diff --git a/packages/server/src/server/createMcpHandler.ts b/packages/server/src/server/createMcpHandler.ts
index 9adbe54fb8..99979fb774 100644
--- a/packages/server/src/server/createMcpHandler.ts
+++ b/packages/server/src/server/createMcpHandler.ts
@@ -686,6 +686,14 @@ export function createMcpHandler(factory: McpServerFactory, options: CreateMcpHa
 
     /** Modern per-request instances with an exchange still in flight (close() tears these down). */
     const inflight = new Set<Server>();
+    /**
+     * Instances whose `onclose` has already been wrapped for inflight tracking below. A factory
+     * that returns the same `Server`/`McpServer` for every session would otherwise have this
+     * wrapping installed once per request, chaining a new closure onto the previous `onclose` each
+     * time — a chain that grows without bound and eventually overflows the stack when it finally
+     * runs (#2607). Installing at most once per instance keeps the chain length at 1.
+     */
+    const oncloseWrapped = new WeakSet<Server>();
     let closed = false;
 
     const reportError = (error: Error) => {
@@ -844,12 +852,15 @@ export function createMcpHandler(factory: McpServerFactory, options: CreateMcpHa
         }
 
         // Track the instance until its exchange tears down so close() can abort it.
-        const previousOnClose = server.onclose;
         inflight.add(server);
-        server.onclose = () => {
-            inflight.delete(server);
-            previousOnClose?.();
-        };
+        if (!oncloseWrapped.has(server)) {
+            oncloseWrapped.add(server);
+            const previousOnClose = server.onclose;
+            server.onclose = () => {
+                inflight.delete(server);
+                previousOnClose?.();
+            };
+        }
 
         try {
             const response = await invoke(product, route.message, {
diff --git a/packages/server/test/server/createMcpHandler.test.ts b/packages/server/test/server/createMcpHandler.test.ts
index 4dae5c0ad7..77343ec4d2 100644
--- a/packages/server/test/server/createMcpHandler.test.ts
+++ b/packages/server/test/server/createMcpHandler.test.ts
@@ -303,6 +303,38 @@ describe('createMcpHandler — modern path', () => {
         expect(closeCalls).toBe(1);
     });
 
+    it('does not grow an unbounded onclose chain when the factory reuses one instance across requests (#2607)', async () => {
+        const shared = new McpServer({ name: 'reused-server', version: '1.0.0' });
+        shared.registerTool('echo', { inputSchema: z.object({ text: z.string() }) }, async ({ text }) => ({
+            content: [{ type: 'text', text }]
+        }));
+
+        // Intercept assignments to the underlying Server's `onclose` field to count how many
+        // times createMcpHandler installs its inflight-tracking wrapper on this one instance.
+        let oncloseSetCount = 0;
+        let currentOnClose: (() => void) | undefined;
+        Object.defineProperty(shared.server, 'onclose', {
+            configurable: true,
+            get: () => currentOnClose,
+            set: (fn: (() => void) | undefined) => {
+                oncloseSetCount += 1;
+                currentOnClose = fn;
+            }
+        });
+
+        const handler = createMcpHandler(() => shared);
+
+        for (let i = 0; i < 5; i++) {
+            const response = await handler.fetch(postRequest(modernToolsCall('echo', { text: 'x' })));
+            expect(response.status).toBe(200);
+        }
+
+        // Wrapped exactly once, no matter how many requests reused this instance: a growing
+        // chain here is what eventually overflows the stack when onclose finally runs.
+        expect(oncloseSetCount).toBe(1);
+        expect(() => currentOnClose?.()).not.toThrow();
+    });
+
     it('rejects a malformed envelope behind a present claim with invalid params naming the offending key', async () => {
         const { factory, state } = testFactory();
         const handler = createMcpHandler(factory);