diff --git a/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/cbor/direct/direct.go b/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/cbor/direct/direct.go
index cd78b1df266..a71a487f9ec 100644
--- a/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/cbor/direct/direct.go
+++ b/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/cbor/direct/direct.go
@@ -23,14 +23,39 @@ import (
 	"k8s.io/apimachinery/pkg/runtime/serializer/cbor/internal/modes"
 )
 
+// Marshal serializes a value to CBOR. If there is more than one way to encode the value, it will
+// make the same choice as the CBOR implementation of runtime.Serializer.
+//
+// Note: Support for CBOR is at an alpha stage. If the value (or, for composite types, any of its
+// nested values) implement any of the interfaces encoding.TextMarshaler, encoding.TextUnmarshaler,
+// encoding/json.Marshaler, or encoding/json.Unmarshaler, a non-nil error will be returned unless
+// the value also implements the corresponding CBOR interfaces. This limitation will ultimately be
+// removed in favor of automatic transcoding to CBOR.
 func Marshal(src interface{}) ([]byte, error) {
+	if err := modes.RejectCustomMarshalers(src); err != nil {
+		return nil, err
+	}
 	return modes.Encode.Marshal(src)
 }
 
+// Unmarshal deserializes from CBOR into an addressable value. If there is more than one way to
+// unmarshal a value, it will make the same choice as the CBOR implementation of runtime.Serializer.
+//
+// Note: Support for CBOR is at an alpha stage. If the value (or, for composite types, any of its
+// nested values) implement any of the interfaces encoding.TextMarshaler, encoding.TextUnmarshaler,
+// encoding/json.Marshaler, or encoding/json.Unmarshaler, a non-nil error will be returned unless
+// the value also implements the corresponding CBOR interfaces. This limitation will ultimately be
+// removed in favor of automatic transcoding to CBOR.
 func Unmarshal(src []byte, dst interface{}) error {
+	if err := modes.RejectCustomMarshalers(dst); err != nil {
+		return err
+	}
 	return modes.Decode.Unmarshal(src, dst)
 }
 
+// Diagnose accepts well-formed CBOR bytes and returns a string representing the same data item in
+// human-readable diagnostic notation (RFC 8949 Section 8). The diagnostic notation is not meant to
+// be parsed.
 func Diagnose(src []byte) (string, error) {
 	return modes.Diagnostic.Diagnose(src)
 }
diff --git a/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/cbor/direct/direct_test.go b/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/cbor/direct/direct_test.go
new file mode 100644
index 00000000000..2986ee3f435
--- /dev/null
+++ b/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/cbor/direct/direct_test.go
@@ -0,0 +1,81 @@
+/*
+Copyright 2024 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 direct_test
+
+import (
+	"encoding"
+	"encoding/json"
+	"fmt"
+	"reflect"
+	"testing"
+
+	"k8s.io/apimachinery/pkg/runtime/serializer/cbor/direct"
+)
+
+var _ json.Marshaler = CustomJSONMarshaler{}
+
+type CustomJSONMarshaler struct{}
+
+func (CustomJSONMarshaler) MarshalJSON() ([]byte, error) {
+	panic("unimplemented")
+}
+
+var _ json.Unmarshaler = CustomJSONUnmarshaler{}
+
+type CustomJSONUnmarshaler struct{}
+
+func (CustomJSONUnmarshaler) UnmarshalJSON([]byte) error {
+	panic("unimplemented")
+}
+
+var _ encoding.TextMarshaler = CustomTextMarshaler{}
+
+type CustomTextMarshaler struct{}
+
+func (CustomTextMarshaler) MarshalText() ([]byte, error) {
+	panic("unimplemented")
+}
+
+var _ encoding.TextUnmarshaler = CustomTextUnmarshaler{}
+
+type CustomTextUnmarshaler struct{}
+
+func (CustomTextUnmarshaler) UnmarshalText([]byte) error {
+	panic("unimplemented")
+}
+
+func TestRejectsCustom(t *testing.T) {
+	for _, tc := range []struct {
+		value interface{}
+		iface reflect.Type
+	}{
+		{value: CustomJSONMarshaler{}, iface: reflect.TypeFor[json.Marshaler]()},
+		{value: CustomJSONUnmarshaler{}, iface: reflect.TypeFor[json.Unmarshaler]()},
+		{value: CustomTextMarshaler{}, iface: reflect.TypeFor[encoding.TextMarshaler]()},
+		{value: CustomTextUnmarshaler{}, iface: reflect.TypeFor[encoding.TextUnmarshaler]()},
+	} {
+		t.Run(fmt.Sprintf("%T", tc.value), func(t *testing.T) {
+			want := fmt.Sprintf("unable to serialize %T: %T implements %s without corresponding cbor interface", tc.value, tc.value, tc.iface.String())
+			if _, err := direct.Marshal(tc.value); err == nil || err.Error() != want {
+				t.Errorf("want error: %q, got: %v", want, err)
+			}
+			if err := direct.Unmarshal(nil, tc.value); err == nil || err.Error() != want {
+				t.Errorf("want error: %q, got: %v", want, err)
+			}
+		})
+	}
+}