diff --git a/proxy.js b/proxy.js
index cd9b5ee..adcf180 100644
--- a/proxy.js
+++ b/proxy.js
@@ -3,8 +3,9 @@
  * the proxy or its target before changing the target's prototype.
  */
 export function createProxy(NativeProxy) {
-  const { getPrototypeOf, setPrototypeOf } = Reflect;
+  const { apply, getPrototypeOf, setPrototypeOf } = Reflect;
   const objectCreate = Object.create;
+  const hasOwnProperty = Object.prototype.hasOwnProperty;
 
   // Proxies are walked like any other object, so a cycle spanning several scriptlet
   // proxies is found. The tortoise ends walks over chains that are already cyclic.
@@ -23,11 +24,20 @@ export function createProxy(NativeProxy) {
   };
 
   return function Proxy(target, handler) {
-    // Inherit from the handler rather than copy it, so traps added to it later still apply.
+    // Compose with the handler's own setPrototypeOf trap, if any: read it once, as an own
+    // property only, so page code cannot inject one through Object.prototype. It runs after
+    // the cycle check below, so it only ever sees non-cyclic prototypes.
+    const trap = apply(hasOwnProperty, handler, ['setPrototypeOf']) ? handler.setPrototypeOf : undefined;
+
+    // Inherit from the handler rather than copy it, so other traps added to it later still apply.
+    // A setPrototypeOf trap added to the handler after construction stays shadowed by the composed one.
     const proxy = new NativeProxy(target, objectCreate(handler, {
-      // Returning false makes Object.setPrototypeOf throw and Reflect.setPrototypeOf
-      // return false, as both do for a native function.
-      setPrototypeOf: { value: (target, proto) => !reaches(proto, proxy, target) && setPrototypeOf(target, proto) },
+      setPrototypeOf: { value(target, proto) {
+        // Returning false makes Object.setPrototypeOf throw and Reflect.setPrototypeOf
+        // return false, as both do for a native function.
+        if (reaches(proto, proxy, target)) return false;
+        return trap === undefined ? setPrototypeOf(target, proto) : apply(trap, this, [target, proto]);
+      } },
     }));
     return proxy;
   };
diff --git a/test.js b/test.js
index b91a519..52a1360 100644
--- a/test.js
+++ b/test.js
@@ -2,6 +2,7 @@ import { test, suite } from "node:test";
 import assert from "node:assert";
 import vm from "node:vm";
 import scriptlets, { run } from "./index.js";
+import { createProxy } from "./proxy.js";
 
 const runSource = run.toString();
 
@@ -138,3 +139,37 @@ suite("proxied functions", () => {
     assert(vm.runInContext(set, page));
   });
 });
+
+suite("createProxy", () => {
+  test("preserves a handler's own setPrototypeOf trap", () => {
+    const Proxy = createProxy(globalThis.Proxy);
+    const target = function fn() {};
+    const calls = [];
+    const proxy = new Proxy(target, {
+      setPrototypeOf(t, proto) {
+        calls.push(proto);
+        return Reflect.setPrototypeOf(t, proto);
+      },
+    });
+
+    // Non-cyclic: the handler's trap runs and its result is honored.
+    const proto = Object.create(Function.prototype);
+    assert.strictEqual(Reflect.setPrototypeOf(proxy, proto), true);
+    assert.deepStrictEqual(calls, [proto]);
+    assert.strictEqual(Object.getPrototypeOf(target), proto);
+
+    // A falsish result from the handler's trap is honored, too.
+    const rejecting = new Proxy(target, {
+      setPrototypeOf() {
+        calls.push("reject");
+        return false;
+      },
+    });
+    assert.strictEqual(Reflect.setPrototypeOf(rejecting, Object.create(Function.prototype)), false);
+
+    // Cyclic: rejected before the handler's trap runs, like a native function.
+    assert.throws(() => Object.setPrototypeOf(proxy, Object.create(proxy)), TypeError);
+    assert.strictEqual(Reflect.setPrototypeOf(proxy, Object.create(proxy)), false);
+    assert.deepStrictEqual(calls, [proto, "reject"]);
+  });
+});
diff --git a/ubo.js b/ubo.js
index 39a0653..186705a 100644
--- a/ubo.js
+++ b/ubo.js
@@ -810,8 +810,9 @@ fn: 'trustedOverrideElementMethod',
 // Proxy shadows the global for every scriptlet in run(), see proxy.js.
 export function run(scriptletGlobals, calls) {
 const Proxy = (function createProxy(NativeProxy) {
-  const { getPrototypeOf, setPrototypeOf } = Reflect;
+  const { apply, getPrototypeOf, setPrototypeOf } = Reflect;
   const objectCreate = Object.create;
+  const hasOwnProperty = Object.prototype.hasOwnProperty;
 
   // Proxies are walked like any other object, so a cycle spanning several scriptlet
   // proxies is found. The tortoise ends walks over chains that are already cyclic.
@@ -830,11 +831,20 @@ const Proxy = (function createProxy(NativeProxy) {
   };
 
   return function Proxy(target, handler) {
-    // Inherit from the handler rather than copy it, so traps added to it later still apply.
+    // Compose with the handler's own setPrototypeOf trap, if any: read it once, as an own
+    // property only, so page code cannot inject one through Object.prototype. It runs after
+    // the cycle check below, so it only ever sees non-cyclic prototypes.
+    const trap = apply(hasOwnProperty, handler, ['setPrototypeOf']) ? handler.setPrototypeOf : undefined;
+
+    // Inherit from the handler rather than copy it, so other traps added to it later still apply.
+    // A setPrototypeOf trap added to the handler after construction stays shadowed by the composed one.
     const proxy = new NativeProxy(target, objectCreate(handler, {
-      // Returning false makes Object.setPrototypeOf throw and Reflect.setPrototypeOf
-      // return false, as both do for a native function.
-      setPrototypeOf: { value: (target, proto) => !reaches(proto, proxy, target) && setPrototypeOf(target, proto) },
+      setPrototypeOf: { value(target, proto) {
+        // Returning false makes Object.setPrototypeOf throw and Reflect.setPrototypeOf
+        // return false, as both do for a native function.
+        if (reaches(proto, proxy, target)) return false;
+        return trap === undefined ? setPrototypeOf(target, proto) : apply(trap, this, [target, proto]);
+      } },
     }));
     return proxy;
   };