fix: exit cleanly on stdin EOF in StdioServerTransport

src/transports/stdio/server.ts

@@ -37,8 +37,23 @@ export class StdioServerTransport implements BaseTransport {
   async start(): Promise<void> {
     await this.transport.start();
     this.running = true;
+
+    // Exit cleanly when the parent process disconnects (stdin EOF). Without
+    // this, the underlying SDK readline poll spins on null reads after the
+    // pipe closes, leaving the server reparented to init at ~99% CPU until
+    // killed manually. Observed daily on macOS Darwin 25.x when the parent
+    // (Claude CLI / Claude Desktop) is hard-killed.
+    process.stdin.on("end", this.handleStdinClose);
+    process.stdin.on("close", this.handleStdinClose);
   }
 
+  private handleStdinClose = (): void => {
+    // Stdio transport has no work to do once stdin is gone. Best-effort
+    // cleanup of the SDK transport, then exit so launchd / parent never sees
+    // a CPU-spinning zombie.
+    this.transport.close().finally(() => process.exit(0));
+  };
+
   async send(message: ExtendedJSONRPCMessage): Promise<void> {
     try {
       if (hasImageContent(message)) {
@@ -73,6 +88,8 @@ export class StdioServerTransport implements BaseTransport {
   }
 
   async close(): Promise<void> {
+    process.stdin.off("end", this.handleStdinClose);
+    process.stdin.off("close", this.handleStdinClose);
     await this.transport.close();
     this.running = false;
   }

tests/transports/stdio/server.test.ts

@@ -0,0 +1,72 @@
+import { describe, it, expect, jest, afterEach, beforeEach } from "@jest/globals";
+import { StdioServerTransport } from "../../../src/transports/stdio/server.js";
+
+/**
+ * Regression tests for stdin-EOF handling.
+ *
+ * Without the listeners installed in StdioServerTransport.start(), the
+ * underlying SDK readline poll spins on null reads after the parent process
+ * disconnects, leaving the MCP server reparented to init at ~99% CPU.
+ *
+ * These tests verify:
+ *  1. process.stdin "close" → process.exit(0)
+ *  2. process.stdin "end"   → process.exit(0)
+ *  3. close() removes the listeners (no leak / re-instantiation safety)
+ */
+describe("StdioServerTransport — stdin EOF handling", () => {
+  let transport: StdioServerTransport | undefined;
+  let exitSpy: ReturnType<typeof jest.spyOn>;
+
+  beforeEach(() => {
+    // Mock process.exit so the test runner survives — record the call instead.
+    exitSpy = jest.spyOn(process, "exit").mockImplementation((() => {
+      // Intentionally a no-op. Real behavior is verified via the spy assertion.
+    }) as never);
+  });
+
+  afterEach(async () => {
+    if (transport?.isRunning()) {
+      await transport.close();
+    }
+    transport = undefined;
+    exitSpy.mockRestore();
+  });
+
+  it('exits with code 0 when process.stdin emits "close"', async () => {
+    transport = new StdioServerTransport();
+    await transport.start();
+
+    process.stdin.emit("close");
+    // Wait one microtask + macrotask for transport.close().finally chain.
+    await new Promise((resolve) => setImmediate(resolve));
+
+    expect(exitSpy).toHaveBeenCalledWith(0);
+  });
+
+  it('exits with code 0 when process.stdin emits "end"', async () => {
+    transport = new StdioServerTransport();
+    await transport.start();
+
+    process.stdin.emit("end");
+    await new Promise((resolve) => setImmediate(resolve));
+
+    expect(exitSpy).toHaveBeenCalledWith(0);
+  });
+
+  it("removes stdin listeners on close() so re-instantiation is leak-free", async () => {
+    const baselineEnd = process.stdin.listenerCount("end");
+    const baselineClose = process.stdin.listenerCount("close");
+
+    transport = new StdioServerTransport();
+    await transport.start();
+
+    expect(process.stdin.listenerCount("end")).toBe(baselineEnd + 1);
+    expect(process.stdin.listenerCount("close")).toBe(baselineClose + 1);
+
+    await transport.close();
+    transport = undefined;
+
+    expect(process.stdin.listenerCount("end")).toBe(baselineEnd);
+    expect(process.stdin.listenerCount("close")).toBe(baselineClose);
+  });
+});