fix(python): preserve agent tool parameter defaults

Author: ump45nose

python/semantic_kernel/functions/kernel_function.py

@@ -452,7 +452,8 @@ def as_agent_framework_tool(
                         param.type_,
                         Field(description=param.description, default=param.default_value),
                     )
-                fields[param.name] = (param.type_, Field(description=param.description))
+                else:
+                    fields[param.name] = (param.type_, Field(description=param.description))
         input_model = create_model("InputModel", **fields)  # type: ignore
 
         async def wrapper(*args: Any, **kwargs: Any) -> Any:

python/tests/unit/functions/test_kernel_function_from_method.py

@@ -140,6 +140,25 @@ def decorated_function() -> None:
     assert len(native_function.parameters) == 0
 
 
+def test_as_agent_framework_tool_preserves_optional_parameter_default(monkeypatch):
+    class MockAIFunction:
+        def __init__(self, *, input_model: Any, **kwargs: Any):
+            self.input_model = input_model
+
+    monkeypatch.setitem(__import__("sys").modules, "agent_framework", Mock(AIFunction=MockAIFunction))
+
+    @kernel_function()
+    def function(count: int = 5) -> int:
+        return count
+
+    native_function = KernelFunction.from_method(method=function, plugin_name="MockPlugin")
+    tool = native_function.as_agent_framework_tool(kernel=Mock())
+    field = tool.input_model.model_fields["count"]
+
+    assert field.default == 5
+    assert not field.is_required()
+
+
 def test_init_method_is_none():
     with pytest.raises(FunctionInitializationError):
         KernelFunction.from_method(method=None, plugin_name="MockPlugin")