling/scheduling_test.go
+++ b/cmd/ateapi/internal/scheduling/scheduling_test.go
@@ -652,7 +652,7 @@ func TestSchedule_EligibleWorkersMetric(t *testing.T) {
 		}
 
 		s := New(flt, WithIntn(firstIntn), WithMeter(meter))
-		_, err := s.Schedule(context.Background(), Constraints{SandboxClass: "kata"})
+		_, err := s.Schedule(context.Background(), Constraints{SandboxClass: "microvm"})
 		if !errors.Is(err, ErrNoCapacity) {
 			t.Fatalf("Schedule() error = %v, want ErrNoCapacity", err)
 		}
@@ -670,8 +670,8 @@ func TestSchedule_EligibleWorkersMetric(t *testing.T) {
 						t.Errorf("datapoint sum = %d, want 0", dp.Sum)
 					}
 					class, _ := dp.Attributes.Value(ateattr.SandboxClassKey)
-					if class.AsString() != "kata" {
-						t.Errorf("got sandbox class %q, want kata", class.AsString())
+					if class.AsString() != "microvm" {
+						t.Errorf("got sandbox class %q, want microvm", class.AsString())
 					}
 				}
 			}
diff --git a/cmd/atelet/metrics.go b/cmd/atelet/metrics.go
index 896c835f5b..b48815b253 100644
--- a/cmd/atelet/metrics.go
+++ b/cmd/atelet/metrics.go
@@ -98,7 +98,7 @@ func (o snapshotOp) attrs() []attribute.KeyValue {
 		attrs = append(attrs, ateattr.SnapshotKindKey.String(o.kind))
 	}
 	if o.sandboxClass != "" {
-		attrs = append(attrs, ateattr.SandboxClassKey.String(ateattr.NormalizeSandboxClass(o.sandboxClass)))
+		attrs = append(attrs, ateattr.SandboxClassAttribute(o.sandboxClass))
 	}
 	return attrs
 }
diff --git a/docs/metrics/registry/metrics.yaml b/docs/metrics/registry/metrics.yaml
index 7548f2cb1f..45c6317310 100644
--- a/docs/metrics/registry/metrics.yaml
+++ b/docs/metrics/registry/metrics.yaml
@@ -140,9 +140,11 @@ groups:
               stability: development
               value: unknown
               brief: >
-                The snapshot manifest has no class, or the class is not in this
-                list. The default is not gvisor. Thus a bad manifest stays
-                visible.
+                The source has no class, or a class that is not in this list.
+                The default is not gvisor. Thus a bad record stays visible.
+                ateapi reports it for a Worker that CreateWorker accepted with
+                no class, and for an ActorTemplate with no sandbox config.
+                atelet reports it for a snapshot manifest with no class.
 
   - id: registry.ate.snapshot
     type: attribute_group
@@ -620,6 +622,9 @@ groups:
       not write. Some moves into this state are not a loss of data. They are
       careful responses to a control plane problem. The ate.failure.reason key
       keeps these groups separate.
+      ate.sandbox.class is unknown when ateapi could not read the class of the
+      worker: the worker record is already gone, or its assignment is already
+      clear.
     annotations:
       substrate:
         emitted_by: [ateapi]
@@ -660,6 +665,10 @@ groups:
       and not a gauge. ateapi sets both states to 0 for each known pool. Thus a
       full pool or an empty pool reports 0. An absent series would stop an alert
       on idle == 0.
+      A worker with the class unknown is not capacity of the pool: the scheduler
+      puts no actor on it. Thus a query that asks how much capacity a pool has
+      must keep ate.sandbox.class, because a sum across the classes adds that
+      worker back into the total.
     annotations:
       substrate:
         emitted_by: [ateapi]
diff --git a/internal/ateattr/ateattr.go b/internal/ateattr/ateattr.go
index 702bf1263b..160f70548e 100644
--- a/internal/ateattr/ateattr.go
+++ b/internal/ateattr/ateattr.go
@@ -343,6 +343,16 @@ func NormalizeSandboxClass(class string) string {
 	}
 }
 
+// SandboxClassAttribute sets ate.sandbox.class. No source of the class is
+// validated, thus an emitter that sets the attribute must set it through this
+// helper and not through SandboxClassKey.
+//
+// To omit the attribute while the class is unknown is a different choice, and
+// two emitters make it: recordSchedulerAssignment and snapshotOp.attrs.
+func SandboxClassAttribute(class string) attribute.KeyValue {
+	return SandboxClassKey.String(NormalizeSandboxClass(class))
+}
+
 // WorkerPoolAttributes returns the namespaced identity of a WorkerPool. A
 // WorkerPool is namespaced, so half the pair identifies no pool: either key
 // missing drops both, rather than emit an empty-string series that merges
@@ -445,7 +455,7 @@ func ActorMetricAttributes(a *ateapipb.Actor, sandboxClass, operationName, reaso
 	attrs := []attribute.KeyValue{
 		TemplateAtespaceKey.String(a.GetActorTemplate().GetAtespace()),
 		TemplateNameKey.String(a.GetActorTemplate().GetName()),
-		SandboxClassKey.String(sandboxClass),
+		SandboxClassAttribute(sandboxClass),
 		ActorOperationNameKey.String(operationName),
 	}
 	attrs = append(attrs, FailureAttributes(reason)...)
diff --git a/internal/ateattr/ateattr_test.go b/internal/ateattr/ateattr_test.go
index 0fd7bdb312..76abffb6b9 100644
--- a/internal/ateattr/ateattr_test.go
+++ b/internal/ateattr/ateattr_test.go
@@ -469,6 +469,24 @@ func TestActorMetricAttributes(t *testing.T) {
 		assertAttrs(t, got, want)
 	})
 
+	// releaseWorker gives an empty class when the worker record is gone.
+	// CreateWorker can also store one. Empty is not a permitted value.
+	t.Run("empty sandbox class is normalized to unknown", func(t *testing.T) {
+		got := toMap(ActorMetricAttributes(actor, "", OperationResume, ReasonCorruptedAssignment))
+		want := map[attribute.Key]any{
+			TemplateAtespaceKey:    "default",
+			TemplateNameKey:        "counter-template",
+			WorkerPoolNamespaceKey: "ate-workers",
+			WorkerPoolNameKey:      "default-pool",
+			SandboxClassKey:        SandboxClassUnknown,
+			ActorOperationNameKey:  OperationResume,
+			FailureReasonKey:       ReasonCorruptedAssignment,
+			FailureDomainKey:       FailureDomainInfrastructure,
+		}
+
+		assertAttrs(t, got, want)
+	})
+
 	t.Run("out of range operation name is normalized to unknown", func(t *testing.T) {
 		got := toMap(ActorMetricAttributes(actor, "gvisor", "invalid_op", ""))
 		want := map[attribute.Key]any{