Fixes to detect optional parameters in tool conversion used by "nat mcp serve"

[willkill07]

Parameters with default values were incorrectly marked as required in MCP tool schemas. Now checking for PydanticUndefined and properly extracting default values from Pydantic fields to correctly identify optional parameters.

This PR also adds comprehensive unit tests for the tool schema conversion

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• New Features

  • Improved detection and handling of optional parameters and default/default-factory values in tool wrapper generation, with unified validation and consistent invocation behavior for workflows and regular functions.
  • Ensures parameter descriptions, order, and type annotations are preserved in generated wrappers.

• Tests

  • Added comprehensive tests covering optional/default combinations, wrapper signatures, execution paths, and observability propagation.

Summary by CodeRabbit

Release Notes

• New Features

  • Added improved support for optional parameters in MCP tools with enhanced default value handling.

• Bug Fixes

  • Fixed inconsistent behavior with nested schema validation and default factory application in tool parameters.

• Tests

  • Added comprehensive test coverage for optional field handling and parameter validation scenarios.

[coderabbitai]

Walkthrough

The changes refactor how optional Pydantic fields are handled in MCP tool conversion. A new is_field_optional() helper function detects optional fields and their defaults. The validation path is unified to use Pydantic's model_validate() instead of ad-hoc reconstruction. Validated model instances or dumps are now passed directly to workflow runners and underlying functions.

Changes

Cohort / File(s)	Summary
Production refactoring of optional field handling src/nat/front_ends/mcp/tool_converter.py	Introduces is_field_optional(field: FieldInfo) helper to detect optional vs. required fields and extract default values (including a _USE_PYDANTIC_DEFAULT sentinel for factory defaults). Extends imports to include Any, FieldInfo, and PydanticUndefined. Integrates the helper into parameter construction and replaces ad-hoc nested-schema handling with uniform Pydantic model_validate() flow. Updates workflow and non-workflow call paths to pass validated model instances or dumps instead of raw kwargs reconstruction.
Comprehensive test coverage for field handling and validation tests/nat/front_ends/mcp/test_tool_converter.py	Adds new mock schema models covering all-required, mixed, all-optional, and optional-with-none Pydantic field configurations. Introduces tests for is_field_optional() utility with various default scenarios (defaults, factories, None, zero). Adds parameter schema validation tests across different model configurations and extended integration tests covering observability context propagation, wrapper execution, error handling, and optional parameter omission/provision scenarios. Imports and uses _USE_PYDANTIC_DEFAULT sentinel.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

• Sentinel value handling: Pay close attention to how _USE_PYDANTIC_DEFAULT is created, propagated through kwargs cleanup, and interpreted by Pydantic during model_validate()
• Validation orchestration: Review the refactored path ensuring that model_validate() correctly applies defaults and factory functions, and that edge cases with nested schemas are handled properly
• Workflow vs. non-workflow paths: Verify that both branches (workflow context present vs. absent) correctly pass model instances or dumps to their respective callees
• Optional field edge cases: Confirm that the detection logic correctly distinguishes between required fields, optional with explicit defaults, optional with factories, and optional with None as default

Pre-merge checks and finishing touches

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title Check	✅ Passed	The pull request title "Fixes to detect optional parameters in tool conversion used by "nat mcp serve"" is fully related to the main changes in the changeset. The title accurately describes the core improvement: enhanced detection of optional parameters in MCP tool schema conversion, which directly aligns with the implementation of is_field_optional(), improved parameter handling for Pydantic fields, and better default value extraction. The title uses imperative mood ("Fixes"), is descriptive, and clearly conveys the purpose of the changes. The title exceeds the stated guideline of 72 characters by 6 characters (78 total), but this minor overage does not materially impact clarity or conciseness given the tilde () indicates an approximate threshold.
Docstring Coverage	✅ Passed	Docstring coverage is 96.00% which is sufficient. The required threshold is 80.00%.

✨ Finishing touches

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🧪 Generate unit tests (beta)

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• Post copyable unit tests in a comment

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[willkill07]

/merge

[coderabbitai]

Actionable comments posted: 0

🧹 Nitpick comments (2)

tests/nat/front_ends/mcp/test_tool_converter.py (2)

547-553: Tighten order assertion

This test claims to guard parameter ordering but only checks membership, so it still passes if the parameters are shuffled. Please assert the exact sequence, e.g., assert param_names == ["required_str", "required_int", "optional_str", "optional_int", "optional_list"].

---

625-631: Verify propagated defaults

We only check that acall_invoke ran once, which wouldn't fail if defaults failed to propagate. Please assert the actual call signature (e.g., via assert_called_once_with(...)) so we prove the factory/default values were forwarded correctly.

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• src/nat/front_ends/mcp/tool_converter.py (4 hunks)
• tests/nat/front_ends/mcp/test_tool_converter.py (6 hunks)

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🧬 Code graph analysis (1)

tests/nat/front_ends/mcp/test_tool_converter.py (1)

src/nat/front_ends/mcp/tool_converter.py (1)

• is_field_optional (41-70)