diff --git a/packages/ast-grep-mcp/src/mcp-lifecycle-log.ts b/packages/ast-grep-mcp/src/mcp-lifecycle-log.ts
new file mode 100644
index 000000000..19fca8d88
--- /dev/null
+++ b/packages/ast-grep-mcp/src/mcp-lifecycle-log.ts
@@ -0,0 +1,33 @@
+import { appendFileSync, renameSync, statSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+
+type LogFieldValue = boolean | number | string | null;
+
+const LOG_FILE_NAME = "omo-ast-grep-mcp.log";
+const MAX_LOG_BYTES = 5 * 1024 * 1024;
+
+export function mcpLifecycleLogPath(): string {
+  return join(tmpdir(), LOG_FILE_NAME);
+}
+
+export function writeMcpLifecycleLog(event: string, fields: Record<string, LogFieldValue> = {}): void {
+  const path = mcpLifecycleLogPath();
+  try {
+    rotateLogIfNeeded(path);
+    appendFileSync(path, `${JSON.stringify({ ts: new Date().toISOString(), event, pid: process.pid, ppid: process.ppid, ...fields })}\n`);
+  } catch (error) {
+    if (error instanceof Error) return;
+    return;
+  }
+}
+
+function rotateLogIfNeeded(path: string): void {
+  try {
+    if (statSync(path).size < MAX_LOG_BYTES) return;
+    renameSync(path, `${path}.1`);
+  } catch (error) {
+    if (error instanceof Error) return;
+    return;
+  }
+}