diff --git a/pkgs/google_generative_ai/CHANGELOG.md b/pkgs/google_generative_ai/CHANGELOG.md
index 86d834f..46dcf67 100644
--- a/pkgs/google_generative_ai/CHANGELOG.md
+++ b/pkgs/google_generative_ai/CHANGELOG.md
@@ -1,4 +1,4 @@
-## 0.3.4-wip
+## 0.4.0
 
 - Add support for parsing Vertex AI specific fields in `CountTokensResponse`.
 - Add named constructors on `Schema` for each value type.
@@ -6,6 +6,8 @@
   `'application/json'` to force the model to reply with JSON parseable output.
 - Add `outputDimensionality` argument support for `embedContent` and
   `batchEmbedContent`.
+- **Breaking** The `Part` class is no longer `sealed`. Exhaustive switches over
+  a `Part` instance will need to add a wildcard case.
 
 ## 0.3.3
 
diff --git a/pkgs/google_generative_ai/lib/src/content.dart b/pkgs/google_generative_ai/lib/src/content.dart
index f7b968b..f63af6b 100644
--- a/pkgs/google_generative_ai/lib/src/content.dart
+++ b/pkgs/google_generative_ai/lib/src/content.dart
@@ -80,7 +80,7 @@ Part _parsePart(Object? jsonObject) {
 }
 
 /// A datatype containing media that is part of a multi-part [Content] message.
-sealed class Part {
+abstract interface class Part {
   Object toJson();
 }
 
diff --git a/pkgs/google_generative_ai/lib/src/version.dart b/pkgs/google_generative_ai/lib/src/version.dart
index 6b556d8..88cf981 100644
--- a/pkgs/google_generative_ai/lib/src/version.dart
+++ b/pkgs/google_generative_ai/lib/src/version.dart
@@ -12,4 +12,4 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-const packageVersion = '0.3.4-wip';
+const packageVersion = '0.4.0';
diff --git a/pkgs/google_generative_ai/pubspec.yaml b/pkgs/google_generative_ai/pubspec.yaml
index d5c862b..bfc9dff 100644
--- a/pkgs/google_generative_ai/pubspec.yaml
+++ b/pkgs/google_generative_ai/pubspec.yaml
@@ -1,6 +1,6 @@
 name: google_generative_ai
 # Update `lib/version.dart` when changing version.
-version: 0.3.4-wip
+version: 0.4.0
 description: >-
   The Google AI Dart SDK enables developers to use Google's state-of-the-art
   generative AI models (like Gemini).
diff --git a/pkgs/google_generative_ai/test/utils/matchers.dart b/pkgs/google_generative_ai/test/utils/matchers.dart
index 0db7f97..416d68e 100644
--- a/pkgs/google_generative_ai/test/utils/matchers.dart
+++ b/pkgs/google_generative_ai/test/utils/matchers.dart
@@ -41,6 +41,7 @@ Matcher matchesPart(Part part) => switch (part) {
             // TODO: When updating min SDK remove ignore.
             // ignore: unused_result, implementation bug
             .having((p) => p.response, 'args', response),
+      _ => throw StateError('Unhandled Part type.'),
     };
 
 Matcher matchesContent(Content content) => isA<Content>()