`outputSchema` does not fully support JSON Schema

[jpmcb]

Describe the bug

Related to spec 2025-06-18, it seems the outputSchema field on tools is not fully supported and seems to always expect type: object which may not always be the shape of the JSON returned.

From the spec:

outputSchema: Optional JSON Schema defining expected output structure

With a valid JSON schema like so for my simple TODO mcp server:

{
  "type": "array",
  "items": {
    "type": "object",
    "required": [ "id", "userId", "title", "completed" ],
    "properties": {
      "id": {
        "type": "integer"
      },
      "userId": {
        "type": "integer"
      },
      "title": {
        "type": "string"
      },
      "completed": {
        "type": "boolean"
      }
    }
  }
}

I can list tools like so:

$ curl http://localhost:9000/mcp \
      -X POST \
      -H 'accept: application/json, text/event-stream' \
      -d '{
    "jsonrpc": "2.0",
    "id": "1",
    "method": "tools/list"
  }'

{
  "jsonrpc": "2.0",
  "id": "1",
  "result": {
    "tools": [
      {
        "name": "get_todos",
        "description": "Gets all the todos in the todo list.",
        "inputSchema": {
          "type": "object",
          "properties": {},
          "required": [],
          "additionalProperties": false
        },
        "outputSchema": {
          "type": "array",
          "items": {
            "type": "object",
            "required": [ "id", "userId", "title", "completed" ],
            "properties": {
              "id": {
                "type": "integer"
              },
              "userId": {
                "type": "integer"
              },
              "title": {
                "type": "string"
              },
              "completed": {
                "type": "boolean"
              }
            }
          }
        }
      }
    ]
  }
}

But in the inspector I get the following error:

{
  "error": [
    {
      "received": "array",
      "code": "invalid_literal",
      "expected": "object",
      "path": [
        "tools",
        1,
        "outputSchema",
        "type"
      ],
      "message": "Invalid literal value, expected \"object\""
    }
  ]
}

Note: this IS valid JSON schema and is validateable on https://www.jsonschemavalidator.net/:

To Reproduce

1. Make a tool with an output schema that has type: array
2. MCP inspector does not work with tools/list

Expected behavior

Per the spec, any JSON Schema would be supported.