diff --git a/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/protobuf/collections_test.go b/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/protobuf/collections_test.go
new file mode 100644
index 00000000000..fa10e290931
--- /dev/null
+++ b/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/protobuf/collections_test.go
@@ -0,0 +1,228 @@
+/*
+Copyright 2025 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package protobuf
+
+import (
+	"bytes"
+	"encoding/base64"
+	"io"
+	"os/exec"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	testapigroupv1 "k8s.io/apimachinery/pkg/apis/testapigroup/v1"
+	"k8s.io/apimachinery/pkg/runtime"
+)
+
+func TestCollectionsEncoding(t *testing.T) {
+	t.Run("Normal", func(t *testing.T) {
+		testCollectionsEncoding(t, NewSerializer(nil, nil))
+	})
+	// Leave place for testing streaming collection serializer proposed as part of KEP-5116
+}
+
+func testCollectionsEncoding(t *testing.T, s *Serializer) {
+	var remainingItems int64 = 1
+	testCases := []struct {
+		name string
+		in   runtime.Object
+		// expect is base64 encoded protobuf bytes
+		expect string
+	}{
+		{
+			name: "CarpList items nil",
+			in: &testapigroupv1.CarpList{
+				Items: nil,
+			},
+			expect: "azhzAAoECgASABIICgYKABIAGgAaACIA",
+		},
+		{
+			name: "CarpList slice nil",
+			in: &testapigroupv1.CarpList{
+				Items: []testapigroupv1.Carp{
+					{
+						Status: testapigroupv1.CarpStatus{
+							Conditions: nil,
+						},
+					},
+				},
+			},
+			expect: "azhzAAoECgASABJBCgYKABIAGgASNwoQCgASABoAIgAqADIAOABCABIXGgBCAEoAUgBYAGAAaACCAQCKAQCaAQAaCgoAGgAiACoAMgAaACIA",
+		},
+		{
+			name: "CarpList map nil",
+			in: &testapigroupv1.CarpList{
+				Items: []testapigroupv1.Carp{
+					{
+						Spec: testapigroupv1.CarpSpec{
+							NodeSelector: nil,
+						},
+					},
+				},
+			},
+			expect: "azhzAAoECgASABJBCgYKABIAGgASNwoQCgASABoAIgAqADIAOABCABIXGgBCAEoAUgBYAGAAaACCAQCKAQCaAQAaCgoAGgAiACoAMgAaACIA",
+		},
+		{
+			name: "CarpList items empty",
+			in: &testapigroupv1.CarpList{
+				Items: []testapigroupv1.Carp{},
+			},
+			expect: "azhzAAoECgASABIICgYKABIAGgAaACIA",
+		},
+		{
+			name: "CarpList slice empty",
+			in: &testapigroupv1.CarpList{
+				Items: []testapigroupv1.Carp{
+					{
+						Status: testapigroupv1.CarpStatus{
+							Conditions: []testapigroupv1.CarpCondition{},
+						},
+					},
+				},
+			},
+			expect: "azhzAAoECgASABJBCgYKABIAGgASNwoQCgASABoAIgAqADIAOABCABIXGgBCAEoAUgBYAGAAaACCAQCKAQCaAQAaCgoAGgAiACoAMgAaACIA",
+		},
+		{
+			name: "CarpList map empty",
+			in: &testapigroupv1.CarpList{
+				Items: []testapigroupv1.Carp{
+					{
+						Spec: testapigroupv1.CarpSpec{
+							NodeSelector: map[string]string{},
+						},
+					},
+				},
+			},
+			expect: "azhzAAoECgASABJBCgYKABIAGgASNwoQCgASABoAIgAqADIAOABCABIXGgBCAEoAUgBYAGAAaACCAQCKAQCaAQAaCgoAGgAiACoAMgAaACIA",
+		},
+		{
+			name: "List just kind",
+			in: &testapigroupv1.CarpList{
+				TypeMeta: metav1.TypeMeta{
+					Kind: "List",
+				},
+			},
+			expect: "azhzAAoICgASBExpc3QSCAoGCgASABoAGgAiAA==",
+		},
+		{
+			name: "List just apiVersion",
+			in: &testapigroupv1.CarpList{
+				TypeMeta: metav1.TypeMeta{
+					APIVersion: "v1",
+				},
+			},
+			expect: "azhzAAoGCgJ2MRIAEggKBgoAEgAaABoAIgA=",
+		},
+		{
+			name: "List no elements",
+			in: &testapigroupv1.CarpList{
+				TypeMeta: metav1.TypeMeta{
+					Kind:       "List",
+					APIVersion: "v1",
+				},
+				ListMeta: metav1.ListMeta{
+					ResourceVersion: "2345",
+				},
+				Items: []testapigroupv1.Carp{},
+			},
+			expect: "azhzAAoKCgJ2MRIETGlzdBIMCgoKABIEMjM0NRoAGgAiAA==",
+		},
+		{
+			name: "List one element with continue",
+			in: &testapigroupv1.CarpList{
+				TypeMeta: metav1.TypeMeta{
+					Kind:       "List",
+					APIVersion: "v1",
+				},
+				ListMeta: metav1.ListMeta{
+					ResourceVersion:    "2345",
+					Continue:           "abc",
+					RemainingItemCount: &remainingItems,
+				},
+				Items: []testapigroupv1.Carp{
+					{TypeMeta: metav1.TypeMeta{APIVersion: "v1", Kind: "Carp"}, ObjectMeta: metav1.ObjectMeta{
+						Name:      "pod",
+						Namespace: "default",
+					}},
+				},
+			},
+			expect: "azhzAAoKCgJ2MRIETGlzdBJUCg8KABIEMjM0NRoDYWJjIAESQQoaCgNwb2QSABoHZGVmYXVsdCIAKgAyADgAQgASFxoAQgBKAFIAWABgAGgAggEAigEAmgEAGgoKABoAIgAqADIAGgAiAA==",
+		},
+		{
+			name: "List two elements",
+			in: &testapigroupv1.CarpList{
+				TypeMeta: metav1.TypeMeta{
+					Kind:       "List",
+					APIVersion: "v1",
+				},
+				ListMeta: metav1.ListMeta{
+					ResourceVersion: "2345",
+				},
+				Items: []testapigroupv1.Carp{
+					{TypeMeta: metav1.TypeMeta{APIVersion: "v1", Kind: "Carp"}, ObjectMeta: metav1.ObjectMeta{
+						Name:      "pod",
+						Namespace: "default",
+					}},
+					{TypeMeta: metav1.TypeMeta{APIVersion: "v1", Kind: "Carp"}, ObjectMeta: metav1.ObjectMeta{
+						Name:      "pod2",
+						Namespace: "default2",
+					}},
+				},
+			},
+			expect: "azhzAAoKCgJ2MRIETGlzdBKUAQoKCgASBDIzNDUaABJBChoKA3BvZBIAGgdkZWZhdWx0IgAqADIAOABCABIXGgBCAEoAUgBYAGAAaACCAQCKAQCaAQAaCgoAGgAiACoAMgASQwocCgRwb2QyEgAaCGRlZmF1bHQyIgAqADIAOABCABIXGgBCAEoAUgBYAGAAaACCAQCKAQCaAQAaCgoAGgAiACoAMgAaACIA",
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			var buf bytes.Buffer
+			if err := s.Encode(tc.in, &buf); err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+			actualBytes := buf.Bytes()
+			expectBytes, err := io.ReadAll(base64.NewDecoder(base64.StdEncoding, bytes.NewBufferString(tc.expect)))
+			if err != nil {
+				t.Fatal(err)
+			}
+			if !bytes.Equal(expectBytes, actualBytes) {
+				t.Errorf("expected:\n%s\ngot:\n%s", tc.expect, base64.StdEncoding.EncodeToString(actualBytes))
+				t.Log(cmp.Diff(dumpProto(t, actualBytes[4:]), dumpProto(t, expectBytes[4:])))
+			}
+		})
+	}
+}
+
+// dumpProto does a best-effort dump of the given proto bytes using protoc if it can be found in the path.
+// This is only used when the test has already failed, to try to give more visibility into the diff of the failure.
+func dumpProto(t *testing.T, data []byte) string {
+	t.Helper()
+	protoc, err := exec.LookPath("protoc")
+	if err != nil {
+		t.Logf("cannot find protoc in path to dump proto contents: %v", err)
+		return ""
+	}
+	cmd := exec.Command(protoc, "--decode_raw")
+	cmd.Stdin = bytes.NewBuffer(data)
+	d, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Logf("protoc invocation failed: %v", err)
+		return ""
+	}
+	return string(d)
+}