claude-code - 💡(How to fix) Fix [BUG] Claude Desktop - Code - MCP InputSchema with nullable arrays has type silently dropped [3 comments, 2 participants]

import json
import sys
from typing import Any


def send_response(result: Any = None, error: Any = None, request_id: Any = None) -> None:
    """Send a JSON-RPC response."""
    response = {"jsonrpc": "2.0"}
    if request_id is not None:
        response["id"] = request_id
    if error is not None:
        response["error"] = error
    else:
        response["result"] = result
    json.dump(response, sys.stdout)
    sys.stdout.write("\n")
    sys.stdout.flush()


def handle_initialize(request_id: int) -> None:
    """Handle initialize request."""
    send_response({
        "protocolVersion": "2025-11-25",
        "capabilities": {},
        "serverInfo": {"name": "echo-stdio", "version": "1.0.0"}
    }, request_id=request_id)


def handle_tools_list(request_id: int) -> None:
    """Handle tools/list request."""
    send_response({
        "tools": [
            {
                "name": "echo_array",
                "description": "Echo an array parameter. Helps observe if arrays are stringified in transit.",
                "inputSchema": {
                    "type": "object",
                    "properties": {
                        "items": {
                            "type": ["array", "null"],
                            "items": {"type": "string"},
                            "description": "A list of strings"
                        }
                    },  
                    "required": ["items"]
                }
            }
        ]
    }, request_id=request_id)


def handle_call_tool(name: str, arguments: dict[str, Any], request_id: int) -> None:
    """Handle tool calls - echo back what was received."""
    if name == "echo_array":
        items = arguments.get('items')
        result = {
            "content": [{
                "type": "text",
                "text": (
                    f"RECEIVED:\n"
                    f"  Value: {json.dumps(items)}\n"
                    f"  Type: {type(items).__name__}\n\n"
                    + ("⚠️ BUG: Array was stringified! Expected type 'list' but got 'str'."
                       if isinstance(items, str) and items.startswith('[')
                       else "✓ OK: Received as proper list.")
                )
            }]
        }
    else:
        send_response(error={"code": -32601, "message": f"Unknown tool: {name}"}, request_id=request_id)
        return
    
    send_response(result, request_id=request_id)


def main() -> None:
    """Main loop - read JSON-RPC requests from stdin."""
    for line in sys.stdin:
        line = line.strip()
        if not line:
            continue
        
        try:
            request = json.loads(line)
        except json.JSONDecodeError:
            send_response(error={"code": -32700, "message": "Parse error"})
            continue
        
        method = request.get("method")
        params = request.get("params", {})
        request_id = request.get("id")
        
        try:
            if method == "initialize":
                handle_initialize(request_id)
            elif method.startswith("notifications/"):
                pass  # notifications must not receive responses
            elif method == "tools/list":
                handle_tools_list(request_id)
            elif method == "tools/call":
                handle_call_tool(params.get("name"), params.get("arguments", {}), request_id)
            elif request_id is not None:
                send_response(error={"code": -32601, "message": f"Method not found: {method}"}, request_id=request_id)
        except Exception as e:
            send_response(error={"code": -32603, "message": str(e)}, request_id=request_id)


if __name__ == "__main__":
    main()