fix(server): restore onclose after modern exchanges

packages/server/src/server/createMcpHandler.ts

@@ -848,6 +848,7 @@ export function createMcpHandler(factory: McpServerFactory, options: CreateMcpHa
         inflight.add(server);
         server.onclose = () => {
             inflight.delete(server);
+            server.onclose = previousOnClose;
             previousOnClose?.();
         };
 

packages/server/test/server/createMcpHandler.test.ts

@@ -274,6 +274,27 @@ describe('createMcpHandler — modern path', () => {
         expect(onerror).toHaveBeenCalledWith(expect.objectContaining({ message: 'factory exploded' }));
     });
 
+    it('restores a reused server onclose handler after each modern exchange', async () => {
+        const reused = new McpServer({ name: 'entry-test-server', version: '1.0.0' });
+        reused.registerTool('echo', { inputSchema: z.object({ text: z.string() }) }, async ({ text }) => ({
+            content: [{ type: 'text', text }]
+        }));
+
+        const originalOnClose = vi.fn();
+        reused.server.onclose = originalOnClose;
+
+        const handler = createMcpHandler(() => reused);
+
+        for (let i = 0; i < 3; i++) {
+            const response = await handler.fetch(postRequest(modernToolsCall('echo', { text: `hello-${i}` })));
+            expect(response.status).toBe(200);
+            await response.text();
+            expect(reused.server.onclose).toBe(originalOnClose);
+        }
+
+        expect(originalOnClose).toHaveBeenCalledTimes(3);
+    });
+
     it('closes and releases the per-request instance when a modern exchange fails internally', async () => {
         const { factory, state } = testFactory();
         const onerror = vi.fn();