diff --git a/google/genai/_interactions/_base_client.py b/google/genai/_interactions/_base_client.py
index c2e8232c3..71453670f 100644
--- a/google/genai/_interactions/_base_client.py
+++ b/google/genai/_interactions/_base_client.py
@@ -101,6 +101,7 @@
     APIConnectionError,
     APIResponseValidationError,
 )
+from ._utils._json import openapi_dumps
 
 log: logging.Logger = logging.getLogger(__name__)
 
@@ -578,8 +579,10 @@ def _build_request(
                 kwargs["content"] = options.content
             elif isinstance(json_data, bytes):
                 kwargs["content"] = json_data
-            else:
-                kwargs["json"] = json_data if is_given(json_data) else None
+            elif not files:
+                # Don't set content when JSON is sent as multipart/form-data,
+                # since httpx's content param overrides other body arguments
+                kwargs["content"] = openapi_dumps(json_data) if is_given(json_data) and json_data is not None else None
             kwargs["files"] = files
         else:
             headers.pop("Content-Type", None)
diff --git a/google/genai/_interactions/_compat.py b/google/genai/_interactions/_compat.py
index 2ad0c9978..25b3af666 100644
--- a/google/genai/_interactions/_compat.py
+++ b/google/genai/_interactions/_compat.py
@@ -154,6 +154,7 @@ def model_dump(
     exclude_defaults: bool = False,
     warnings: bool = True,
     mode: Literal["json", "python"] = "python",
+    by_alias: bool | None = None,
 ) -> dict[str, Any]:
     if (not PYDANTIC_V1) or hasattr(model, "model_dump"):
         return model.model_dump(
@@ -163,13 +164,12 @@ def model_dump(
             exclude_defaults=exclude_defaults,
             # warnings are not supported in Pydantic v1
             warnings=True if PYDANTIC_V1 else warnings,
+            by_alias=by_alias,
         )
     return cast(
         "dict[str, Any]",
         model.dict(  # pyright: ignore[reportDeprecated, reportUnnecessaryCast]
-            exclude=exclude,
-            exclude_unset=exclude_unset,
-            exclude_defaults=exclude_defaults,
+            exclude=exclude, exclude_unset=exclude_unset, exclude_defaults=exclude_defaults, by_alias=bool(by_alias)
         ),
     )
 
diff --git a/google/genai/_interactions/_utils/_json.py b/google/genai/_interactions/_utils/_json.py
new file mode 100644
index 000000000..d54cafcc3
--- /dev/null
+++ b/google/genai/_interactions/_utils/_json.py
@@ -0,0 +1,50 @@
+# Copyright 2025 Google LLC
+#
+# 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.
+#
+
+import json
+from typing import Any
+from datetime import datetime
+from typing_extensions import override
+
+import pydantic
+
+from .._compat import model_dump
+
+
+def openapi_dumps(obj: Any) -> bytes:
+    """
+    Serialize an object to UTF-8 encoded JSON bytes.
+
+    Extends the standard json.dumps with support for additional types
+    commonly used in the SDK, such as `datetime`, `pydantic.BaseModel`, etc.
+    """
+    return json.dumps(
+        obj,
+        cls=_CustomEncoder,
+        # Uses the same defaults as httpx's JSON serialization
+        ensure_ascii=False,
+        separators=(",", ":"),
+        allow_nan=False,
+    ).encode()
+
+
+class _CustomEncoder(json.JSONEncoder):
+    @override
+    def default(self, o: Any) -> Any:
+        if isinstance(o, datetime):
+            return o.isoformat()
+        if isinstance(o, pydantic.BaseModel):
+            return model_dump(o, exclude_unset=True, mode="json", by_alias=True)
+        return super().default(o)
diff --git a/google/genai/_interactions/types/image_content.py b/google/genai/_interactions/types/image_content.py
index f775ba185..0f78a53dc 100644
--- a/google/genai/_interactions/types/image_content.py
+++ b/google/genai/_interactions/types/image_content.py
@@ -29,6 +29,8 @@ class ImageContent(BaseModel):
 
     type: Literal["image"]
 
+    caption: Optional[str] = None
+
     data: Optional[str] = None
     """The image content."""
 
diff --git a/google/genai/_interactions/types/image_content_param.py b/google/genai/_interactions/types/image_content_param.py
index cb247f318..951fa6dab 100644
--- a/google/genai/_interactions/types/image_content_param.py
+++ b/google/genai/_interactions/types/image_content_param.py
@@ -33,6 +33,8 @@ class ImageContentParam(TypedDict, total=False):
 
     type: Required[Literal["image"]]
 
+    caption: str
+
     data: Annotated[Union[str, Base64FileInput], PropertyInfo(format="base64")]
     """The image content."""