fix: handle EPIPE and ERR_STREAM_DESTROYED in StdioServerTransport

.changeset/fix-stdio-epipe.md

@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/server': patch
+---
+
+Handle EPIPE errors gracefully in stdio transport to prevent crashes when the connected process terminates unexpectedly.

packages/server/src/server/stdio.ts

@@ -37,6 +37,14 @@ export class StdioServerTransport implements Transport {
     _onerror = (error: Error) => {
         this.onerror?.(error);
     };
+    _onstdouterror = (error: NodeJS.ErrnoException) => {
+        if (error.code === 'EPIPE' || error.code === 'ERR_STREAM_DESTROYED') {
+            // Client disconnected — close gracefully instead of crashing.
+            void this.close();
+        } else {
+            this.onerror?.(error);
+        }
+    };
 
     /**
      * Starts listening for messages on `stdin`.
@@ -51,6 +59,7 @@ export class StdioServerTransport implements Transport {
         this._started = true;
         this._stdin.on('data', this._ondata);
         this._stdin.on('error', this._onerror);
+        this._stdout.on('error', this._onstdouterror);
     }
 
     private processReadBuffer() {
@@ -72,6 +81,7 @@ export class StdioServerTransport implements Transport {
         // Remove our event listeners first
         this._stdin.off('data', this._ondata);
         this._stdin.off('error', this._onerror);
+        this._stdout.off('error', this._onstdouterror);
 
         // Check if we were the only data listener
         const remainingDataListeners = this._stdin.listenerCount('data');
@@ -87,12 +97,26 @@ export class StdioServerTransport implements Transport {
     }
 
     send(message: JSONRPCMessage): Promise<void> {
-        return new Promise(resolve => {
+        return new Promise((resolve, reject) => {
             const json = serializeMessage(message);
-            if (this._stdout.write(json)) {
-                resolve();
-            } else {
-                this._stdout.once('drain', resolve);
+            if (!this._stdout.writable) {
+                reject(new Error('stdout is not writable'));
+                return;
+            }
+            try {
+                if (this._stdout.write(json)) {
+                    resolve();
+                } else {
+                    this._stdout.once('drain', resolve);
+                }
+            } catch (error: unknown) {
+                const errno = error as NodeJS.ErrnoException;
+                if (errno.code === 'EPIPE' || errno.code === 'ERR_STREAM_DESTROYED') {
+                    void this.close();
+                    reject(error);
+                } else {
+                    reject(error);
+                }
             }
         });
     }

packages/server/test/server/stdio.test.ts

@@ -102,3 +102,87 @@ test('should read multiple messages', async () => {
     await finished;
     expect(readMessages).toEqual(messages);
 });
+
+test('should handle EPIPE error on stdout gracefully', async () => {
+    const server = new StdioServerTransport(input, output);
+
+    let didClose = false;
+    server.onclose = () => {
+        didClose = true;
+    };
+
+    await server.start();
+
+    // Simulate EPIPE error on stdout
+    const epipeError = new Error('write EPIPE') as NodeJS.ErrnoException;
+    epipeError.code = 'EPIPE';
+    output.emit('error', epipeError);
+
+    // Should trigger graceful close, not crash
+    expect(didClose).toBeTruthy();
+});
+
+test('should handle ERR_STREAM_DESTROYED error on stdout gracefully', async () => {
+    const server = new StdioServerTransport(input, output);
+
+    let didClose = false;
+    server.onclose = () => {
+        didClose = true;
+    };
+
+    await server.start();
+
+    const destroyedError = new Error('stream destroyed') as NodeJS.ErrnoException;
+    destroyedError.code = 'ERR_STREAM_DESTROYED';
+    output.emit('error', destroyedError);
+
+    expect(didClose).toBeTruthy();
+});
+
+test('should forward non-EPIPE stdout errors to onerror', async () => {
+    const server = new StdioServerTransport(input, output);
+
+    let reportedError: Error | undefined;
+    server.onerror = error => {
+        reportedError = error;
+    };
+
+    await server.start();
+
+    const otherError = new Error('some other error') as NodeJS.ErrnoException;
+    otherError.code = 'ENOSPC';
+    output.emit('error', otherError);
+
+    expect(reportedError).toBe(otherError);
+});
+
+test('should reject send when stdout is not writable', async () => {
+    const closedOutput = new Writable({
+        write(_chunk, _encoding, callback) {
+            callback();
+        }
+    });
+    closedOutput.destroy();
+
+    const server = new StdioServerTransport(input, closedOutput);
+    await server.start();
+
+    const message: JSONRPCMessage = {
+        jsonrpc: '2.0',
+        id: 1,
+        method: 'ping'
+    };
+
+    await expect(server.send(message)).rejects.toThrow('stdout is not writable');
+});
+
+test('should remove stdout error listener on close', async () => {
+    const server = new StdioServerTransport(input, output);
+    await server.start();
+
+    const listenersBefore = output.listenerCount('error');
+    await server.close();
+    const listenersAfter = output.listenerCount('error');
+
+    expect(listenersAfter).toBeLessThan(listenersBefore);
+});