diff --git a/changelog.d/11514-callback-this-undefined.md b/changelog.d/11514-callback-this-undefined.md
new file mode 100644
index 0000000000..59e31540f6
--- /dev/null
+++ b/changelog.d/11514-callback-this-undefined.md
@@ -0,0 +1,17 @@
+Fixed `sort` comparators and `reduce` / `reduceRight` callbacks seeing the
+enclosing method's receiver as `this` (#11419). The runtime invoked them
+through plain `js_closure_callN` calls without resetting the per-thread
+implicit-`this` cell, so a comparator called from inside `obj.m()` read `obj`
+instead of `undefined`. The `forEach` / `map` / `filter` / `find*` / `some` /
+`every` / `flatMap` family already bound `undefined` around their callbacks;
+the sort and reduce entry points now do the same through a new
+`ImplicitThisScope::bind_undefined`, covering arrays, typed arrays (including
+BigInt lanes and `toSorted`) and generic array-likes
+(`Array.prototype.sort/reduce/reduceRight.call(obj, …)`). The guard restores
+the caller's receiver from a rooted slot on return and on unwind.
+
+The structural fix (receiver as a calling-convention parameter, deleting the
+cell) remains the `this`-as-parameter plan; this closes the observable leak
+for these callbacks. Regression test:
+`test-files/test_gap_11419_callback_this_undefined.ts` (every
+surface leaks on `main`; all pass with this change).
diff --git a/crates/perry-runtime/src/array/generic.rs b/crates/perry-runtime/src/array/generic.rs
index 1f1d12e20f..372a6d0088 100644
--- a/crates/perry-runtime/src/array/generic.rs
+++ b/crates/perry-runtime/src/array/generic.rs
@@ -873,6 +873,8 @@ pub extern "C" fn js_arraylike_findLastIndex(recv: f64, cb: f64, this_arg: f64)
 #[no_mangle]
 pub extern "C" fn js_arraylike_reduce(recv: f64, cb: f64, has_init: i32, init: f64) -> f64 {
     let scope = crate::gc::RuntimeHandleScope::new();
+    // #11419: `this` is undefined in the callback (no thisArg parameter).
+    let _this = crate::object::ImplicitThisScope::bind_undefined(&scope);
     let cb_h = scope.root_nanbox_f64(cb);
     let recv_h = scope.root_nanbox_f64(to_object(recv));
     // Spec order: LengthOfArrayLike(O) is read *before* the IsCallable(cb)
@@ -916,6 +918,8 @@ pub extern "C" fn js_arraylike_reduce(recv: f64, cb: f64, has_init: i32, init: f
 #[no_mangle]
 pub extern "C" fn js_arraylike_reduceRight(recv: f64, cb: f64, has_init: i32, init: f64) -> f64 {
     let scope = crate::gc::RuntimeHandleScope::new();
+    // #11419: `this` is undefined in the callback (no thisArg parameter).
+    let _this = crate::object::ImplicitThisScope::bind_undefined(&scope);
     let cb_h = scope.root_nanbox_f64(cb);
     let recv_h = scope.root_nanbox_f64(to_object(recv));
     // Spec order: LengthOfArrayLike(O) is read *before* the IsCallable(cb)
diff --git a/crates/perry-runtime/src/array/generic_object.rs b/crates/perry-runtime/src/array/generic_object.rs
index cbf4f74cd2..534c676ddf 100644
--- a/crates/perry-runtime/src/array/generic_object.rs
+++ b/crates/perry-runtime/src/array/generic_object.rs
@@ -382,6 +382,8 @@ pub(crate) fn object_sort(recv: f64, cmp_validated: *const ClosureHeader) -> f64
     // Length and indexed getters may collect before comparison begins.
     let scope = crate::gc::RuntimeHandleScope::new();
     let recv_handle = scope.root_nanbox_f64(recv);
+    // #11419: the comparator runs with `this` undefined.
+    let _this = crate::object::ImplicitThisScope::bind_undefined(&scope);
     let cmp_handle = scope.root_raw_const_ptr(cmp_validated);
     let cmp = if cmp_validated.is_null() {
         None
diff --git a/crates/perry-runtime/src/array/iter_methods.rs b/crates/perry-runtime/src/array/iter_methods.rs
index e59c1b4709..0e704d1f5c 100644
--- a/crates/perry-runtime/src/array/iter_methods.rs
+++ b/crates/perry-runtime/src/array/iter_methods.rs
@@ -166,7 +166,7 @@ mod rooted_iter_array_tests {
 fn bind_undefined_this(
     scope: &crate::gc::RuntimeHandleScope,
 ) -> crate::object::ImplicitThisScope<'_> {
-    crate::object::ImplicitThisScope::bind(scope, undefined_value())
+    crate::object::ImplicitThisScope::bind_undefined(scope)
 }
 
 /// #5989/#8117: `.forEach` on a receiver codegen could not prove is a
@@ -1199,6 +1199,10 @@ pub extern "C" fn js_array_reduce(
     has_initial: i32,
     initial: f64,
 ) -> f64 {
+    // #11419: the callback runs with `this` undefined, not the receiver of
+    // whatever method dispatch encloses this call.
+    let this_scope = crate::gc::RuntimeHandleScope::new();
+    let _this = crate::object::ImplicitThisScope::bind_undefined(&this_scope);
     // #8137: a Buffer-backed `Uint8Array` receiver. Perry's
     // `new Uint8Array([…])` is a `BufferHeader`, absent from the typed-array
     // registry, so the `lookup_typed_array_kind` re-dispatch below never
diff --git a/crates/perry-runtime/src/array/reduce_right.rs b/crates/perry-runtime/src/array/reduce_right.rs
index d77ab55453..a2653d4d5c 100644
--- a/crates/perry-runtime/src/array/reduce_right.rs
+++ b/crates/perry-runtime/src/array/reduce_right.rs
@@ -22,6 +22,10 @@ pub extern "C" fn js_array_reduce_right(
     has_initial: i32,
     initial: f64,
 ) -> f64 {
+    // #11419: the callback runs with `this` undefined, not the receiver of
+    // whatever method dispatch encloses this call.
+    let this_scope = crate::gc::RuntimeHandleScope::new();
+    let _this = crate::object::ImplicitThisScope::bind_undefined(&this_scope);
     // #8137: a Buffer-backed `Uint8Array` receiver reads as an `ArrayHeader`
     // below — correct `length`, GARBAGE elements. `reduceRight` is the widest
     // case in the family: it is wrong even for a STATICALLY typed
diff --git a/crates/perry-runtime/src/array/sort.rs b/crates/perry-runtime/src/array/sort.rs
index c9f4628791..afefd91184 100644
--- a/crates/perry-runtime/src/array/sort.rs
+++ b/crates/perry-runtime/src/array/sort.rs
@@ -767,6 +767,10 @@ pub extern "C" fn js_array_sort_with_comparator(
     if comparator.is_null() {
         return js_array_sort_default(arr);
     }
+    // #11419: the callback runs with `this` undefined, not the receiver of
+    // whatever method dispatch encloses this call.
+    let this_scope = crate::gc::RuntimeHandleScope::new();
+    let _this = crate::object::ImplicitThisScope::bind_undefined(&this_scope);
     unsafe {
         // Runtime plain-object receiver behind a statically-Array variable —
         // probe the RAW pointer before the array-plausibility clean.
diff --git a/crates/perry-runtime/src/object/this_binding.rs b/crates/perry-runtime/src/object/this_binding.rs
index 45fe20d7c8..f996dd19f4 100644
--- a/crates/perry-runtime/src/object/this_binding.rs
+++ b/crates/perry-runtime/src/object/this_binding.rs
@@ -280,6 +280,15 @@ impl<'scope> ImplicitThisScope<'scope> {
             previous: scope.root_nanbox_f64(js_implicit_this_set(receiver)),
         }
     }
+
+    /// Bind `this` to `undefined`: OrdinaryCallBindThis for a callback the
+    /// runtime invokes with no receiver (`sort` comparators, `reduce`
+    /// callbacks, an absent `thisArg`). Without it the callee reads whatever
+    /// the enclosing method dispatch left in the cell (#11419).
+    #[inline]
+    pub fn bind_undefined(scope: &'scope crate::gc::RuntimeHandleScope) -> Self {
+        Self::bind(scope, f64::from_bits(crate::value::TAG_UNDEFINED))
+    }
 }
 
 impl Drop for ImplicitThisScope<'_> {
diff --git a/crates/perry-runtime/src/typedarray/iterate.rs b/crates/perry-runtime/src/typedarray/iterate.rs
index d782f59a1c..af0ab949d6 100644
--- a/crates/perry-runtime/src/typedarray/iterate.rs
+++ b/crates/perry-runtime/src/typedarray/iterate.rs
@@ -240,6 +240,10 @@ pub extern "C" fn js_typed_array_reduce(
     has_initial: i32,
     initial: f64,
 ) -> f64 {
+    // #11419: the callback runs with `this` undefined, not the receiver of
+    // whatever method dispatch encloses this call.
+    let this_scope = crate::gc::RuntimeHandleScope::new();
+    let _this = crate::object::ImplicitThisScope::bind_undefined(&this_scope);
     let ta = clean_ta_ptr(ta);
     if ta.is_null() {
         if has_initial != 0 {
@@ -296,6 +300,10 @@ pub extern "C" fn js_typed_array_reduce_right(
     has_initial: i32,
     initial: f64,
 ) -> f64 {
+    // #11419: the callback runs with `this` undefined, not the receiver of
+    // whatever method dispatch encloses this call.
+    let this_scope = crate::gc::RuntimeHandleScope::new();
+    let _this = crate::object::ImplicitThisScope::bind_undefined(&this_scope);
     let ta = clean_ta_ptr(ta);
     if ta.is_null() {
         if has_initial != 0 {
diff --git a/crates/perry-runtime/src/typedarray/transform.rs b/crates/perry-runtime/src/typedarray/transform.rs
index 2b39fe2185..6c6f168dfe 100644
--- a/crates/perry-runtime/src/typedarray/transform.rs
+++ b/crates/perry-runtime/src/typedarray/transform.rs
@@ -187,6 +187,10 @@ pub extern "C" fn js_typed_array_sort_with_comparator(
     if comparator.is_null() {
         return js_typed_array_sort_default(ta);
     }
+    // #11419: the callback runs with `this` undefined, not the receiver of
+    // whatever method dispatch encloses this call.
+    let this_scope = crate::gc::RuntimeHandleScope::new();
+    let _this = crate::object::ImplicitThisScope::bind_undefined(&this_scope);
     let ta_clean = clean_ta_ptr(ta as *const TypedArrayHeader) as *mut TypedArrayHeader;
     if ta_clean.is_null() {
         return ta_clean;
@@ -277,6 +281,10 @@ pub extern "C" fn js_typed_array_to_sorted_with_comparator(
     if comparator.is_null() {
         return js_typed_array_to_sorted_default(ta);
     }
+    // #11419: the callback runs with `this` undefined, not the receiver of
+    // whatever method dispatch encloses this call.
+    let this_scope = crate::gc::RuntimeHandleScope::new();
+    let _this = crate::object::ImplicitThisScope::bind_undefined(&this_scope);
     let ta = clean_ta_ptr(ta);
     if ta.is_null() {
         return typed_array_alloc(KIND_FLOAT64, 0);
diff --git a/test-files/test_gap_11419_callback_this_undefined.ts b/test-files/test_gap_11419_callback_this_undefined.ts
new file mode 100644
index 0000000000..79236f4afd
--- /dev/null
+++ b/test-files/test_gap_11419_callback_this_undefined.ts
@@ -0,0 +1,74 @@
+"use strict";
+// #11419: a callback the runtime invokes with no receiver — a `sort`
+// comparator, a `reduce` / `reduceRight` callback — and a plain call of a
+// function value must see `this === undefined`, even when the call happens
+// inside a method whose receiver is still sitting in the implicit-`this` cell.
+// Before the fix the comparator / reducer saw the enclosing method's receiver.
+
+const seen: string[] = [];
+function note(tag: string, self: any): void {
+  if (self !== undefined) seen.push(tag + ":" + typeof self);
+}
+
+function cmp(this: any, a: number, b: number) { note("sort", this); return a - b; }
+function cmpBig(this: any, a: bigint, b: bigint) { note("sortBig", this); return a < b ? -1 : a > b ? 1 : 0; }
+function red(this: any, acc: number, v: number) { note("reduce", this); return acc + v; }
+function zero(this: any) { return typeof this; }
+function one(this: any, a: number) { return typeof this; }
+function two(this: any, a: number, b: number) { return typeof this; }
+function three(this: any, a: number, b: number, c: number) { return typeof this; }
+
+const holder: any = {
+  k: 1,
+  arrays(this: any) {
+    const a = [3, 1, 2];
+    a.sort(cmp);
+    const s = [3, 1, 2].toSorted(cmp);
+    const r1 = [1, 2, 3].reduce(red, 0);
+    const r2 = [1, 2, 3].reduce(red);
+    const r3 = [1, 2, 3].reduceRight(red, 0);
+    const r4 = [1, 2, 3].reduceRight(red);
+    return a.join() + "|" + s.join() + "|" + r1 + "," + r2 + "," + r3 + "," + r4;
+  },
+  typed(this: any) {
+    const f = new Float64Array([3, 1, 2]);
+    f.sort(cmp);
+    const t = new Int32Array([3, 1, 2]).toSorted(cmp);
+    const b = new BigInt64Array([3n, 1n, 2n]);
+    b.sort(cmpBig);
+    const r1 = new Int32Array([1, 2, 3]).reduce(red, 0);
+    const r2 = new Int32Array([1, 2, 3]).reduceRight(red);
+    return f.join(",") + "|" + t.join(",") + "|" + b.join(",") + "|" + r1 + "," + r2;
+  },
+  arrayLike(this: any) {
+    const o: any = { length: 3, 0: 3, 1: 1, 2: 2 };
+    Array.prototype.sort.call(o, cmp);
+    const r1 = Array.prototype.reduce.call({ length: 2, 0: 1, 1: 2 }, red, 0);
+    const r2 = Array.prototype.reduceRight.call({ length: 2, 0: 1, 1: 2 }, red, 0);
+    return o[0] + "," + o[1] + "," + o[2] + "|" + r1 + "," + r2;
+  },
+  plain(this: any) {
+    const f0: any = zero;
+    const f1: any = one;
+    const f2: any = two;
+    const f3: any = three;
+    return [f0(), f1(1), f2(1, 2), f3(1, 2, 3), zero(), two(1, 2)].join(",");
+  },
+};
+
+console.log("arrays", holder.arrays());
+console.log("typed", holder.typed());
+console.log("arrayLike", holder.arrayLike());
+console.log("plain", holder.plain());
+
+class Box {
+  v = 7;
+  run() {
+    const out = [5, 4].sort(cmp).join() + "|" + [1, 2].reduce(red, 0);
+    // The method's own `this` must survive the callbacks.
+    return out + "|" + this.v;
+  }
+}
+console.log("class", new Box().run());
+
+console.log("leaks", seen.length === 0 ? "none" : seen.join(" "));