[Eduard van Valkenburg (eavanvalkenburg)]

Motivation and Context

Parameters declared with {"include_in_function_choices": False} are left out of the tool schema that is sent to the model, and the docs describe them as values the host supplies through KernelArguments. Kernel.invoke_function_call still accepted those names in a model tool call, so a value from the model took precedence over the invocation-level value.

This change makes argument handling match the schema: a tool call can only set the parameters that were advertised to the model. It is the counterpart of #14625, which stopped advertising these parameters in the Anthropic, Bedrock, Google AI and Vertex AI tool formats. There is no related open issue.

Description

• Kernel.invoke_function_call builds the set of accepted argument names from the parameters that have include_in_function_choices=True. A tool call that includes a hidden parameter gets the existing "Received unexpected argument(s)" result and the function is not invoked, so the model can retry without it.
• Hidden parameters still get their value from the invocation-level KernelArguments or from their default. The required-parameter check is unchanged.
• The kernel_function docstring now states that a tool call from the model cannot set such a parameter.
• Tests in tests/unit/kernel/test_kernel.py cover a hidden name in dict and JSON-string arguments (the function must not run and the result must be the unexpected-argument message) and a host-supplied hidden value that still reaches the function. The two rejection tests fail without the change.

Compatibility: Bedrock agents create their function schema server-side when the action group is created, so action groups created before #14625 (released in python 1.46.0) still list hidden parameters. Calls from such agents that include a hidden name now get the unexpected-argument result until the action group is re-created.

Validation, run on Python 3.13 (macOS) in an environment built like the unit-test workflow (uv pip install --editable ".[<all extras>]" --group dev), from python/ unless noted:

• uv run --no-sync python -u -X faulthandler -m pytest -p no:cacheprovider -n auto ./tests/unit --ignore=tests/unit/processes/dapr_runtime: 4983 passed, 1 skipped, 5 xfailed. The dapr suite was not run locally.
• uv run --no-sync pytest -p no:cacheprovider tests/unit/kernel/test_kernel.py -k hidden_parameter: 3 passed.
• uv run --no-sync pre-commit run --config python/.pre-commit-config.yaml --files <the three changed files> (from the repo root): all hooks passed or had no files to check.
• uv run --no-sync mypy -p semantic_kernel --config-file mypy.ini: 42 errors in 7 files, none in the changed files. Unmodified main reports the same total (42 errors in 7 files) in this environment.

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 😄

[Copilot]

🟢 Approval recommended

No active review issues were identified for the change rejecting model-supplied arguments for hidden parameters.

No open issues 🎉

What changed in this PR

This PR aligns Python auto function calling with tool schemas by rejecting model-supplied arguments for parameters marked include_in_function_choices=False. It complements #14625, which excluded these parameters from additional providers’ tool schemas.

Changes:

• Reject hidden parameter names through the existing unexpected-argument response.
• Document that hidden values come from invocation-level arguments or parameter defaults.
• Add tests for rejection of dict and JSON-string arguments and preservation of host-supplied values.

File	Description
python/​tests/​unit/​kernel/​test_kernel.py	Adds hidden-parameter rejection and host-value preservation tests.
python/​semantic_kernel/​kernel.py	Restricts accepted model arguments to visible parameters.
python/​semantic_kernel/​functions/​kernel_function_decorator.py	Documents hidden-parameter handling during auto function calling.

_{Review effort: Balanced}