fix(mcp): detect orphaned stdio host and self-terminate

StdioServerTransport only listens for stdin data/error events, so a
stdio-mode server never notices when its host (Claude Code) dies
without sending SIGTERM/SIGINT first. The child is left running as
an orphan (ppid 1) with nothing to talk to, observed spinning at
sustained high CPU indefinitely until manually killed.

Detect this two ways - stdin closing and a periodic ppid check - and
run the existing shutdownFn immediately once either fires.

Author: geunwoobaek

mcp/src/index.ts

@@ -88,7 +88,9 @@ function resolveTransport(env: TransportMode): TransportMode {
  * For stdio/http, installs `SIGINT`/`SIGTERM` handlers invoking the
  * transport's `shutdownFn`, plus `unhandledRejection`/`uncaughtException`
  * handlers logging via {@link Logger} — the latter sets `process.exitCode = 1`
- * without exiting immediately, letting in-flight work finish.
+ * without exiting immediately, letting in-flight work finish. Stdio mode also
+ * self-terminates on stdin close or a detected ppid-1 orphan, since a dead
+ * host process never sends a signal in that case.
  */
 async function main() {
   const config = loadConfig();
@@ -118,6 +120,34 @@ async function main() {
       await stdioTransport.close?.();
       await server.close();
     };
+
+    // The SDK's StdioServerTransport only listens for stdin "data"/"error"
+    // events, so it never notices when the host (Claude Code) dies without
+    // sending SIGTERM/SIGINT first: stdin just closes silently and this
+    // process lingers as an orphan (ppid 1) — observed spinning at high CPU
+    // indefinitely with nothing left to talk to. Detect that two ways —
+    // stdin closing and a periodic ppid check, since either can fire first
+    // depending on how the host exited — and self-terminate immediately.
+    let orphanHandled = false;
+    const exitIfOrphaned = (reason: string) => {
+      if (orphanHandled) return;
+      orphanHandled = true;
+      clearInterval(orphanCheckInterval);
+      logger.info("Host process gone; shutting down orphaned stdio server", { reason });
+      shutdownFn?.()
+        .catch((error) => {
+          logger.error("Error during orphan shutdown", {
+            error: error instanceof Error ? error.message : String(error),
+          });
+        })
+        .finally(() => process.exit(0));
+    };
+    process.stdin.on("end", () => exitIfOrphaned("stdin_end"));
+    process.stdin.on("close", () => exitIfOrphaned("stdin_close"));
+    const orphanCheckInterval = setInterval(() => {
+      if (process.ppid === 1) exitIfOrphaned("ppid_orphaned");
+    }, 5000);
+    orphanCheckInterval.unref();
   }
 
   // ── HTTP mode (SSE + Streamable HTTP) ───────────────────────