PostgreSQL MCP Server

Instructions

Implementation Reference

• src/index.js:209-259 (handler)

  The core handler function that implements the describe_table tool. It queries the PostgreSQL information_schema.columns for column details of the specified table and schema, formats them into a readable text table, and returns as MCP content.

  async describeTable(tableName, schema = 'public') {
    const client = await this.connectToDatabase();
    
    try {
      const query = `
        SELECT 
          column_name,
          data_type,
          is_nullable,
          column_default,
          character_maximum_length,
          numeric_precision,
          numeric_scale,
          ordinal_position
        FROM information_schema.columns 
        WHERE table_schema = $1 AND table_name = $2
        ORDER BY ordinal_position;
      `;
      
      const result = await client.query(query, [schema, tableName]);
      
      if (result.rows.length === 0) {
        throw new Error(`Table "${tableName}" not found in schema "${schema}"`);
      }
  
      const tableInfo = result.rows.map(row => ({
        column: row.column_name,
        type: row.data_type,
        nullable: row.is_nullable === 'YES',
        default: row.column_default,
        maxLength: row.character_maximum_length,
        precision: row.numeric_precision,
        scale: row.numeric_scale,
        position: row.ordinal_position
      }));
  
      return {
        content: [
          {
            type: 'text',
            text: `Schema for table "${schema}.${tableName}":\n\n` + 
                  tableInfo.map(col => 
                    `${col.column} | ${col.type}${col.maxLength ? `(${col.maxLength})` : ''}${col.precision ? `(${col.precision}${col.scale ? `,${col.scale}` : ''})` : ''} | ${col.nullable ? 'NULL' : 'NOT NULL'}${col.default ? ` | DEFAULT ${col.default}` : ''}`
                  ).join('\n'),
          },
        ],
      };
    } finally {
      await client.end();
    }
  }

• src/index.js:74-88 (schema)

  Input schema for the describe_table tool, defining parameters table_name (required, string) and schema (optional string, defaults to 'public').

  inputSchema: {
    type: 'object',
    properties: {
      table_name: {
        type: 'string',
        description: 'Name of the table to describe',
      },
      schema: {
        type: 'string',
        description: 'Schema name (default: public)',
        default: 'public'
      }
    },
    required: ['table_name'],
  },

• src/index.js:71-89 (registration)

  Registration of the describe_table tool in the ListToolsRequestSchema response, including name, description, and input schema.

  {
    name: 'describe_table',
    description: 'Get detailed schema information for a specific table',
    inputSchema: {
      type: 'object',
      properties: {
        table_name: {
          type: 'string',
          description: 'Name of the table to describe',
        },
        schema: {
          type: 'string',
          description: 'Schema name (default: public)',
          default: 'public'
        }
      },
      required: ['table_name'],
    },
  },

• src/index.js:148-149 (registration)

  Dispatch registration in the CallToolRequestSchema switch statement, mapping tool calls to the describeTable handler.

  case 'describe_table':
    return await this.describeTable(args.table_name, args?.schema || 'public');