diff --git a/python/packages/core/agent_framework/_tools.py b/python/packages/core/agent_framework/_tools.py
index 6a6354419b..588ae93d5c 100644
--- a/python/packages/core/agent_framework/_tools.py
+++ b/python/packages/core/agent_framework/_tools.py
@@ -655,8 +655,14 @@ async def invoke(
                 if isinstance(arguments, Mapping):
                     parsed_arguments = dict(arguments)
                     if self.input_model is not None and not self._schema_supplied:
+                        # exclude_unset (not exclude_none): keep arguments the model
+                        # explicitly provided even when their value is null, and drop
+                        # only the ones it left out, so the function's own defaults
+                        # apply. Excluding null instead would strip a required nullable
+                        # parameter the model deliberately set to null, failing the
+                        # invocation on the missing argument (#5934).
                         parsed_arguments = self.input_model.model_validate(parsed_arguments).model_dump(
-                            exclude_none=True
+                            exclude_unset=True
                         )
                 elif isinstance(arguments, BaseModel):
                     if (
@@ -665,7 +671,7 @@ 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 = arguments.model_dump(exclude_unset=True)
                 else:
                     raise TypeError(
                         f"Expected mapping-like arguments for tool '{self.name}', got {type(arguments).__name__}"
@@ -1480,7 +1486,10 @@ 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)
+            # exclude_unset (not exclude_none) so an argument the model explicitly set
+            # to null still reaches the function; see FunctionTool.invoke for the full
+            # rationale. This is the auto-calling path #5934 actually hits.
+            args = tool.input_model.model_validate(parsed_args).model_dump(exclude_unset=True)
         else:
             args = dict(parsed_args)
         args = _validate_arguments_against_schema(
diff --git a/python/packages/core/tests/core/test_tools.py b/python/packages/core/tests/core/test_tools.py
index da632b6074..a24e20cacd 100644
--- a/python/packages/core/tests/core/test_tools.py
+++ b/python/packages/core/tests/core/test_tools.py
@@ -17,8 +17,10 @@
 )
 from agent_framework._middleware import FunctionInvocationContext
 from agent_framework._tools import (
+    _auto_invoke_function,
     _parse_annotation,
     _parse_inputs,
+    normalize_function_invocation_configuration,
 )
 from agent_framework.observability import OtelAttr
 
@@ -151,6 +153,59 @@ def search(query: str) -> str:
         await search.invoke(arguments={})
 
 
+async def test_invoke_preserves_explicit_null_argument():
+    """A required nullable argument the model sets to null must reach the function.
+
+    Regression for #5934: exclude_none dropped the explicit null, so the required
+    ``unit`` went missing and the invocation failed.
+    """
+
+    @tool
+    def get_weather(location: str, unit: Literal["C", "F"] | 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"
+
+
+async def test_invoke_omitted_optional_uses_function_default():
+    """An omitted optional argument still falls back to the function's own default."""
+
+    @tool
+    def get_weather(location: str, unit: str = "C") -> str:
+        return f"{location}:{unit}"
+
+    result = await get_weather.invoke(arguments={"location": "Seattle"})
+    assert result[0].text == "Seattle:C"
+
+
+async def test_auto_invoke_preserves_explicit_null_argument():
+    """The auto function-calling path must preserve an explicit null argument too.
+
+    Regression for #5934: ``FunctionTool.invoke`` was fixed, but ``_auto_invoke_function``
+    (the path a model's ``function_call`` actually takes) still ran ``exclude_none`` and
+    dropped the required ``unit``, so the invocation failed with a missing argument.
+    """
+
+    @tool
+    def get_weather(location: str, unit: Literal["C", "F"] | None) -> str:
+        return f"{location}:{unit}"
+
+    function_call = Content.from_function_call(
+        call_id="call-1",
+        name=get_weather.name,
+        arguments='{"location": "Seattle", "unit": null}',
+    )
+    result = await _auto_invoke_function(
+        function_call,
+        config=normalize_function_invocation_configuration(None),
+        tool_map={get_weather.name: get_weather},
+    )
+    assert result.type == "function_result"
+    assert result.result == "Seattle:None"
+
+
 async def test_tool_decorator_with_json_schema_invoke_invalid_type():
     """Test schema type checks run for mapping arguments."""