diff --git a/moli-benchmark/wpt-cross-current/failed-cases.txt b/moli-benchmark/wpt-cross-current/failed-cases.txt
index 3849307eaa..58344fe010 100644
--- a/moli-benchmark/wpt-cross-current/failed-cases.txt
+++ b/moli-benchmark/wpt-cross-current/failed-cases.txt
@@ -2924,7 +2924,6 @@ html/browsers/the-window-object/window-indexed-properties-strict.html
 html/browsers/the-window-object/window-indexed-properties.html
 html/browsers/the-window-object/window-open-noopener.html?indexed
 html/browsers/the-window-object/window-open-noreferrer.html
-html/browsers/the-windowproxy-exotic-object/windowproxy-prototype-setting-same-origin.html
 html/browsers/windows/browsing-context-names/choose-default-001.html
 html/browsers/windows/browsing-context.html
 html/browsers/windows/document-domain-removed-iframe.html
diff --git a/moli-benchmark/wpt-cross-current/passed-cases.txt b/moli-benchmark/wpt-cross-current/passed-cases.txt
index 2e131b70ec..72c5f44883 100644
--- a/moli-benchmark/wpt-cross-current/passed-cases.txt
+++ b/moli-benchmark/wpt-cross-current/passed-cases.txt
@@ -5248,6 +5248,7 @@ html/browsers/the-window-object/window-open-noopener.html?_top
 html/browsers/the-window-object/window-prototype-chain.html
 html/browsers/the-windowproxy-exotic-object/windowproxy-define-own-property-unforgeable-same-origin.html
 html/browsers/the-windowproxy-exotic-object/windowproxy-prevent-extensions.html
+html/browsers/the-windowproxy-exotic-object/windowproxy-prototype-setting-same-origin.html
 html/browsers/windows/browsing-context-names/choose-_blank-001.html
 html/browsers/windows/browsing-context-names/choose-default-002.html
 html/browsers/windows/browsing-context-window.html
diff --git a/moli-renderer-v8/src/context_bootstrap/assets/global.rs b/moli-renderer-v8/src/context_bootstrap/assets/global.rs
index fa8b63f167..275ed0e684 100644
--- a/moli-renderer-v8/src/context_bootstrap/assets/global.rs
+++ b/moli-renderer-v8/src/context_bootstrap/assets/global.rs
@@ -42,6 +42,14 @@ impl ContextBootstrapAssets {
             .id_by_name("Window")
             .ok_or_else(|| anyhow!("missing constructor template metadata `Window`"))?;
         let window_template = registry.get_or_build_template(scope, window_id)?;
+        // WindowProxy's [[SetPrototypeOf]] only succeeds when the requested
+        // prototype is already its current prototype. Set the invariant on
+        // Window's instance template before deriving the same- and
+        // cross-origin global templates so it remains attached to V8's global
+        // proxy rather than only to the inner global object.
+        window_template
+            .instance_template(scope)
+            .set_immutable_proto();
         // A cross-origin WindowProxy shell must retain Window's V8 wrapper
         // identity so that detaching and reusing it for the committed child
         // realm discards the facade's temporary own properties. It must not,
diff --git a/moli-renderer-v8/src/script_vm/tests/dom_elements/dom_surface.rs b/moli-renderer-v8/src/script_vm/tests/dom_elements/dom_surface.rs
index b72d484819..d10c1cd8a1 100644
--- a/moli-renderer-v8/src/script_vm/tests/dom_elements/dom_surface.rs
+++ b/moli-renderer-v8/src/script_vm/tests/dom_elements/dom_surface.rs
@@ -8827,6 +8827,72 @@ fn window_named_properties_respect_later_prototype_properties_and_descriptor_fla
     );
 }
 
+#[test]
+fn main_and_child_window_proxies_have_immutable_prototypes() {
+    let mut vm = new_storage_test_vm("https://window-proxy-prototype.test/");
+
+    let result = vm
+        .eval(
+            r#"
+            (() => {
+                "use strict";
+                const frame = document.createElement("iframe");
+                (document.body || document.documentElement || document).appendChild(frame);
+
+                const probe = target => {
+                    const original = Object.getPrototypeOf(target);
+                    const replacement = {};
+                    const outcome = callback => {
+                        try {
+                            callback();
+                            return "returned";
+                        } catch (error) {
+                            return error && error.name;
+                        }
+                    };
+
+                    const objectDifferent = outcome(() => {
+                        Object.setPrototypeOf(target, replacement);
+                    });
+                    const dunderDifferent = outcome(() => {
+                        target.__proto__ = replacement;
+                    });
+                    const reflectDifferent = Reflect.setPrototypeOf(target, replacement);
+                    const unchanged = Object.getPrototypeOf(target) === original;
+                    const objectSame = Object.setPrototypeOf(target, original) === target;
+                    const dunderSame = outcome(() => {
+                        target.__proto__ = original;
+                    });
+                    const reflectSame = Reflect.setPrototypeOf(target, original);
+
+                    return {
+                        objectDifferent,
+                        dunderDifferent,
+                        reflectDifferent,
+                        unchanged,
+                        objectSame,
+                        dunderSame,
+                        reflectSame,
+                    };
+                };
+
+                const observations = {
+                    main: probe(window),
+                    child: probe(frame.contentWindow),
+                };
+                frame.remove();
+                return JSON.stringify(observations);
+            })()
+            "#,
+        )
+        .expect("WindowProxy immutable prototype probe should evaluate");
+
+    assert_eq!(
+        result,
+        r#"{"main":{"objectDifferent":"TypeError","dunderDifferent":"TypeError","reflectDifferent":false,"unchanged":true,"objectSame":true,"dunderSame":"returned","reflectSame":true},"child":{"objectDifferent":"TypeError","dunderDifferent":"TypeError","reflectDifferent":false,"unchanged":true,"objectSame":true,"dunderSame":"returned","reflectSame":true}}"#
+    );
+}
+
 #[test]
 fn window_internal_child_context_identity_is_not_read_from_web_properties() {
     let mut vm = new_storage_test_vm("https://window-private-identity.test/");