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Python: Fix Agent Framework tool parameter defaults #14346

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ff7fbf2
fix(python): preserve optional agent-framework tool parameter defaults
ump45nose Oct 4, 2026
5403348
Merge branch 'main' into fix/agent-framework-tool-default
ump45nose Oct 11, 2026
0a9db0f
Merge branch 'main' into fix/agent-framework-tool-default
ump45nose Oct 17, 2026
084034f
Merge branch 'main' into fix/agent-framework-tool-default
ump45nose Oct 31, 2026
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7 changes: 5 additions & 2 deletions python/semantic_kernel/functions/kernel_function.py
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Original file line number Diff line number Diff line change
Expand Up @@ -451,12 +451,15 @@ def as_agent_framework_tool(
fields = {}
for param in self.parameters:
if param.include_in_function_choices:
if param.default_value is not None:
# Requiredness is explicit metadata: None can be a legitimate
# default for optional parameters and must remain a Pydantic default.
if param.is_required:

@eavanvalkenburg Eduard van Valkenburg (eavanvalkenburg) Nov 2, 2026

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param.is_required does not distinguish x: str | None from x: str | None = None: the decorator marks both optional. With a nullable annotation but no Python default, this branch now creates Field(default=None), and tool.input_model() accepts the missing required argument. I reproduced this with a decorated def f(x: str | None). Preserve the missing-default distinction from the callable signature (or metadata) and add a regression for nullable-but-required parameters.

fields[param.name] = (param.type_, Field(description=param.description))
else:
fields[param.name] = (
param.type_,

@eavanvalkenburg Eduard van Valkenburg (eavanvalkenburg) Nov 2, 2026

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Preserving the None default also needs a nullable field annotation. For the new test function, metadata stores label as type_="str"; the generated model therefore advertises a string-only field with default null, and tool.input_model(label=None) raises ValidationError. The current test checks the default object but never validates an explicit None. Please retain the nullable annotation and cover both omitted and explicitly-null values.

Field(description=param.description, default=param.default_value),
)
fields[param.name] = (param.type_, Field(description=param.description))
input_model = create_model("InputModel", **fields) # type: ignore
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async def wrapper(*args: Any, **kwargs: Any) -> Any:
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22 changes: 22 additions & 0 deletions python/tests/unit/functions/test_kernel_function_from_method.py
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Expand Up @@ -171,6 +171,28 @@ 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, label: str | None = None) -> int:
return count

native_function = KernelFunction.from_method(method=function, plugin_name="MockPlugin")
tool = native_function.as_agent_framework_tool(kernel=Mock())
count_field = tool.input_model.model_fields["count"]
label_field = tool.input_model.model_fields["label"]

assert count_field.default == 5
assert not count_field.is_required()
assert label_field.default is None
assert not label_field.is_required()


def test_init_method_is_none():
with pytest.raises(FunctionInitializationError):
KernelFunction.from_method(method=None, plugin_name="MockPlugin")
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