diff --git a/python/packages/gemini/agent_framework_gemini/_chat_client.py b/python/packages/gemini/agent_framework_gemini/_chat_client.py
index 72f82bbdf41..b9870544530 100644
--- a/python/packages/gemini/agent_framework_gemini/_chat_client.py
+++ b/python/packages/gemini/agent_framework_gemini/_chat_client.py
@@ -890,9 +890,14 @@ def _prepare_config(
             kwargs["response_schema"] = response_schema
         elif (schema := self._extract_response_schema(response_format)) is not None:
             kwargs["response_schema"] = schema
-        if tools := self._prepare_tools(options):
+        tools = self._prepare_tools(options)
+        if tools:
             kwargs["tools"] = tools
-        if tool_config := self._prepare_tool_config(options.get("tool_choice")):
+        tool_config = self._prepare_tool_config(options.get("tool_choice"))
+        if tools and not self._vertexai and self._requires_server_side_tool_invocations(tools):
+            tool_config = tool_config or types.ToolConfig()
+            tool_config.include_server_side_tool_invocations = True
+        if tool_config:
             kwargs["tool_config"] = tool_config
         if thinking_config := options.get("thinking_config"):
             thinking_config_kwargs = {k: v for k, v in thinking_config.items() if v is not None}
@@ -975,7 +980,7 @@ def _prepare_tools(self, options: Mapping[str, Any]) -> list[types.Tool] | None:
             types.FunctionDeclaration(
                 name=tool.name,
                 description=tool.description or "",
-                parameters=tool.parameters(),  # type: ignore[arg-type]
+                parameters_json_schema=tool.parameters(),
             )
             for tool in tools_option
             if isinstance(tool, FunctionTool)
@@ -988,6 +993,20 @@ def _prepare_tools(self, options: Mapping[str, Any]) -> list[types.Tool] | None:
 
         return result or None
 
+    @staticmethod
+    def _requires_server_side_tool_invocations(tools: Sequence[types.Tool]) -> bool:
+        """Return whether Gemini Developer API requires server-side tool invocation reporting."""
+        has_function_declarations = any(tool.function_declarations for tool in tools)
+        return has_function_declarations and any(RawGeminiChatClient._has_server_side_tool(tool) for tool in tools)
+
+    @staticmethod
+    def _has_server_side_tool(tool: types.Tool) -> bool:
+        """Return whether the Gemini tool declares a native server-side tool."""
+        return any(
+            field_name != "function_declarations" and getattr(tool, field_name, None) is not None
+            for field_name in type(tool).model_fields
+        )
+
     def _prepare_tool_config(self, tool_choice: Any) -> types.ToolConfig | None:
         """Build a Gemini ``ToolConfig`` from the framework ``tool_choice`` value.
 
diff --git a/python/packages/gemini/tests/test_gemini_client.py b/python/packages/gemini/tests/test_gemini_client.py
index f2af3cfd7ae..dceb7b72aac 100644
--- a/python/packages/gemini/tests/test_gemini_client.py
+++ b/python/packages/gemini/tests/test_gemini_client.py
@@ -14,6 +14,7 @@
 from agent_framework import Agent, Content, FunctionTool, Message
 from google.genai import types
 from pydantic import BaseModel
+from typing_extensions import NotRequired, TypedDict
 
 from agent_framework_gemini import GeminiChatClient, GeminiChatOptions, RawGeminiChatClient, ThinkingConfig
 
@@ -40,6 +41,12 @@ def _has_gemini_integration_credentials() -> bool:
 
 _TEST_MODEL = os.getenv("GOOGLE_MODEL") or os.getenv("GEMINI_MODEL", "gemini-2.5-flash-lite")
 
+
+class _ToolListItem(TypedDict):
+    title: str
+    description: NotRequired[str]
+
+
 # stub helpers
 
 
@@ -1733,6 +1740,31 @@ def get_weather(city: str) -> str:
     assert function_declaration.name == "get_weather"
 
 
+async def test_function_tool_json_schema_forwarded_to_parameters_json_schema() -> None:
+    """Forwards full JSON Schema from FunctionTool instead of coercing it into Gemini's Schema subset."""
+
+    def add_items(items: list[_ToolListItem]) -> str:
+        """Add items."""
+        return "ok"
+
+    tool = FunctionTool(name="add_items", func=add_items)
+    schema = tool.parameters()
+    assert "$defs" in schema
+
+    client, mock = _make_gemini_client()
+    mock.aio.models.generate_content = AsyncMock(return_value=_make_response([_make_part(text="Done")]))
+
+    await client.get_response(
+        messages=[Message(role="user", contents=[Content.from_text("Add items")])],
+        options={"tools": [tool]},
+    )
+
+    config: types.GenerateContentConfig = mock.aio.models.generate_content.call_args.kwargs["config"]
+    function_declaration = _first_function_declaration(config)
+    assert function_declaration.parameters is None
+    assert function_declaration.parameters_json_schema == schema
+
+
 async def test_callable_tool_resolved_via_validate_options() -> None:
     """Raw callables passed as tools must be normalized by _validate_options into FunctionTools
     and reach the Gemini config as function declarations.
@@ -1756,6 +1788,63 @@ def get_weather(city: str) -> str:
     assert function_declaration.name == "get_weather"
 
 
+async def test_developer_api_mixed_native_and_function_tools_enable_server_side_invocations() -> None:
+    """Enables Gemini Developer API server-side tool invocations when built-ins and function tools are mixed."""
+    tool = _make_dummy_tool()
+    search_tool = GeminiChatClient.get_web_search_tool()
+    client, mock = _make_gemini_client()
+    mock.aio.models.generate_content = AsyncMock(return_value=_make_response([_make_part(text="Done")]))
+
+    await client.get_response(
+        messages=[Message(role="user", contents=[Content.from_text("Search and call a tool")])],
+        options={"tools": [search_tool, tool]},
+    )
+
+    config: types.GenerateContentConfig = mock.aio.models.generate_content.call_args.kwargs["config"]
+    assert config.tool_config is not None
+    assert config.tool_config.include_server_side_tool_invocations is True
+
+
+async def test_developer_api_mixed_native_and_function_tools_preserve_tool_choice() -> None:
+    """Preserves function calling mode while enabling Developer API server-side built-in tool invocations."""
+    tool = _make_dummy_tool()
+    search_tool = GeminiChatClient.get_web_search_tool()
+    client, mock = _make_gemini_client()
+    mock.aio.models.generate_content = AsyncMock(return_value=_make_response([_make_part(text="Done")]))
+
+    await client.get_response(
+        messages=[Message(role="user", contents=[Content.from_text("Search and call a tool")])],
+        options={"tools": [search_tool, tool], "tool_choice": "auto"},
+    )
+
+    config: types.GenerateContentConfig = mock.aio.models.generate_content.call_args.kwargs["config"]
+    function_calling_config = _function_calling_config(config)
+    assert function_calling_config.mode == "AUTO"
+    assert config.tool_config is not None
+    assert config.tool_config.include_server_side_tool_invocations is True
+
+
+async def test_vertex_ai_mixed_native_and_function_tools_do_not_enable_server_side_invocations() -> None:
+    """Does not set the Developer API-only server-side invocation flag for Vertex AI."""
+    tool = _make_dummy_tool()
+    search_tool = GeminiChatClient.get_web_search_tool()
+    mock = MagicMock()
+    mock._api_client.vertexai = True
+    client = GeminiChatClient(client=mock, model="gemini-2.5-flash")
+    mock.aio.models.generate_content = AsyncMock(return_value=_make_response([_make_part(text="Done")]))
+
+    await client.get_response(
+        messages=[Message(role="user", contents=[Content.from_text("Search and call a tool")])],
+        options={"tools": [search_tool, tool], "tool_choice": "auto"},
+    )
+
+    config: types.GenerateContentConfig = mock.aio.models.generate_content.call_args.kwargs["config"]
+    function_calling_config = _function_calling_config(config)
+    assert function_calling_config.mode == "AUTO"
+    assert config.tool_config is not None
+    assert config.tool_config.include_server_side_tool_invocations is None
+
+
 # _coerce_to_dict