Python: Preserve MCP array items schema in Pydantic field generation

[giles17]

Motivation and Context

Fixes an issue where MCP tools with complex array parameters (like browser_fill_form from Playwright MCP) were losing their array items schema during the MCP → Pydantic → JSON Schema conversion pipeline, causing LLMs to receive incomplete parameter information.

When converting MCP tool schemas to Pydantic models, array parameters with complex items schemas were being converted to simple list types without preserving the detailed structure of array items. This resulted in LLMs receiving incomplete schemas and making wrong function calls.

Before Fix:

{
  "type": "function",
  "function": {
    "name": "browser_fill_form",
    "description": "Fill multiple form fields",
    "parameters": {
      "properties": {
        "fields": {
          "description": "Fields to fill in",
          "items": {},
          "title": "Fields",
          "type": "array"
        }
      },
      "required": [
        "fields"
      ],
      "title": "browser_fill_form_input",
      "type": "object"
    }
  }
}

After Fix:

{
  "type": "function",
  "function": {
    "name": "browser_fill_form",
    "description": "Fill multiple form fields",
    "parameters": {
      "properties": {
        "fields": {
          "description": "Fields to fill in",
          "items": {
            "additionalProperties": false,
            "properties": {
              "name": {
                "description": "Human-readable field name",
                "type": "string"
              },
              "type": {
                "description": "Type of the field",
                "enum": [
                  "textbox",
                  "checkbox",
                  "radio",
                  "combobox",
                  "slider"
                ],
                "type": "string"
              },
              "ref": {
                "description": "Exact target field reference from the page snapshot",
                "type": "string"
              },
              "value": {
                "description": "Value to fill in the field. If the field is a checkbox, the value should be `true` or `false`. If the field is a combobox, the value should be the text of the option.",  
                "type": "string"
              }
            },
            "required": [
              "name",
              "type",
              "ref",
              "value"
            ],
            "type": "object"
          },
          "title": "Fields",
          "type": "array"
        }
      },
      "required": [
        "fields"
      ],
      "title": "browser_fill_form_input",
      "type": "object"
    }
  }
}

Resolves #1869

Description

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[markwallace-microsoft]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/core/agent_framework
_mcp.py	379	58	84%	184, 194–195, 216, 241, 256, 262, 266, 302, 371, 398, 432–433, 435–443, 445–447, 450–451, 497, 512, 530, 571, 584, 587–588, 592, 602, 626, 629–630, 634, 644, 675, 694, 696, 703–704, 723, 725, 731–734, 751–755, 883
TOTAL	15473	2352	84%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
2219	127 💤	0 ❌	0 🔥	56.934s ⏱️

[Copilot]

Pull request overview

This PR fixes an issue where MCP tools with complex array parameters were losing their items schema during the MCP → Pydantic → JSON Schema conversion pipeline. The fix preserves array item schemas by storing them in Pydantic's json_schema_extra field attribute.

Key Changes:

• Modified _get_input_model_from_mcp_tool() to preserve array items schema using json_schema_extra
• Refactored field definition creation logic to use a field_kwargs dictionary approach
• Added test for simple array types to verify items schema preservation

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
python/packages/core/agent_framework/_mcp.py	Modified field definition logic to preserve array items schema via json_schema_extra and refactored field creation to use a kwargs-based approach
python/packages/core/tests/core/test_mcp.py	Added test case for simple array types to verify items schema is preserved in generated JSON schema