MySQL MCP Server

Instructions

Implementation Reference

• src/index.ts:208-255 (handler)

  Core handler for 'run_sql_query' tool: validates arguments and query type (SELECT only), executes query on MySQL connection pool, returns JSON-stringified rows or error response.

  // Handle read-only queries (SELECT)
  private async handleReadQuery(request: any, transactionId: string) {
    if (!isValidSqlQueryArgs(request.params.arguments)) {
      throw new McpError(
        ErrorCode.InvalidParams,
        'Invalid SQL query arguments.'
      );
    }
  
    const query = request.params.arguments.query;
    
    if (!isReadOnlyQuery(query)) {
      throw new McpError(
        ErrorCode.InvalidParams,
        'Only SELECT queries are allowed with run_sql_query tool.'
      );
    }
  
    console.error(`[${transactionId}] Executing SELECT query: ${query}`);
    
    try {
      const [rows] = await this.pool.query(query);
      console.error(`[${transactionId}] Query executed successfully`);
      
      return {
        content: [
          {
            type: 'text',
            text: JSON.stringify(rows, null, 2),
          },
        ],
      };
    } catch (error) {
      console.error(`[${transactionId}] Query error:`, error);
      if (error instanceof Error) {
        return {
          content: [
            {
              type: 'text',
              text: `MySQL error: ${error.message}`,
            },
          ],
          isError: true,
        };
      }
      throw error;
    }
  }

• src/index.ts:97-106 (schema)

  Input schema definition for the 'run_sql_query' tool, specifying a required 'query' string parameter.

  inputSchema: {
    type: 'object',
    properties: {
      query: {
        type: 'string',
        description: 'The SQL SELECT query to execute.',
      },
    },
    required: ['query'],
  },

• src/index.ts:94-107 (registration)

  Tool registration in ListToolsRequestSchema response: defines name, description, and input schema for 'run_sql_query'.

  {
    name: 'run_sql_query',
    description: 'Executes a read-only SQL query (SELECT statements only) against the MySQL database.',
    inputSchema: {
      type: 'object',
      properties: {
        query: {
          type: 'string',
          description: 'The SQL SELECT query to execute.',
        },
      },
      required: ['query'],
    },
  },

• src/index.ts:187-188 (registration)

  Dispatch logic in CallToolRequestSchema handler: routes 'run_sql_query' calls to the handleReadQuery method.

  case 'run_sql_query':
    return this.handleReadQuery(request, transactionId);

• src/index.ts:29-31 (helper)

  Helper function used by handler to validate that the query is a read-only SELECT statement.

  // Check if query is read-only (SELECT)
  const isReadOnlyQuery = (query: string): boolean => 
    query.trim().toLowerCase().startsWith('select');