Python: Reject tool call arguments for parameters with include_in_function_choices=False

python/semantic_kernel/functions/kernel_function_decorator.py

@@ -35,6 +35,8 @@ def kernel_function(
     if you set `{"include_in_function_choices": False}` in the annotation, that parameter will not be included in
     the representation of the function towards LLM's or MCP Servers. If you do set this and the parameter is required
     but you do not set it in a invoke level arguments, the function will raise an error.
+    During auto function calling, a tool call from the model that sets such a parameter is rejected as an
+    unexpected argument, so the value can only come from invoke level arguments or the parameter default.
 
     After the annotations, it checks recursively until it reaches the lowest level, and it combines
     the types into a single comma-separated string, a forwardRef is also supported.

python/semantic_kernel/kernel.py

@@ -376,10 +376,14 @@ async def invoke_function_call(
             required_param_names = {
                 param.name for param in function_to_call.parameters if param.name is not None and param.is_required
             }
+            # Parameters hidden from the model (include_in_function_choices=False) can only be set by the host.
+            model_param_names = {
+                param.name for param in function_to_call.parameters if param.include_in_function_choices
+            }
             received_param_names = set(parsed_args or {})
 
             missing_params = required_param_names - received_param_names
-            unexpected_params = received_param_names - {param.name for param in function_to_call.parameters}
+            unexpected_params = received_param_names - model_param_names
 
             if missing_params or unexpected_params:
                 msg_parts = []

python/tests/unit/kernel/test_kernel.py

@@ -7,7 +7,7 @@
 from copy import deepcopy
 from dataclasses import dataclass
 from pathlib import Path
-from typing import Union
+from typing import Annotated, Union
 from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
@@ -682,6 +682,62 @@ async def test_invoke_function_call_with_missing_or_unexpected_args(kernel: Kern
     ), "Expected fallback message not found in chat history."
 
 
+@pytest.mark.parametrize(
+    "model_arguments",
+    [{"value": "visible", "scope": "model-value"}, '{"value": "visible", "scope": "model-value"}'],
+    ids=["dict", "json"],
+)
+async def test_invoke_function_call_rejects_model_supplied_hidden_parameter(kernel: Kernel, model_arguments):
+    calls: list[str] = []
+
+    class TestPlugin:
+        @kernel_function
+        def echo(
+            self,
+            value: str,
+            scope: Annotated[str, "Host supplied", {"include_in_function_choices": False}] = "default",
+        ) -> str:
+            calls.append(scope)
+            return f"{value}:{scope}"
+
+    kernel.add_plugin(TestPlugin(), plugin_name="test")
+    chat_history = ChatHistory()
+
+    result = await kernel.invoke_function_call(
+        function_call=FunctionCallContent(name="test-echo", arguments=model_arguments),
+        chat_history=chat_history,
+        arguments=KernelArguments(scope="host-value"),
+        function_behavior=FunctionChoiceBehavior.Auto(),
+    )
+
+    assert result is None
+    assert calls == []
+    assert "Received unexpected argument(s): ['scope']" in chat_history.messages[-1].items[0].result
+
+
+async def test_invoke_function_call_keeps_host_supplied_hidden_parameter(kernel: Kernel):
+    class TestPlugin:
+        @kernel_function
+        def echo(
+            self,
+            value: str,
+            scope: Annotated[str, "Host supplied", {"include_in_function_choices": False}] = "default",
+        ) -> str:
+            return f"{value}:{scope}"
+
+    kernel.add_plugin(TestPlugin(), plugin_name="test")
+    chat_history = ChatHistory()
+
+    await kernel.invoke_function_call(
+        function_call=FunctionCallContent(name="test-echo", arguments={"value": "visible"}),
+        chat_history=chat_history,
+        arguments=KernelArguments(scope="host-value"),
+        function_behavior=FunctionChoiceBehavior.Auto(),
+    )
+
+    assert chat_history.messages[-1].items[0].result == "visible:host-value"
+
+
 async def test_invoke_function_call_with_filters_blocks_unallowed_function(kernel: Kernel):
     """Verify that when function_behavior has filters, an unallowed function is blocked."""
     tool_call_mock = MagicMock(spec=FunctionCallContent)