[John Karam (johnkaram02)]

Motivation and Context

KernelFunctionFromMethod throws ArgumentOutOfRangeException when an argument is present with a null value and the target parameter is a non-nullable value type with a default value, for example int limit = 10.

With AllowStrictSchemaAdherence = true, the OpenAI connector marks optional parameters as required and nullable, so a model can send null for an optional parameter it skips. A plugin method with an optional value-type parameter then fails on that call, and the model receives an error instead of the function result.

Fixes #14652

Description

When the argument value is null, the parameter has a default value, and the parameter type is a non-nullable value type, the marshaler now skips the argument and uses the default value, the same as when the argument is missing.

The change only affects the case that currently throws. These cases keep their current behavior:

• null for a non-nullable value type without a default value still throws ArgumentOutOfRangeException.
• null for a reference type or Nullable<T> parameter is still passed as null.

A new unit test, ItShouldUseDefaultValueWhenNullIsPassedForValueTypeParameterWithDefaultValueAsync, covers the fix. It fails on main with ArgumentOutOfRangeException and passes with this change. All 1615 tests in SemanticKernel.UnitTests pass.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

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[Copilot]

🟢 Approval recommended

The narrow condition preserves other argument-handling paths, and the regression test covers the reported failure.

No open issues 🎉

What changed in this PR

Fixes #14652 by using a parameter’s default value when an explicit null targets an optional, non-nullable value type. This supports strict-schema function calls without changing null handling for other parameter types.

Changes:

• Routes eligible null arguments through the existing default-value fallback.
• Adds a regression test verifying that an optional int receives its default value.

File	Description
dotnet/​src/​SemanticKernel.UnitTests/​Functions/​KernelFunctionFromMethodTests1.cs	Tests default-value handling for an explicit null argument.
dotnet/​src/​SemanticKernel.Core/​Functions/​KernelFunctionFromMethod.cs	Adjusts argument marshaling to use the default for eligible null values.

_{Review effort: Balanced}