diff --git a/python/packages/core/agent_framework/_tools.py b/python/packages/core/agent_framework/_tools.py
index 93722a8987..dc8a38ded8 100644
--- a/python/packages/core/agent_framework/_tools.py
+++ b/python/packages/core/agent_framework/_tools.py
@@ -237,6 +237,14 @@ def _annotation_includes_function_invocation_context(annotation: Any) -> bool:
 ClassT = TypeVar("ClassT", bound="SerializationMixin")
 
 
+def _dump_tool_arguments(model: BaseModel, *, include_none_from: Iterable[str] | None = None) -> dict[str, Any]:
+    arguments = model.model_dump(exclude_none=True)
+    for name in include_none_from if include_none_from is not None else model.model_fields_set:
+        if getattr(model, name, None) is None:
+            arguments[name] = None
+    return arguments
+
+
 class FunctionTool(SerializationMixin):
     """A tool that wraps a Python function to make it callable by AI models.
 
@@ -635,8 +643,9 @@ async def invoke(
                 if isinstance(arguments, Mapping):
                     parsed_arguments = dict(arguments)
                     if self.input_model is not None and not self._schema_supplied:
-                        parsed_arguments = self.input_model.model_validate(parsed_arguments).model_dump(
-                            exclude_none=True
+                        parsed_arguments = _dump_tool_arguments(
+                            self.input_model.model_validate(parsed_arguments),
+                            include_none_from=parsed_arguments,
                         )
                 elif isinstance(arguments, BaseModel):
                     if (
@@ -645,7 +654,7 @@ async def invoke(
                         and not isinstance(arguments, self.input_model)
                     ):
                         raise TypeError(f"Expected {self.input_model.__name__}, got {type(arguments).__name__}")
-                    parsed_arguments = arguments.model_dump(exclude_none=True)
+                    parsed_arguments = _dump_tool_arguments(arguments)
                 else:
                     raise TypeError(
                         f"Expected mapping-like arguments for tool '{self.name}', got {type(arguments).__name__}"
@@ -1492,7 +1501,7 @@ async def _auto_invoke_function(
         runtime_kwargs["session"] = invocation_session
     try:
         if not cast(bool, getattr(tool, "_schema_supplied", False)) and tool.input_model is not None:
-            args = tool.input_model.model_validate(parsed_args).model_dump(exclude_none=True)
+            args = _dump_tool_arguments(tool.input_model.model_validate(parsed_args), include_none_from=parsed_args)
         else:
             args = dict(parsed_args)
         args = _validate_arguments_against_schema(
diff --git a/python/packages/core/tests/core/test_function_invocation_logic.py b/python/packages/core/tests/core/test_function_invocation_logic.py
index 3d20a26080..774f41fef9 100644
--- a/python/packages/core/tests/core/test_function_invocation_logic.py
+++ b/python/packages/core/tests/core/test_function_invocation_logic.py
@@ -87,6 +87,40 @@ def ai_func(arg1: str) -> str:
     assert response.messages[2].text == "done"
 
 
+async def test_base_client_with_function_calling_preserves_null_arguments(chat_client_base: SupportsChatGetResponse):
+    seen_unit = "not-called"
+
+    @tool(name="get_weather", approval_mode="never_require")
+    def get_weather(location: str, unit: str | None) -> str:
+        nonlocal seen_unit
+        seen_unit = unit
+        return f"{location}:{unit}"
+
+    chat_client_base.run_responses = [
+        ChatResponse(
+            messages=Message(
+                role="assistant",
+                contents=[
+                    Content.from_function_call(
+                        call_id="1",
+                        name="get_weather",
+                        arguments='{"location": "Seattle", "unit": null}',
+                    )
+                ],
+            )
+        ),
+        ChatResponse(messages=Message(role="assistant", contents=["done"])),
+    ]
+
+    response = await chat_client_base.get_response(
+        [Message(role="user", contents=["weather?"])], options={"tool_choice": "auto", "tools": [get_weather]}
+    )
+
+    assert seen_unit is None
+    assert response.messages[1].contents[0].type == "function_result"
+    assert response.messages[1].contents[0].result == "Seattle:None"
+
+
 async def test_base_client_with_function_calling_string_input(chat_client_base: SupportsChatGetResponse):
     exec_counter = 0
 
diff --git a/python/packages/core/tests/core/test_tools.py b/python/packages/core/tests/core/test_tools.py
index b3762bf4ef..3d62e76a93 100644
--- a/python/packages/core/tests/core/test_tools.py
+++ b/python/packages/core/tests/core/test_tools.py
@@ -183,6 +183,17 @@ def search(query: str, max_results: int = 10) -> str:
         await search.invoke(arguments={"query": "hello", "max_results": "three"})
 
 
+async def test_tool_invoke_preserves_required_nullable_argument() -> None:
+    @tool
+    def get_weather(location: str, unit: str | None) -> str:
+        return f"{location}:{unit}"
+
+    result = await get_weather.invoke(arguments={"location": "Seattle", "unit": None})
+
+    assert isinstance(result, list)
+    assert result[0].text == "Seattle:None"
+
+
 def test_tool_decorator_with_json_schema_preserves_custom_properties():
     """Test schema passthrough keeps custom JSON schema properties."""