diff --git a/source/units/Goccia.Values.FunctionBase.pas b/source/units/Goccia.Values.FunctionBase.pas
index e3f0006fa..7e38cb9d3 100644
--- a/source/units/Goccia.Values.FunctionBase.pas
+++ b/source/units/Goccia.Values.FunctionBase.pas
@@ -521,6 +521,8 @@ function OrdinaryHasInstance(const AConstructor, AInstance: TGocciaValue): Boole
   ConstructorPrototype: TGocciaValue;
   CurrentObject: TGocciaObjectValue;
   CurrentPrototype: TGocciaValue;
+  Roots: TGocciaActiveRootFrame;
+  HopRoots: TGocciaActiveRootFrame;
 begin
   if not IsCallableForHasInstance(AConstructor) then
     Exit(False);
@@ -541,18 +543,37 @@ function OrdinaryHasInstance(const AConstructor, AInstance: TGocciaValue): Boole
     ThrowTypeError('Function has non-object prototype',
       'set the constructor prototype property to an object');
 
-  CurrentObject := TGocciaObjectValue(AInstance);
-  while True do
-  begin
-    CurrentPrototype := GetPrototypeOfObject(CurrentObject);
-    if (CurrentPrototype = nil) or
-       (CurrentPrototype is TGocciaNullLiteralValue) then
-      Exit(False);
-    if CurrentPrototype = ConstructorPrototype then
-      Exit(True);
-    if not (CurrentPrototype is TGocciaObjectValue) then
-      Exit(False);
-    CurrentObject := TGocciaObjectValue(CurrentPrototype);
+  // GetPrototypeOfObject can invoke a proxy getPrototypeOf trap — guest code
+  // that may force a collection. The walked prototype is held only in a Pascal
+  // local across that trap, and DispatchProxyGetPrototype dereferences the
+  // proxy's internal target for the post-trap invariant check, so an
+  // intermediate prototype reachable only through this walk (e.g. a fresh proxy
+  // returned by an outer trap) would be swept before use. Root the target
+  // prototype for the whole walk and the current object across each hop.
+  Roots.Initialize;
+  Roots.Add(ConstructorPrototype);
+  try
+    CurrentObject := TGocciaObjectValue(AInstance);
+    while True do
+    begin
+      HopRoots.Initialize;
+      HopRoots.Add(CurrentObject);
+      try
+        CurrentPrototype := GetPrototypeOfObject(CurrentObject);
+      finally
+        HopRoots.Clear;
+      end;
+      if (CurrentPrototype = nil) or
+         (CurrentPrototype is TGocciaNullLiteralValue) then
+        Exit(False);
+      if CurrentPrototype = ConstructorPrototype then
+        Exit(True);
+      if not (CurrentPrototype is TGocciaObjectValue) then
+        Exit(False);
+      CurrentObject := TGocciaObjectValue(CurrentPrototype);
+    end;
+  finally
+    Roots.Clear;
   end;
 end;
 
diff --git a/tests/language/expressions/instanceof/prototype-chain-gc-roots.js b/tests/language/expressions/instanceof/prototype-chain-gc-roots.js
new file mode 100644
index 000000000..03a06b620
--- /dev/null
+++ b/tests/language/expressions/instanceof/prototype-chain-gc-roots.js
@@ -0,0 +1,87 @@
+/*---
+description: instanceof keeps the walked prototype reachable across a proxy getPrototypeOf trap
+features: [Goccia.gc, Proxy, Reflect]
+---*/
+
+const hasGoccia = typeof Goccia !== "undefined";
+
+const range = (n) => Array.from({ length: n }, (_, i) => i);
+
+// A bare gc() usually leaves a freed slot readable; allocating a run of objects
+// of the same GC class as the collected temporary is what overwrites the slot
+// and makes a dangling read observable. The walked prototype is a Proxy, so the
+// churn allocates proxies.
+const churn = () => {
+  Goccia.gc();
+  Goccia.gc();
+  let sink = null;
+  for (const i of range(200)) {
+    sink = new Proxy({ a: i, b: i + 1 }, { get(t, k) { return t[k]; } });
+  }
+  return sink;
+};
+
+describe.runIf(hasGoccia)("instanceof prototype-chain GC roots", () => {
+  // OrdinaryHasInstance walks the instance's prototype chain with
+  // GetPrototypeOfObject, which invokes a proxy getPrototypeOf trap — guest code
+  // that can force a collection. An intermediate prototype reachable only through
+  // the walk (here, a fresh proxy returned by an outer trap) is held solely in
+  // the loop variable; DispatchProxyGetPrototype dereferences the proxy's
+  // internal target for its post-trap invariant check, so without a root the
+  // proxy is swept mid-walk and the invariant check reads freed memory.
+  test("an engine-only intermediate proxy survives its getPrototypeOf trap", () => {
+    let outerRuns = 0;
+    let innerRuns = 0;
+    class C {}
+    const obj = {};
+
+    Reflect.setPrototypeOf(
+      obj,
+      new Proxy(
+        {},
+        {
+          getPrototypeOf() {
+            outerRuns += 1;
+            const innerTarget = Object.preventExtensions(Object.create(null));
+            return new Proxy(innerTarget, {
+              getPrototypeOf(t) {
+                innerRuns += 1;
+                churn();
+                return Reflect.getPrototypeOf(t);
+              },
+            });
+          },
+        }
+      )
+    );
+
+    const result = obj instanceof C;
+
+    expect(outerRuns).toBe(1);
+    expect(innerRuns).toBe(1);
+    expect(result).toBe(false);
+  });
+
+  // The direct case: the walked proxy is orphaned by the trap that runs on it and
+  // must survive to the post-trap extensibility invariant check.
+  test("a directly walked proxy survives a collecting getPrototypeOf trap", () => {
+    let trapRuns = 0;
+    class C {}
+    const obj = {};
+    const target = Object.preventExtensions(Object.create(null));
+
+    Reflect.setPrototypeOf(
+      obj,
+      new Proxy(target, {
+        getPrototypeOf(t) {
+          trapRuns += 1;
+          churn();
+          return Reflect.getPrototypeOf(t);
+        },
+      })
+    );
+
+    expect(obj instanceof C).toBe(false);
+    expect(trapRuns).toBe(1);
+  });
+});