fix: async onclose, stdin EOF detection, SIGTERM in examples

Three related improvements to server lifecycle handling:

1. Allow async onclose callbacks on Transport and Protocol.
   MCP servers that hold external resources (browser sessions,
   database connections) need to await cleanup before the process
   exits. The onclose signature changes from `() => void` to
   `() => void | Promise<void>`, matching the existing pattern
   used by onsessionclosed in StreamableHTTPServerTransport.
   All transports and Protocol._onclose now await the callback.

2. Close StdioServerTransport when stdin ends. The transport
   listened for data and error but not EOF. When the MCP client
   disconnects, the transport stays open and onclose never fires.
   This is especially visible with containerized servers using
   docker run with automatic removal: without onclose the server
   never exits and the container accumulates.

3. Add SIGTERM handlers alongside SIGINT in all examples. MCP
   servers run as background processes spawned by clients, not
   interactively. SIGTERM is what container runtimes and process
   managers send to stop a process.

Author: MayCXC

.changeset/async-onclose-stdin-eof.md

@@ -0,0 +1,11 @@
+---
+"@modelcontextprotocol/core": patch
+"@modelcontextprotocol/server": patch
+"@modelcontextprotocol/client": patch
+---
+
+Allow async `onclose` callbacks on Transport and Protocol. The signature changes from `() => void` to `() => void | Promise<void>`, and all call sites now await the callback. This lets MCP servers perform async cleanup (e.g., releasing browser sessions or database connections) when the transport closes.
+
+Close `StdioServerTransport` when stdin reaches EOF, so containerized servers exit cleanly on client disconnect.
+
+Add SIGTERM handlers alongside SIGINT in all examples, since MCP servers run as background processes stopped by SIGTERM, not interactively via Ctrl+C.

examples/client/src/elicitationUrlExample.ts

@@ -813,6 +813,10 @@ process.on('SIGINT', async () => {
     console.log('\nReceived SIGINT. Cleaning up...');
     await cleanup();
 });
+process.on('SIGTERM', async () => {
+    console.log('\nReceived SIGINT. Cleaning up...');
+    await cleanup();
+});
 
 // Start the interactive client
 try {

examples/client/src/simpleOAuthClient.ts

@@ -448,6 +448,11 @@ async function main(): Promise<void> {
         client.close();
         process.exit(0);
     });
+    process.on('SIGTERM', () => {
+        console.log('\n\n👋 Goodbye!');
+        client.close();
+        process.exit(0);
+    });
 
     try {
         await client.connect();

examples/client/src/simpleStreamableHttp.ts

@@ -997,6 +997,10 @@ process.on('SIGINT', async () => {
     console.log('\nReceived SIGINT. Cleaning up...');
     await cleanup();
 });
+process.on('SIGTERM', async () => {
+    console.log('\nReceived SIGINT. Cleaning up...');
+    await cleanup();
+});
 
 // Start the interactive client
 try {

examples/server/src/elicitationFormExample.ts

@@ -453,6 +453,22 @@ async function main() {
     process.on('SIGINT', async () => {
         console.log('Shutting down server...');
 
+        // Close all active transports to properly clean up resources
+        for (const sessionId in transports) {
+            try {
+                console.log(`Closing transport for session ${sessionId}`);
+                await transports[sessionId]!.close();
+                delete transports[sessionId];
+            } catch (error) {
+                console.error(`Error closing transport for session ${sessionId}:`, error);
+            }
+        }
+        console.log('Server shutdown complete');
+        process.exit(0);
+    });
+    process.on('SIGTERM', async () => {
+        console.log('Shutting down server...');
+
         // Close all active transports to properly clean up resources
         for (const sessionId in transports) {
             try {

examples/server/src/elicitationUrlExample.ts

@@ -731,3 +731,20 @@ process.on('SIGINT', async () => {
     console.log('Server shutdown complete');
     process.exit(0);
 });
+process.on('SIGTERM', async () => {
+    console.log('Shutting down server...');
+
+    // Close all active transports to properly clean up resources
+    for (const sessionId in transports) {
+        try {
+            console.log(`Closing transport for session ${sessionId}`);
+            await transports[sessionId]!.close();
+            delete transports[sessionId];
+            delete sessionsNeedingElicitation[sessionId];
+        } catch (error) {
+            console.error(`Error closing transport for session ${sessionId}:`, error);
+        }
+    }
+    console.log('Server shutdown complete');
+    process.exit(0);
+});

examples/server/src/jsonResponseStreamableHttp.ts

@@ -163,3 +163,7 @@ process.on('SIGINT', async () => {
     console.log('Shutting down server...');
     process.exit(0);
 });
+process.on('SIGTERM', async () => {
+    console.log('Shutting down server...');
+    process.exit(0);
+});

examples/server/src/simpleStatelessStreamableHttp.ts

@@ -169,3 +169,8 @@ process.on('SIGINT', async () => {
     // eslint-disable-next-line unicorn/no-process-exit
     process.exit(0);
 });
+process.on('SIGTERM', async () => {
+    console.log('Shutting down server...');
+    // eslint-disable-next-line unicorn/no-process-exit
+    process.exit(0);
+});

examples/server/src/simpleStreamableHttp.ts

@@ -815,3 +815,19 @@ process.on('SIGINT', async () => {
     console.log('Server shutdown complete');
     process.exit(0);
 });
+process.on('SIGTERM', async () => {
+    console.log('Shutting down server...');
+
+    // Close all active transports to properly clean up resources
+    for (const sessionId in transports) {
+        try {
+            console.log(`Closing transport for session ${sessionId}`);
+            await transports[sessionId]!.close();
+            delete transports[sessionId];
+        } catch (error) {
+            console.error(`Error closing transport for session ${sessionId}:`, error);
+        }
+    }
+    console.log('Server shutdown complete');
+    process.exit(0);
+});

examples/server/src/simpleTaskInteractive.ts

@@ -741,3 +741,18 @@ process.on('SIGINT', async () => {
     console.log('Server shutdown complete');
     process.exit(0);
 });
+process.on('SIGTERM', async () => {
+    console.log('\nShutting down server...');
+    for (const sessionId of Object.keys(transports)) {
+        try {
+            await transports[sessionId]!.close();
+            delete transports[sessionId];
+        } catch (error) {
+            console.error(`Error closing session ${sessionId}:`, error);
+        }
+    }
+    taskStore.cleanup();
+    messageQueue.cleanup();
+    console.log('Server shutdown complete');
+    process.exit(0);
+});