diff --git a/CHANGELOG.md b/CHANGELOG.md
index 66532502..7b9cb41a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 1.8.0-dev
+
+* [BREAKING] stop exporting the internal `Undefined` sentinel from the public API
+
 ## 1.7.6
 
 * [CHORE] improve decode internals readability with clearer helper extraction, comments, formatting, and local type annotations
diff --git a/README.md b/README.md
index 45a6908b..f06260ef 100644
--- a/README.md
+++ b/README.md
@@ -923,20 +923,6 @@ expect(
 );
 ```
 
-Properties set to [Undefined] are omitted entirely:
-
-```dart
-expect(
-  QS.encode(
-    {
-      'a': null,
-      'b': const Undefined(),
-    },
-  ),
-  equals('a='),
-);
-```
-
 The query string may optionally be prepended with a question mark:
 
 ```dart
@@ -1032,7 +1018,6 @@ expect(
     EncodeOptions(
       encode: false,
       filter: (prefix, value) => switch (prefix) {
-        'b' => const Undefined(),
         'e[f]' => (value as DateTime).millisecondsSinceEpoch,
         'e[g][0]' => (value as num) * 2,
         _ => value,
diff --git a/example/example.dart b/example/example.dart
index 038250cd..33179c96 100644
--- a/example/example.dart
+++ b/example/example.dart
@@ -477,9 +477,6 @@ void main() {
       equals(''),
     );
 
-    /// Properties that are `Undefined` will be omitted entirely:
-    expect(QS.encode({'a': null, 'b': const Undefined()}), equals('a='));
-
     /// The query string may optionally be prepended with a question mark:
     expect(
       QS.encode({
@@ -541,7 +538,6 @@ void main() {
         EncodeOptions(
           encode: false,
           filter: (prefix, value) => switch (prefix) {
-            'b' => const Undefined(),
             'e[f]' => (value as DateTime).millisecondsSinceEpoch,
             'e[g][0]' => (value as num) * 2,
             _ => value,
diff --git a/lib/qs_dart.dart b/lib/qs_dart.dart
index 70156132..0e5236da 100644
--- a/lib/qs_dart.dart
+++ b/lib/qs_dart.dart
@@ -27,6 +27,5 @@ export 'src/enums/sentinel.dart';
 export 'src/methods.dart' show decode, encode;
 export 'src/models/decode_options.dart';
 export 'src/models/encode_options.dart';
-export 'src/models/undefined.dart';
 export 'src/qs.dart';
 export 'src/uri.dart';
diff --git a/test/unit/encode_test.dart b/test/unit/encode_test.dart
index 66748cf2..1227c5aa 100644
--- a/test/unit/encode_test.dart
+++ b/test/unit/encode_test.dart
@@ -5,6 +5,7 @@ import 'dart:typed_data' show ByteBuffer, Uint8List;
 
 import 'package:euc/jis.dart';
 import 'package:qs_dart/qs_dart.dart';
+import 'package:qs_dart/src/models/undefined.dart';
 import 'package:qs_dart/src/utils.dart';
 import 'package:test/test.dart';
 
diff --git a/test/unit/example.dart b/test/unit/example.dart
index 120624d1..2ddf5f99 100644
--- a/test/unit/example.dart
+++ b/test/unit/example.dart
@@ -629,17 +629,6 @@ void main() {
         ),
         equals(''));
 
-    /// Properties that are `Undefined` will be omitted entirely:
-    expect(
-      QS.encode(
-        {
-          'a': null,
-          'b': const Undefined(),
-        },
-      ),
-      equals('a='),
-    );
-
     /// The query string may optionally be prepended with a question mark:
     expect(
       QS.encode(
@@ -721,7 +710,6 @@ void main() {
         EncodeOptions(
           encode: false,
           filter: (prefix, value) => switch (prefix) {
-            'b' => const Undefined(),
             'e[f]' => (value as DateTime).millisecondsSinceEpoch,
             'e[g][0]' => (value as num) * 2,
             _ => value,
diff --git a/test/unit/extensions/extensions_test.dart b/test/unit/extensions/extensions_test.dart
index 3868a001..ebb5b69e 100644
--- a/test/unit/extensions/extensions_test.dart
+++ b/test/unit/extensions/extensions_test.dart
@@ -1,5 +1,5 @@
-import 'package:qs_dart/qs_dart.dart';
 import 'package:qs_dart/src/extensions/extensions.dart';
+import 'package:qs_dart/src/models/undefined.dart';
 import 'package:test/test.dart';
 
 void main() {
diff --git a/test/unit/utils_test.dart b/test/unit/utils_test.dart
index 6a735d16..1c429d68 100644
--- a/test/unit/utils_test.dart
+++ b/test/unit/utils_test.dart
@@ -4,6 +4,7 @@ import 'dart:convert' show latin1, utf8;
 import 'dart:typed_data' show Uint8List;
 
 import 'package:qs_dart/qs_dart.dart';
+import 'package:qs_dart/src/models/undefined.dart';
 import 'package:qs_dart/src/utils.dart';
 import 'package:test/test.dart';