/**
* Logger that writes to stderr only.
*
* CRITICAL: MCP servers using stdio transport MUST NOT use console.log
* because it corrupts the JSON-RPC message stream on stdout.
* All logging must go to stderr.
*/
type LogLevel = 'debug' | 'info' | 'warn' | 'error';
interface LogEntry {
level: LogLevel;
message: string;
timestamp: string;
data?: unknown;
}
function formatLog(entry: LogEntry): string {
const prefix = `[${entry.timestamp}] [${entry.level.toUpperCase()}]`;
const dataStr = entry.data ? ` ${JSON.stringify(entry.data)}` : '';
return `${prefix} ${entry.message}${dataStr}`;
}
function log(level: LogLevel, message: string, data?: unknown): void {
const entry: LogEntry = {
level,
message,
timestamp: new Date().toISOString(),
data,
};
// CRITICAL: Write to stderr, never stdout
console.error(formatLog(entry));
}
export const logger = {
debug: (message: string, data?: unknown) => log('debug', message, data),
info: (message: string, data?: unknown) => log('info', message, data),
warn: (message: string, data?: unknown) => log('warn', message, data),
error: (message: string, data?: unknown) => log('error', message, data),
};
