Ensure stdio client closes underlying process

src/client/stdio.ts

@@ -138,7 +138,6 @@ export class StdioClientTransport implements Transport {
       this._process.on("error", (error) => {
         if (error.name === "AbortError") {
           // Expected when close() is called.
-          this.onclose?.();
           return;
         }
 
@@ -214,8 +213,33 @@ export class StdioClientTransport implements Transport {
   }
 
   async close(): Promise<void> {
-    this._abortController.abort();
-    this._process = undefined;
+    if (this._process) {
+      const processToClose = this._process;
+      this._process = undefined;
+
+      const closePromise = new Promise<void>((resolve) => {
+        processToClose.once("close", () => {
+          resolve();
+        });
+      });
+
+      this._abortController.abort();
+
+      // waits the underlying process to exit cleanly otherwise after 1s kills it
+      await Promise.race([
+        closePromise,
+        new Promise((resolve) => setTimeout(resolve, 1_000).unref()),
+      ]);
+
+      if (processToClose.exitCode === null) {
+        try {
+          processToClose.kill("SIGKILL");
+        } catch {
+          // we did our best
+        }
+      }
+    }
+
     this._readBuffer.clear();
   }
 

src/integration-tests/process-cleanup.test.ts

@@ -1,10 +1,13 @@
+import { execSync } from "node:child_process";
+import { Client } from "../client/index.js";
+import { StdioClientTransport } from "../client/stdio.js";
 import { Server } from "../server/index.js";
 import { StdioServerTransport } from "../server/stdio.js";
 
 describe("Process cleanup", () => {
   jest.setTimeout(5000); // 5 second timeout
 
-  it("should exit cleanly after closing transport", async () => {
+  it("server should exit cleanly after closing transport", async () => {
     const server = new Server(
       {
         name: "test-server",
@@ -25,4 +28,62 @@ describe("Process cleanup", () => {
     // The test runner will fail if the process hangs
     expect(true).toBe(true);
   });
-});
+
+  it("client should exit cleanly after closing transport", async () => {
+    const isProcessRunning = (pid: number) => {
+      try {
+        execSync(`ps -p ${pid}`, { stdio: "ignore", windowsHide: true });
+        return true;
+      } catch {
+        return false;
+      }
+    };
+
+    const client = new Client({
+      name: "test-client",
+      version: "1.0.0",
+    });
+
+    const transport = new StdioClientTransport({
+      command: process.argv0,
+      args: ["server-that-hangs.js"],
+      cwd: __dirname,
+    });
+
+    await client.connect(transport);
+    const pid = transport.pid;
+
+    await client.close();
+
+    // delay so OS can cleanup the process
+    await new Promise((resolve) => setTimeout(resolve, 50));
+
+    expect(isProcessRunning(pid!)).toBe(false);
+  });
+
+  it("onclose should be called exactly once", async () => {
+    const client = new Client({
+      name: "test-client",
+      version: "1.0.0",
+    });
+
+    const transport = new StdioClientTransport({
+      command: process.argv0,
+      args: ["test-server.js"],
+      cwd: __dirname,
+    });
+
+    let onCloseWasCalled = 0;
+    client.onclose = () => {
+      onCloseWasCalled++;
+    };
+
+    await client.connect(transport);
+    await client.close();
+
+    // A short delay to allow the close event to propagate
+    await new Promise((resolve) => setTimeout(resolve, 50));
+
+    expect(onCloseWasCalled).toBe(1);
+  });
+});

src/integration-tests/server-that-hangs.js

@@ -0,0 +1,21 @@
+import { setTimeout } from 'node:timers'
+import process from 'node:process'
+import { McpServer } from "../../dist/esm/server/mcp.js";
+import { StdioServerTransport } from "../../dist/esm/server/stdio.js";
+
+const transport = new StdioServerTransport();
+
+const server = new McpServer({
+  name: "server-that-hangs",
+  version: "1.0.0"
+});
+
+await server.connect(transport);
+
+const doNotExitImmediately = async () => {
+  setTimeout(() => process.exit(0), 30 * 1000);
+};
+
+process.stdin.on('close', doNotExitImmediately);
+process.on('SIGINT', doNotExitImmediately);
+process.on('SIGTERM', doNotExitImmediately);

src/integration-tests/test-server.js

@@ -0,0 +1,11 @@
+import { McpServer } from "../../dist/esm/server/mcp.js";
+import { StdioServerTransport } from "../../dist/esm/server/stdio.js";
+
+const transport = new StdioServerTransport();
+
+const server = new McpServer({
+  name: "test-server",
+  version: "1.0.0",
+});
+
+await server.connect(transport);