Python: Preserve null arguments during tool invocation

[shusingh]

Summary

• Preserve explicitly provided null values when dumping validated tool arguments
• Apply the same behavior to direct FunctionTool.invoke() and automatic function calling
• Add regression coverage for required nullable tool parameters

Fixes #5934

Tests

• python -m pytest tests/core/test_tools.py::test_tool_invoke_preserves_explicit_null_for_required_nullable_argument tests/core/test_function_invocation_logic.py::test_auto_function_calling_preserves_explicit_null_arguments -q
• python -m pytest tests/core/test_tools.py tests/core/test_function_invocation_logic.py -q
• python -m ruff check agent_framework/_tools.py tests/core/test_tools.py tests/core/test_function_invocation_logic.py

[Copilot]

Pull request overview

This PR updates Python core tool invocation to preserve explicitly supplied null values through Pydantic argument validation, addressing required nullable tool parameters during direct and automatic invocation.

Changes:

• Adds a helper for dumping Pydantic argument models while restoring explicit top-level None values.
• Applies the helper in FunctionTool.invoke() and automatic function calling.
• Adds regression tests for direct and automatic nullable required arguments.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
python/packages/core/agent_framework/_tools.py	Updates argument dumping in tool invocation paths to preserve explicit None values.
python/packages/core/tests/core/test_tools.py	Adds direct FunctionTool.invoke() regression coverage.
python/packages/core/tests/core/test_function_invocation_logic.py	Adds automatic function calling regression coverage.

[shusingh]

Updated the PR to handle nested explicit null values as well. The argument dump helper now recursively restores explicitly provided nulls in nested Pydantic models, mappings, and lists/tuples, and I added regression coverage for both direct tool invocation and automatic function calling with nested nullable arguments.

[moonbox3]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/core/agent_framework
_tools.py	1182	83	92%	215, 264–265, 442, 444, 457, 482–484, 492, 510, 524, 531, 538, 561, 563, 570, 578, 707, 741–743, 746–748, 750, 756, 807–809, 834, 860, 864, 902–904, 908, 930, 1081, 1093, 1100–1103, 1124, 1132, 1146–1148, 1498, 1590, 1618, 1640, 1648, 1737, 1744–1745, 1804, 1808, 1854, 1915–1916, 1927, 1999, 2013, 2016, 2029, 2032, 2055, 2062, 2072, 2076, 2153, 2206, 2228, 2284, 2363, 2560, 2626–2627, 2793–2794, 2885
TOTAL	44517	5347	87%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
8819	33 💤	0 ❌	0 🔥	2m 20s ⏱️

[shusingh]

Pushed a follow-up that keeps the explicit-null helper behavior unchanged while making the mapping and sequence narrowing explicit for Pyright. Locally verified the touched file with Pyright 1.1.408 and reran the focused nullable-argument regression tests.

[moonbox3]

@shusingh please fix the failing CI/CD checks.

[shusingh]

Fixed the failing type checks by changing the helper narrowing from Any container casts to concrete object container casts, which keeps Pyright from seeing unknown element types while avoiding Mypy redundant-cast errors. Locally verified the touched file with Pyright 1.1.408, Mypy on �gent_framework/_tools.py, and the focused nullable-argument regression tests.

[shusingh]

Hi! Just checking in on this PR. The review feedback has been addressed, and it’s ready for another look when you have a chance. I’m happy to update the branch or make any further adjustments. Thanks!

[shusingh]

Hi again! Just following up on this PR when you have a chance. It should be ready for review, and I’m happy to update the branch or address anything else needed. Thanks!

[eavanvalkenburg]

there is also a exclude_defaults keyword on model_dump could that be used instead? I am not a fan of this extra function doing a bunch of dict manipulation.

[moonbox3]

Please re-open when ready to address comments and move forward. Thanks.