mcp-logger.js•2.86 kB
/**
* MCP-Compliant Logger
*
* ALL output goes to stderr (console.error) to maintain MCP protocol compliance.
* The MCP specification requires that stdout ONLY contains JSON-RPC messages.
*
* Environment Configuration:
* - MCP_LOG_LEVEL: silent | error | warn | info | debug
* - Default: info
*
* Usage:
* import { logger } from './mcp-logger.js';
* logger.debug('Debug message');
* logger.info('Info message');
* logger.warn('Warning message');
* logger.error('Error message');
*/
const LOG_LEVELS = {
silent: 0,
error: 1,
warn: 2,
info: 3,
debug: 4
};
class MCPLogger {
constructor() {
// Read from environment, default to 'info' for production
const envLevel = process.env.MCP_LOG_LEVEL || process.env.LOG_LEVEL || 'info';
this.level = envLevel.toLowerCase();
this.threshold = LOG_LEVELS[this.level] !== undefined ? LOG_LEVELS[this.level] : LOG_LEVELS.info;
// Validate log level on startup
if (LOG_LEVELS[this.level] === undefined) {
console.error(`⚠️ Invalid log level '${envLevel}', defaulting to 'info'`);
this.level = 'info';
this.threshold = LOG_LEVELS.info;
}
}
/**
* Check if a message at the given level should be logged
*/
_shouldLog(level) {
return LOG_LEVELS[level] <= this.threshold;
}
/**
* Internal logging method - ALWAYS uses console.error for MCP compliance
*/
_log(level, ...args) {
if (this._shouldLog(level)) {
// Use console.error for MCP compliance (stderr)
// This ensures we NEVER write to stdout
console.error(...args);
}
}
/**
* Log error-level messages (always shown except in silent mode)
*/
error(...args) {
this._log('error', ...args);
}
/**
* Log warning-level messages
*/
warn(...args) {
this._log('warn', ...args);
}
/**
* Log info-level messages (default for production)
*/
info(...args) {
this._log('info', ...args);
}
/**
* Log debug-level messages (verbose, for development)
*/
debug(...args) {
this._log('debug', ...args);
}
/**
* Check if logger is in silent mode
*/
isSilent() {
return this.threshold === LOG_LEVELS.silent;
}
/**
* Get current log level name
*/
getLevel() {
return this.level;
}
/**
* Set log level programmatically
*/
setLevel(level) {
const normalizedLevel = level.toLowerCase();
if (LOG_LEVELS[normalizedLevel] !== undefined) {
this.level = normalizedLevel;
this.threshold = LOG_LEVELS[normalizedLevel];
return true;
}
return false;
}
}
// Singleton instance
const logger = new MCPLogger();
// Log the current level on startup (unless silent)
if (!logger.isSilent()) {
logger.debug(`📋 MCP Logger initialized (level: ${logger.getLevel()})`);
}
export { logger, MCPLogger, LOG_LEVELS };