fix: handle EPIPE errors in StdioServerTransport to prevent process crash

packages/server/src/server/mcp.ts

@@ -166,15 +166,16 @@ export class McpServer {
         );
 
         this.server.setRequestHandler('tools/call', async (request, ctx): Promise<CallToolResult | CreateTaskResult> => {
-            try {
-                const tool = this._registeredTools[request.params.name];
-                if (!tool) {
-                    throw new ProtocolError(ProtocolErrorCode.InvalidParams, `Tool ${request.params.name} not found`);
-                }
-                if (!tool.enabled) {
-                    throw new ProtocolError(ProtocolErrorCode.InvalidParams, `Tool ${request.params.name} disabled`);
-                }
+            // Tool lookup errors are JSON-RPC errors, not tool errors
+            const tool = this._registeredTools[request.params.name];
+            if (!tool) {
+                throw new ProtocolError(ProtocolErrorCode.InvalidParams, `Tool ${request.params.name} not found`);
+            }
+            if (!tool.enabled) {
+                throw new ProtocolError(ProtocolErrorCode.InvalidParams, `Tool ${request.params.name} disabled`);
+            }
 
+            try {
                 const isTaskRequest = !!request.params.task;
                 const taskSupport = tool.execution?.taskSupport;
                 const isTaskHandler = 'createTask' in (tool.handler as AnyToolHandler<AnySchema>);

packages/server/src/server/stdio.ts

@@ -37,6 +37,14 @@ export class StdioServerTransport implements Transport {
     _onerror = (error: Error) => {
         this.onerror?.(error);
     };
+    _onstdouterror = (error: Error) => {
+        // Handle stdout errors (e.g., EPIPE when client disconnects)
+        // Trigger close to clean up gracefully
+        this.close().catch(() => {
+            // Ignore errors during close
+        });
+        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 +97,25 @@ export class StdioServerTransport implements Transport {
     }
 
     send(message: JSONRPCMessage): Promise<void> {
-        return new Promise(resolve => {
+        return new Promise((resolve, reject) => {
             const json = serializeMessage(message);
+
+            // Handle write errors (e.g., EPIPE when client disconnects)
+            const onError = (error: Error) => {
+                this._stdout.off('error', onError);
+                reject(error);
+            };
+
+            this._stdout.once('error', onError);
+
             if (this._stdout.write(json)) {
+                this._stdout.off('error', onError);
                 resolve();
             } else {
-                this._stdout.once('drain', resolve);
+                this._stdout.once('drain', () => {
+                    this._stdout.off('error', onError);
+                    resolve();
+                });
             }
         });
     }

test/integration/test/server/mcp.test.ts

@@ -1837,25 +1837,17 @@ describe('Zod v4', () => {
 
             await Promise.all([client.connect(clientTransport), mcpServer.server.connect(serverTransport)]);
 
-            const result = await client.request(
-                {
-                    method: 'tools/call',
-                    params: {
-                        name: 'nonexistent-tool'
-                    }
-                },
-                CallToolResultSchema
-            );
-
-            expect(result.isError).toBe(true);
-            expect(result.content).toEqual(
-                expect.arrayContaining([
+            await expect(
+                client.request(
                     {
-                        type: 'text',
-                        text: expect.stringContaining('Tool nonexistent-tool not found')
-                    }
-                ])
-            );
+                        method: 'tools/call',
+                        params: {
+                            name: 'nonexistent-tool'
+                        }
+                    },
+                    CallToolResultSchema
+                )
+            ).rejects.toThrow(/Tool nonexistent-tool not found/);
         });
 
         /***