Python: fix(claude): preserve $defs in JSON schema for nested Pydantic models

[Dineshsuriya D (droideronline)]

Summary

This PR fixes a bug where _function_tool_to_sdk_mcp_tool() dropped the $defs section from Pydantic JSON schemas, breaking any tool that uses nested Pydantic models.

Problem

When converting a FunctionTool to an SDK MCP tool, the code was only extracting properties and required from the Pydantic model's JSON schema, but dropping the $defs section which contains definitions for nested types:

input_schema: dict[str, Any] = {
    "type": "object",
    "properties": schema.get("properties", {}),
    "required": schema.get("required", []),
}
# $defs was not being preserved!

This caused tools with nested Pydantic models to produce invalid/unresolvable JSON schemas. The schema would contain $ref references like "$ref": "#/$defs/SomeNestedType" but the $defs section was missing, so Claude could not resolve the type and refused to call the tool.

Changes

_agent.py

• Preserve $defs section from Pydantic JSON schema when present:

  if "$defs" in schema:
      input_schema["$defs"] = schema["$defs"]

test_claude_agent.py

• Added test_function_tool_to_sdk_mcp_tool_preserves_defs_for_nested_types - verifies that nested Pydantic models generate schemas with $defs preserved

Testing

All 45 tests pass:

pytest tests/test_claude_agent.py -v
============================== 45 passed in 0.31s ==============================

Fixes #3654

[Copilot]

Pull request overview

This PR fixes the Claude agent’s FunctionTool → SDK MCP conversion so that Pydantic nested models produce valid JSON schemas by preserving the $defs section, and adds a regression test to prevent regressions for nested types.

Changes:

• Updated _function_tool_to_sdk_mcp_tool to carry over the $defs section from the Pydantic model’s model_json_schema() into the MCP tool input schema.
• Added a dedicated test ensuring that tools with nested Pydantic models have $defs preserved and that the top-level property referencing the nested model is present.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
python/packages/claude/agent_framework_claude/_agent.py	Extends the generated MCP input schema to include $defs from the underlying Pydantic schema so nested $ref types resolve correctly.
python/packages/claude/tests/test_claude_agent.py	Adds a regression test using nested Pydantic models to verify that $defs is preserved and that the nested model reference appears in properties.

[Mark Wallace (markwallace-microsoft)]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/core/agent_framework
_tools.py	769	76	90%	226, 272, 323, 325, 353, 523, 558–559, 661, 663, 683, 701, 715, 727, 732, 734, 741, 774, 845–847, 888, 913–922, 928–937, 973, 981, 1222, 1559, 1646–1650, 1672–1673, 1789, 1791, 1857, 1949, 1955, 1997–1998, 2011–2012, 2055, 2139, 2177–2178, 2217–2219, 2257–2258, 2268, 2325–2326, 2333–2334
TOTAL	16338	1982	87%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
3822	221 💤	0 ❌	0 🔥	1m 9s ⏱️

[Dineshsuriya D (droideronline)]

Evan Mattson (@moonbox3) - Can we have please have a look at this ?

[Evan Mattson (moonbox3)]

I need to get another PR merged that fixes a bug, and thus allows the failing unit test to pass.

[Dineshsuriya D (droideronline)]

I need to get another PR merged that fixes a bug, and thus allows the failing unit test to pass.

sure, I just cherry-picked your test failure fixes here