`create_work_item_property` fails through MCP with an output validation error, even though the underlying Plane API request succeeds and the property is actually created.

[Echo-AE]

## Summary

`create_work_item_property` fails through MCP with an output validation error, even though the underlying Plane API request succeeds and the property is actually created.

## Environment

- `plane-mcp-server` `0.2.5`
- `FastMCP` `2.14.1`
- self-hosted Plane workspace with paid features enabled

## Reproduction

1. Create a project
2. Enable `work_item_types`
3. Create a custom work item type
4. Call MCP tool:

```json
{
  "name": "create_work_item_property",
  "arguments": {
    "project_id": "<project_uuid>",
    "type_id": "<custom_type_uuid>",
    "display_name": "Risk",
    "property_type": "TEXT",
    "settings": {
      "display_format": "single-line"
    }
  }
}

Actual Result

MCP returns:

{
  "content": [
    {
      "type": "text",
      "text": "Output validation error: None is not of type 'string'"
    }
  ],
  "isError": true
}

Expected Result

The tool should return the created WorkItemProperty.

Additional Evidence

The direct Plane REST API call succeeds with the same payload.

Request:

{
  "display_name": "Risk",
  "property_type": "TEXT",
  "settings": {
    "display_format": "single-line"
  }
}

Response example from Plane:

{
  "id": "5496bd03-1137-4eda-974a-766f19dbd458",
  "deleted_at": null,
  "relation_type": null,
  "created_at": "2026-03-08T18:50:35.983383Z",
  "updated_at": "2026-03-08T18:50:35.983390Z",
  "name": "risk-direct-api-schema-debug-3",
  "display_name": "Risk Direct API Schema Debug 3",
  "description": null,
  "property_type": "TEXT",
  "settings": {
    "display_format": "single-line"
  },
  "external_source": null,
  "external_id": null,
  "updated_by": null,
  "issue_type": "e1603494-a7be-488b-bd25-9fb8c20ee938"
}

Notes

The nullable fields in the successful Plane response include at least:

• deleted_at
• relation_type
• description
• external_source
• external_id
• updated_by

The Plane SDK model WorkItemProperty accepts and serializes this response successfully, so this appears to be an issue in plane-mcp-server / FastMCP output validation rather than in the request payload itself.