diff --git a/packages/kosong/src/kosong/message.py b/packages/kosong/src/kosong/message.py
index a658f36992..5351c8d25f 100644
--- a/packages/kosong/src/kosong/message.py
+++ b/packages/kosong/src/kosong/message.py
@@ -1,7 +1,13 @@
 from abc import ABC
 from typing import Any, ClassVar, Literal, cast, override
 
-from pydantic import BaseModel, GetCoreSchemaHandler, field_serializer, field_validator
+from pydantic import (
+    BaseModel,
+    GetCoreSchemaHandler,
+    SerializerFunctionWrapHandler,
+    field_serializer,
+    field_validator,
+)
 from pydantic_core import core_schema
 
 from kosong.utils.typing import JsonType
@@ -262,11 +268,15 @@ class Message(BaseModel):
 
     partial: bool | None = None
 
-    @field_serializer("content")
-    def _serialize_content(self, content: list[ContentPart]) -> str | list[dict[str, Any]] | None:
+    @field_serializer("content", mode="wrap")
+    def _serialize_content(
+        self,
+        content: list[ContentPart],
+        serializer: SerializerFunctionWrapHandler,
+    ) -> str | list[dict[str, Any]] | None:
         if len(content) == 1 and isinstance(content[0], TextPart):
             return content[0].text
-        return [part.model_dump() for part in content]
+        return serializer(content)
 
     @field_validator("content", mode="before")
     @classmethod
diff --git a/packages/kosong/tests/api_snapshot_tests/test_kimi.py b/packages/kosong/tests/api_snapshot_tests/test_kimi.py
index 369f3c32f9..6e28409bdd 100644
--- a/packages/kosong/tests/api_snapshot_tests/test_kimi.py
+++ b/packages/kosong/tests/api_snapshot_tests/test_kimi.py
@@ -133,7 +133,6 @@ async def test_kimi_message_conversion():
                                     "type": "image_url",
                                     "image_url": {
                                         "url": "https://example.com/image.png",
-                                        "id": None,
                                     },
                                 },
                             ],
@@ -204,7 +203,6 @@ async def test_kimi_message_conversion():
                                     "type": "image_url",
                                     "image_url": {
                                         "url": "https://example.com/image.png",
-                                        "id": None,
                                     },
                                 },
                             ],
diff --git a/packages/kosong/tests/api_snapshot_tests/test_openai_legacy.py b/packages/kosong/tests/api_snapshot_tests/test_openai_legacy.py
index 78114ff9ae..e773555a72 100644
--- a/packages/kosong/tests/api_snapshot_tests/test_openai_legacy.py
+++ b/packages/kosong/tests/api_snapshot_tests/test_openai_legacy.py
@@ -57,7 +57,6 @@ async def test_openai_legacy_message_conversion():
                                     "type": "image_url",
                                     "image_url": {
                                         "url": "https://example.com/image.png",
-                                        "id": None,
                                     },
                                 },
                             ],
@@ -128,7 +127,6 @@ async def test_openai_legacy_message_conversion():
                                     "type": "image_url",
                                     "image_url": {
                                         "url": "https://example.com/image.png",
-                                        "id": None,
                                     },
                                 },
                             ],
diff --git a/packages/kosong/tests/test_message.py b/packages/kosong/tests/test_message.py
index 7dfa1056db..ba6a01f907 100644
--- a/packages/kosong/tests/test_message.py
+++ b/packages/kosong/tests/test_message.py
@@ -30,7 +30,7 @@ def test_message_with_single_part():
             "content": [
                 {
                     "type": "image_url",
-                    "image_url": {"url": "https://example.com/image.png", "id": None},
+                    "image_url": {"url": "https://example.com/image.png"},
                 }
             ],
         }
@@ -38,6 +38,46 @@ def test_message_with_single_part():
     assert Message.model_validate(dumped) == message
 
 
+def test_exclude_none_applies_to_nested_media_content_parts():
+    message = Message(
+        role="user",
+        content=[
+            ImageURLPart(image_url=ImageURLPart.ImageURL(url="https://example.com/image.png")),
+            AudioURLPart(audio_url=AudioURLPart.AudioURL(url="https://example.com/audio.mp3")),
+            VideoURLPart(
+                video_url=VideoURLPart.VideoURL(
+                    url="https://example.com/video.mp4",
+                    id="video-1",
+                )
+            ),
+        ],
+    )
+
+    dumped_with_none = message.model_dump()
+    assert dumped_with_none["content"][0]["image_url"]["id"] is None
+
+    assert message.model_dump(exclude_none=True) == {
+        "role": "user",
+        "content": [
+            {
+                "type": "image_url",
+                "image_url": {"url": "https://example.com/image.png"},
+            },
+            {
+                "type": "audio_url",
+                "audio_url": {"url": "https://example.com/audio.mp3"},
+            },
+            {
+                "type": "video_url",
+                "video_url": {
+                    "url": "https://example.com/video.mp4",
+                    "id": "video-1",
+                },
+            },
+        ],
+    }
+
+
 def test_message_with_tool_calls():
     message = Message(
         role="assistant",
@@ -107,22 +147,18 @@ def test_message_with_complex_content():
             "role": "user",
             "content": [
                 {"type": "text", "text": "Hello, world!"},
-                {
-                    "type": "think",
-                    "think": "I think I need to think about this.",
-                    "encrypted": None,
-                },
+                {"type": "think", "think": "I think I need to think about this."},
                 {
                     "type": "image_url",
-                    "image_url": {"url": "https://example.com/image.png", "id": None},
+                    "image_url": {"url": "https://example.com/image.png"},
                 },
                 {
                     "type": "audio_url",
-                    "audio_url": {"url": "https://example.com/audio.mp3", "id": None},
+                    "audio_url": {"url": "https://example.com/audio.mp3"},
                 },
                 {
                     "type": "video_url",
-                    "video_url": {"url": "https://example.com/video.mp4", "id": None},
+                    "video_url": {"url": "https://example.com/video.mp4"},
                 },
             ],
             "tool_calls": [