Fixes an issue where MCP tools with complex array parameters (likebrowser_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 complexitems schemas were being converted to simplelist 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 theContribution 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.

mcp tool parameter fix

3b6c0b0

CopilotAI review requested due to automatic review settings

November 21, 2025 17:34

markwallace-microsoft added the python label

Nov 21, 2025

github-actionsbot changed the title~~Preserve MCP array items schema in Pydantic field generation~~Python: Preserve MCP array items schema in Pydantic field generation

Nov 21, 2025

Copilotstarted reviewing on behalf ofgiles17

November 21, 2025 17:34

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markwallace-microsoft commentedNov 21, 2025•
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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 ⏱️

Copilotfinished reviewing on behalf ofgiles17

November 21, 2025 17:36

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Pull request overview

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

Key Changes:

Modified_get_input_model_from_mcp_tool() to preserve array items schema usingjson_schema_extra
Refactored field definition creation logic to use afield_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