MCP Protocol Validation Error During Initialization

[ahmed98Osama]

Summary

The Java MCP SQL Server successfully connects to the database and all tools are functional, but Cursor IDE shows a protocol validation error during initialization with a red error indicator. The error occurs during the MCP handshake but the server recovers and continues to work normally.

Environment

• OS: Windows 10 (Build 19045)
• Java Version: openjdk version "21.0.4" 2024-07-16 LTS
• MCP Server Version: 1.0.0
• JAR Location: C:\java-mcp-sql-server-1.0.0.jar
• Database: Oracle
• Cursor IDE: 2.2.43

Configuration

{
  "java-mcp-sql-server": {
    "command": "java",
    "args": ["-jar", "C:\\java-mcp-sql-server-1.0.0.jar"],
    "env": {
      "JMCP_JDBC_URL": "jdbc:oracle:thin:@//your-server:1521/your-database",
      "JMCP_DB_USERNAME": "YOUR_USERNAME",
      "JMCP_DB_PASSWORD": "YOUR_PASSWORD",
      "JMCP_MODE": "stdio",
      "JMCP_QUERY_SELECT_ONLY": "false"
    }
  }
}

Error Details

Server Startup (Successful)

23:47:21.222 [main] INFO  com.dgdev91.mcpsql.SqlMcpServer - Starting SQL MCP Server...
23:47:21.481 [main] INFO  com.dgdev91.mcpsql.service.DatabaseService - Oracle driver loaded successfully
23:47:23.111 [main] INFO  com.dgdev91.mcpsql.service.DatabaseService - Successfully connected to database: ORACLE
23:47:23.242 [main] INFO  com.dgdev91.mcpsql.SqlMcpServer - Database connection successful. Type: ORACLE, Query SELECT only: false

Protocol Validation Error

Cursor IDE reports a Zod validation error during initialization:

{
  "code": "invalid_union",
  "unionErrors": [
    {
      "issues": [
        {
          "code": "invalid_union",
          "unionErrors": [
            {
              "issues": [
                {
                  "code": "invalid_type",
                  "expected": "string",
                  "received": "null",
                  "path": ["id"],
                  "message": "Expected string, received null"
                }
              ],
              "name": "ZodError"
            },
            {
              "issues": [
                {
                  "code": "invalid_type",
                  "expected": "number",
                  "received": "null",
                  "path": ["id"],
                  "message": "Expected number, received null"
                }
              ],
              "name": "ZodError"
            }
          ],
          "path": ["id"],
          "message": "Invalid input"
        },
        {
          "code": "invalid_type",
          "expected": "string",
          "received": "undefined",
          "path": ["method"],
          "message": "Required"
        },
        {
          "code": "unrecognized_keys",
          "keys": ["error"],
          "path": [],
          "message": "Unrecognized key(s) in object: 'error'"
        }
      ],
      "name": "ZodError"
    }
  ],
  "path": [],
  "message": "Invalid input"
}

Recovery (Successful)

After the error, the server recovers:

23:47:24.394 [info] Handling ListOfferings action, server stored: true
23:47:24.402 [info] listOfferings: Found 5 tools
23:47:24.402 [info] Found 5 tools, 0 prompts, and 0 resources

Expected Behavior

The MCP server should initialize without protocol validation errors. All messages sent during initialization should conform to the MCP protocol specification.

Actual Behavior

• Server starts successfully ✅
• Database connection established ✅
• Tools are available and functional ✅
• Protocol validation error during initialization ❌
• Red error indicator in Cursor IDE ❌

Impact

• Severity: Low (cosmetic issue)
• Functionality: All tools work correctly after initialization
• User Experience: Red error indicator is misleading and concerning

Steps to Reproduce

1. Configure mcp.json with java-mcp-sql-server
2. Restart Cursor IDE
3. Check MCP server logs in Cursor
4. Observe protocol validation error during initialization
5. Verify tools are still functional (they are)

Additional Notes

• The error appears to be related to a message sent during initialization that contains an "error" key with a null id
• The server appears to send a non-standard message format that doesn't match MCP protocol expectations
• This might be related to error handling or logging during the initialization phase
• All 5 tools (list_schemas, list_tables, get_table_structure, query_table, execute_query) work correctly after initialization

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Note: This issue was identified while using the server with Oracle database, but it may affect other database types as well.