Failed to validate tool f1e_GameObject_Modify: Error: tool parameters array type must have items

[IvanMurzak]

Does this issue occur when all extensions are disabled?: Yes/No

• VS Code Version: 1.100.0 (user setup)
• OS Version: Windows_NT x64 10.0.26100

Steps to Reproduce:

1. To have the MCP tool with name GameObject_Modify with the specified schema down below.
2. Be in "Agent" mode and have "GPT-4.1" model.
3. Type any prompt.

As you may notice from the json schema, each "array" has an "item" which describe the data model.
Same model GPT-4.1 works in Cursor with the same tool and the same schema. That is why I believe it is VS Code's issue.
I am using my Unity-MCP.

Json Schema

{
  "name": "GameObject_Modify",
  "description": "Modify GameObjects and/or attached component\u0027s field and properties.\r\nYou can modify multiple GameObjects at once. Just provide the same number of GameObject references and SerializedMember objects.",
  "inputSchema": {
    "type": "object",
    "properties": {
      "gameObjectDiffs": {
        "type": "array",
        "items": {
          "type": "object",
          "properties": {
            "typeName": {
              "type": "string",
              "description": "Full type name. Eg: \u0027System.String\u0027, \u0027System.Int32\u0027, \u0027UnityEngine.Vector3\u0027, etc."
            },
            "name": {
              "type": "string"
            },
            "value": {
              "type": "object"
            },
            "fields": {
              "type": "array",
              "items": {
                "type": "object",
                "properties": {
                  "typeName": {
                    "type": "string",
                    "description": "Full type name. Eg: \u0027System.String\u0027, \u0027System.Int32\u0027, \u0027UnityEngine.Vector3\u0027, etc."
                  },
                  "name": {
                    "type": "string"
                  },
                  "value": {
                    "type": "object"
                  },
                  "fields": {
                    "type": "array",
                    "items": {
                      "type": "object"
                    }
                  },
                  "props": {
                    "type": "array",
                    "items": {
                      "type": "object"
                    }
                  }
                },
                "required": [
                  "typeName",
                  "name",
                  "value"
                ]
              }
            },
            "props": {
              "type": "array",
              "items": {
                "type": "object",
                "properties": {
                  "typeName": {
                    "type": "string",
                    "description": "Full type name. Eg: \u0027System.String\u0027, \u0027System.Int32\u0027, \u0027UnityEngine.Vector3\u0027, etc."
                  },
                  "name": {
                    "type": "string"
                  },
                  "value": {
                    "type": "object"
                  },
                  "fields": {
                    "type": "array",
                    "items": {
                      "type": "object"
                    }
                  },
                  "props": {
                    "type": "array",
                    "items": {
                      "type": "object"
                    }
                  }
                },
                "required": [
                  "typeName",
                  "name",
                  "value"
                ]
              }
            }
          },
          "required": [
            "typeName",
            "value"
          ]
        },
        "description": "Json Object with required readonly \u0027instanceID\u0027 and \u0027type\u0027 fields.\r\nEach field and property requires to have \u0027type\u0027 and \u0027name\u0027 fields to identify the exact modification target.\r\nFollow the object schema to specify what to change, ignore values that should not be modified. Keep the original data structure.\r\nAny unknown or wrong located fields and properties will be ignored.\r\nCheck the result of this command to see what was changed. The ignored fields and properties will be listed."
      },
      "gameObjectRefs": {
        "type": "array",
        "items": {
          "type": "object",
          "properties": {
            "instanceID": {
              "type": "integer"
            },
            "path": {
              "type": "string"
            },
            "name": {
              "type": "string"
            }
          }
        },
        "description": "GameObject references. Used to specify GameObjects in opened Prefab or in a Scene.\r\nUse one of the following properties:\r\n1. \u0027instanceID\u0027 (int) - recommended. It finds the exact GameObject.\r\n2. \u0027path\u0027 (string) - finds GameObject by path. It may find a wrong GameObject.\r\n3. \u0027name\u0027 (string) - finds GameObject by name. It may find a wrong GameObject."
      }
    },
    "required": [
      "gameObjectDiffs",
      "gameObjectRefs"
    ]
  }
}

[connor4312]

I do not repro this, can you share a standalone example?

Code:

import { Server } from '@modelcontextprotocol/sdk/server/index.js';
import { StdioServerTransport } from '@modelcontextprotocol/sdk/server/stdio.js';
import { ListToolsRequestSchema } from '@modelcontextprotocol/sdk/types.js';

// Create an MCP server
const server = new Server(
  {
    name: 'Demo',
    version: '1.0.0',
  },
  { capabilities: { tools: {} } },
);

server.setRequestHandler(ListToolsRequestSchema, async () => {
  return {
    tools: [
      {
        name: 'some_test_tool',
        description: 'call this when asked by the user',
        inputSchema: {
          type: 'object',
          properties: {
            gameObjectDiffs: {
              type: 'array',
              items: {
                type: 'object',
                properties: {
                  typeName: {
                    type: 'string',
                    description:
                      'Full type name. Eg: \u0027System.String\u0027, \u0027System.Int32\u0027, \u0027UnityEngine.Vector3\u0027, etc.',
                  },
                  name: {
                    type: 'string',
                  },
                  value: {
                    type: 'object',
                  },
                  fields: {
                    type: 'array',
                    items: {
                      type: 'object',
                      properties: {
                        typeName: {
                          type: 'string',
                          description:
                            'Full type name. Eg: \u0027System.String\u0027, \u0027System.Int32\u0027, \u0027UnityEngine.Vector3\u0027, etc.',
                        },
                        name: {
                          type: 'string',
                        },
                        value: {
                          type: 'object',
                        },
                        fields: {
                          type: 'array',
                          items: {
                            type: 'object',
                          },
                        },
                        props: {
                          type: 'array',
                          items: {
                            type: 'object',
                          },
                        },
                      },
                      required: ['typeName', 'name', 'value'],
                    },
                  },
                  props: {
                    type: 'array',
                    items: {
                      type: 'object',
                      properties: {
                        typeName: {
                          type: 'string',
                          description:
                            'Full type name. Eg: \u0027System.String\u0027, \u0027System.Int32\u0027, \u0027UnityEngine.Vector3\u0027, etc.',
                        },
                        name: {
                          type: 'string',
                        },
                        value: {
                          type: 'object',
                        },
                        fields: {
                          type: 'array',
                          items: {
                            type: 'object',
                          },
                        },
                        props: {
                          type: 'array',
                          items: {
                            type: 'object',
                          },
                        },
                      },
                      required: ['typeName', 'name', 'value'],
                    },
                  },
                },
                required: ['typeName', 'value'],
              },
              description:
                'Json Object with required readonly \u0027instanceID\u0027 and \u0027type\u0027 fields.\r\nEach field and property requires to have \u0027type\u0027 and \u0027name\u0027 fields to identify the exact modification target.\r\nFollow the object schema to specify what to change, ignore values that should not be modified. Keep the original data structure.\r\nAny unknown or wrong located fields and properties will be ignored.\r\nCheck the result of this command to see what was changed. The ignored fields and properties will be listed.',
            },
            gameObjectRefs: {
              type: 'array',
              items: {
                type: 'object',
                properties: {
                  instanceID: {
                    type: 'integer',
                  },
                  path: {
                    type: 'string',
                  },
                  name: {
                    type: 'string',
                  },
                },
              },
              description:
                'GameObject references. Used to specify GameObjects in opened Prefab or in a Scene.\r\nUse one of the following properties:\r\n1. \u0027instanceID\u0027 (int) - recommended. It finds the exact GameObject.\r\n2. \u0027path\u0027 (string) - finds GameObject by path. It may find a wrong GameObject.\r\n3. \u0027name\u0027 (string) - finds GameObject by name. It may find a wrong GameObject.',
            },
          },
          required: ['gameObjectDiffs', 'gameObjectRefs'],
        },
      },
    ],
  };
});

const transport = new StdioServerTransport();
await server.connect(transport);

[IvanMurzak]

Hi @connor4312 , please make sure you explicitly select the model "GPT-4.1". Also you may install the entire MCP server itself to make the the real battle test which reproduces the issue 100% of a time.

It could be installed only from the local source, because I am using csharp-sdk + .net9.0 to make it work. Please let me know if it is way to install from a some remote registry without making a build like I do.

unity-mcp.zip

this is the Windows build, if you have mac it may not work. Please let me know, I will use mac to make the build for it.

You may unzip it anywhere you want, but just to match with the json config I added down below please unzip it to disk C.

{
  "servers": {
    "Unity-MCP": {
      "command": "C:/Unity-MCP/net9.0/com.IvanMurzak.Unity.MCP.Server",
      "args": [
        "60606"
      ]
    }
  }
}

[connor4312]

Thanks. I don't have access to a Windows machine right at this moment, it may be easier just to capture some logs real quick

• Run Developer: Set Log Level > Debug
• MCP: List Servers > Your Server > Start Server, it'll open a log output
• Repro the issue in chat
• Share the contents of the log that was created

[IvanMurzak]

Thanks for the instruction. Just captured the logs and discovered that it runs in infinity update loop. I don't know what is wrong.

The server is using dynamic tools. And there is a tool update happens at least once at the server start.
Tested it with Claude Desktop and Cursor, it works well there, just in case. It still could be a my mistake - let me know please if so, I just don't where where is it.

The server starts with predefined tools and immediately connects to Unity Editor to fetch the list of available tools. And push the notification to MCP client about the tools update once as well.

unity-mcp-logs.json

[IvanMurzak]

If Unity is not running, the MCP server doesn't do a dynamic tool update, and there is no issue in VS Code. Seems like the VS Code error happens only after a dynamic tool update.

unity-mcp-logs-2.txt

[IvanMurzak]

Please take a look at the predefined json schema and dynamically updated json schema. They have a bit different format. Would that cause the problem?

• predefined-schema.json
• dynamically-updated-schema.json