diff --git a/pkg/app/pipedv1/plugin/kubernetes/provider/manifest_test.go b/pkg/app/pipedv1/plugin/kubernetes/provider/manifest_test.go
new file mode 100644
index 0000000000..8f3f5469f2
--- /dev/null
+++ b/pkg/app/pipedv1/plugin/kubernetes/provider/manifest_test.go
@@ -0,0 +1,112 @@
+// Copyright 2024 The PipeCD 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 provider
+
+import (
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"k8s.io/apimachinery/pkg/apis/meta/v1/unstructured"
+)
+
+func TestManifest_AddStringMapValues(t *testing.T) {
+	tests := []struct {
+		name     string
+		initial  map[string]interface{}
+		values   map[string]string
+		fields   []string
+		expected map[string]interface{}
+	}{
+		{
+			name: "add new values to empty map",
+			initial: map[string]interface{}{
+				"metadata": map[string]interface{}{
+					"annotations": map[string]interface{}{},
+				},
+			},
+			values: map[string]string{
+				"key1": "value1",
+				"key2": "value2",
+			},
+			fields: []string{"metadata", "annotations"},
+			expected: map[string]interface{}{
+				"metadata": map[string]interface{}{
+					"annotations": map[string]interface{}{
+						"key1": "value1",
+						"key2": "value2",
+					},
+				},
+			},
+		},
+		{
+			name: "override existing values",
+			initial: map[string]interface{}{
+				"metadata": map[string]interface{}{
+					"annotations": map[string]interface{}{
+						"key1": "oldvalue1",
+					},
+				},
+			},
+			values: map[string]string{
+				"key1": "newvalue1",
+				"key2": "value2",
+			},
+			fields: []string{"metadata", "annotations"},
+			expected: map[string]interface{}{
+				"metadata": map[string]interface{}{
+					"annotations": map[string]interface{}{
+						"key1": "newvalue1",
+						"key2": "value2",
+					},
+				},
+			},
+		},
+		{
+			name: "add values to non-existing map",
+			initial: map[string]interface{}{
+				"metadata": map[string]interface{}{},
+			},
+			values: map[string]string{
+				"key1": "value1",
+			},
+			fields: []string{"metadata", "annotations"},
+			expected: map[string]interface{}{
+				"metadata": map[string]interface{}{
+					"annotations": map[string]interface{}{
+						"key1": "value1",
+					},
+				},
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			manifest := Manifest{
+				Body: &unstructured.Unstructured{
+					Object: tt.initial,
+				},
+			}
+			err := manifest.AddStringMapValues(tt.values, tt.fields...)
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+
+			if diff := cmp.Diff(tt.expected, manifest.Body.Object); diff != "" {
+				t.Errorf("unexpected result (-want +got):\n%s", diff)
+			}
+		})
+	}
+}