diff --git a/extension/pytree/aten_util/targets.bzl b/extension/pytree/aten_util/targets.bzl
index d09ffe48cbd..e1793080203 100644
--- a/extension/pytree/aten_util/targets.bzl
+++ b/extension/pytree/aten_util/targets.bzl
@@ -17,6 +17,7 @@ def define_common_targets():
         ],
         exported_deps = [
             "//executorch/extension/pytree:pytree",
+            "//executorch/runtime/platform:platform",
         ],
         compiler_flags = ["-Wno-missing-prototypes"],
         fbcode_deps = [
diff --git a/extension/pytree/function_ref.h b/extension/pytree/function_ref.h
new file mode 100644
index 00000000000..01d2988597a
--- /dev/null
+++ b/extension/pytree/function_ref.h
@@ -0,0 +1,158 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+//===- llvm/ADT/STLFunctionalExtras.h - Extras for <functional> -*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file contains some extension to <functional>.
+//
+// No library is required when using these functions.
+//
+//===----------------------------------------------------------------------===//
+//     Extra additions to <functional>
+//===----------------------------------------------------------------------===//
+
+/// An efficient, type-erasing, non-owning reference to a callable. This is
+/// intended for use as the type of a function parameter that is not used
+/// after the function in question returns.
+///
+/// This class does not own the callable, so it is not in general safe to store
+/// a FunctionRef.
+
+// torch::executor: modified from llvm::function_ref
+// see https://www.foonathan.net/2017/01/function-ref-implementation/
+
+#pragma once
+
+#include <cstdint>
+#include <type_traits>
+#include <utility>
+
+namespace torch {
+namespace executor {
+namespace pytree {
+
+//===----------------------------------------------------------------------===//
+//     Features from C++20
+//===----------------------------------------------------------------------===//
+
+template <typename T>
+struct remove_cvref {
+  using type =
+      typename std::remove_cv<typename std::remove_reference<T>::type>::type;
+};
+
+template <typename T>
+using remove_cvref_t = typename remove_cvref<T>::type;
+
+template <typename Fn>
+class FunctionRef;
+
+template <typename Ret, typename... Params>
+class FunctionRef<Ret(Params...)> {
+  Ret (*callback_)(const void* memory, Params... params) = nullptr;
+  union Storage {
+    void* callable;
+    Ret (*function)(Params...);
+  } storage_;
+
+ public:
+  FunctionRef() = default;
+  explicit FunctionRef(std::nullptr_t) {}
+
+  /**
+   * Case 1: A callable object passed by lvalue reference.
+   * Taking rvalue reference is error prone because the object will be always
+   * be destroyed immediately.
+   */
+  template <
+      typename Callable,
+      // This is not the copy-constructor.
+      typename std::enable_if<
+          !std::is_same<remove_cvref_t<Callable>, FunctionRef>::value,
+          int32_t>::type = 0,
+      // Avoid lvalue reference to non-capturing lambda.
+      typename std::enable_if<
+          !std::is_convertible<Callable, Ret (*)(Params...)>::value,
+          int32_t>::type = 0,
+      // Functor must be callable and return a suitable type.
+      // To make this container type safe, we need to ensure either:
+      // 1. The return type is void.
+      // 2. Or the resulting type from calling the callable is convertible to
+      // the declared return type.
+      typename std::enable_if<
+          std::is_void<Ret>::value ||
+              std::is_convertible<
+                  decltype(std::declval<Callable>()(std::declval<Params>()...)),
+                  Ret>::value,
+          int32_t>::type = 0>
+  explicit FunctionRef(Callable& callable)
+      : callback_([](const void* memory, Params... params) {
+          auto& storage = *static_cast<const Storage*>(memory);
+          auto& callable = *static_cast<Callable*>(storage.callable);
+          return static_cast<Ret>(callable(std::forward<Params>(params)...));
+        }) {
+    storage_.callable = &callable;
+  }
+
+  /**
+   * Case 2: A plain function pointer.
+   * Instead of storing an opaque pointer to underlying callable object,
+   * store a function pointer directly.
+   * Note that in the future a variant which coerces compatible function
+   * pointers could be implemented by erasing the storage type.
+   */
+  /* implicit */ FunctionRef(Ret (*ptr)(Params...))
+      : callback_([](const void* memory, Params... params) {
+          auto& storage = *static_cast<const Storage*>(memory);
+          return storage.function(std::forward<Params>(params)...);
+        }) {
+    storage_.function = ptr;
+  }
+
+  /**
+   * Case 3: Implicit conversion from lambda to FunctionRef.
+   * A common use pattern is like:
+   * void foo(FunctionRef<...>) {...}
+   * foo([](...){...})
+   * Here constructors for non const lvalue reference or function pointer
+   * would not work because they do not cover implicit conversion from rvalue
+   * lambda.
+   * We need to define a constructor for capturing temporary callables and
+   * always try to convert the lambda to a function pointer behind the scene.
+   */
+  template <
+      typename Function,
+      // This is not the copy-constructor.
+      typename std::enable_if<
+          !std::is_same<Function, FunctionRef>::value,
+          int32_t>::type = 0,
+      // Function is convertible to pointer of (Params...) -> Ret.
+      typename std::enable_if<
+          std::is_convertible<Function, Ret (*)(Params...)>::value,
+          int32_t>::type = 0>
+  /* implicit */ FunctionRef(const Function& function)
+      : FunctionRef(static_cast<Ret (*)(Params...)>(function)) {}
+
+  Ret operator()(Params... params) const {
+    return callback_(&storage_, std::forward<Params>(params)...);
+  }
+
+  explicit operator bool() const {
+    return callback_;
+  }
+};
+
+} // namespace pytree
+} // namespace executor
+} // namespace torch
diff --git a/extension/pytree/pytree.h b/extension/pytree/pytree.h
index 19dedb25814..4d6116fb4b5 100644
--- a/extension/pytree/pytree.h
+++ b/extension/pytree/pytree.h
@@ -16,7 +16,8 @@
 #include <memory>
 #include <string>
 
-#include <executorch/runtime/core/function_ref.h>
+// NB: This is a local, pytree FunctionRef and not from the ExecuTorch runtime.
+#include <executorch/extension/pytree/function_ref.h>
 
 namespace torch {
 namespace executor {
diff --git a/extension/pytree/targets.bzl b/extension/pytree/targets.bzl
index 5bb8bae1f52..02779c26106 100644
--- a/extension/pytree/targets.bzl
+++ b/extension/pytree/targets.bzl
@@ -10,13 +10,9 @@ def define_common_targets():
     runtime.cxx_library(
         name = "pytree",
         srcs = [],
-        exported_headers = ["pytree.h"],
+        exported_headers = ["pytree.h", "function_ref.h"],
         visibility = [
             "//executorch/...",
             "@EXECUTORCH_CLIENTS",
         ],
-        exported_deps = [
-            "//executorch/runtime/platform:platform",
-            "//executorch/runtime/core:core",
-        ],
     )
diff --git a/extension/pytree/test/TARGETS b/extension/pytree/test/TARGETS
index 765f297b381..d1a4eb54590 100644
--- a/extension/pytree/test/TARGETS
+++ b/extension/pytree/test/TARGETS
@@ -11,6 +11,13 @@ cpp_unittest(
     deps = ["//executorch/extension/pytree:pytree"],
 )
 
+cpp_unittest(
+    name = "function_ref_test",
+    srcs = ["function_ref_test.cpp"],
+    supports_static_listing = True,
+    deps = ["//executorch/extension/pytree:pytree"],
+)
+
 python_unittest(
     name = "test",
     srcs = [
diff --git a/extension/pytree/test/function_ref_test.cpp b/extension/pytree/test/function_ref_test.cpp
new file mode 100644
index 00000000000..f847c8ebd78
--- /dev/null
+++ b/extension/pytree/test/function_ref_test.cpp
@@ -0,0 +1,90 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#include <gtest/gtest.h>
+
+#include <executorch/extension/pytree/function_ref.h>
+
+using namespace ::testing;
+
+namespace torch {
+namespace executor {
+namespace pytree {
+
+namespace {
+class Item {
+ private:
+  int32_t val_;
+  FunctionRef<void(int32_t&)> ref_;
+
+ public:
+  /* implicit */ Item(int32_t val, FunctionRef<void(int32_t&)> ref)
+      : val_(val), ref_(ref) {}
+
+  int32_t get() {
+    ref_(val_);
+    return val_;
+  }
+};
+
+void one(int32_t& i) {
+  i = 1;
+}
+
+} // namespace
+
+TEST(FunctionRefTest, CapturingLambda) {
+  auto one = 1;
+  auto f = [&](int32_t& i) { i = one; };
+  Item item(0, FunctionRef<void(int32_t&)>{f});
+  EXPECT_EQ(item.get(), 1);
+  // ERROR:
+  // Item item1(0, f);
+  // Item item2(0, [&](int32_t& i) { i = 2; });
+  // FunctionRef<void(int32_t&)> ref([&](int32_t&){});
+}
+
+TEST(FunctionRefTest, NonCapturingLambda) {
+  int32_t val = 0;
+  FunctionRef<void(int32_t&)> ref([](int32_t& i) { i = 1; });
+  ref(val);
+  EXPECT_EQ(val, 1);
+
+  val = 0;
+  auto lambda = [](int32_t& i) { i = 1; };
+  FunctionRef<void(int32_t&)> ref1(lambda);
+  ref1(val);
+  EXPECT_EQ(val, 1);
+
+  Item item(0, [](int32_t& i) { i = 1; });
+  EXPECT_EQ(item.get(), 1);
+
+  auto f = [](int32_t& i) { i = 1; };
+  Item item1(0, f);
+  EXPECT_EQ(item1.get(), 1);
+
+  Item item2(0, std::move(f));
+  EXPECT_EQ(item2.get(), 1);
+}
+
+TEST(FunctionRefTest, FunctionPointer) {
+  int32_t val = 0;
+  FunctionRef<void(int32_t&)> ref(one);
+  ref(val);
+  EXPECT_EQ(val, 1);
+
+  Item item(0, one);
+  EXPECT_EQ(item.get(), 1);
+
+  Item item1(0, &one);
+  EXPECT_EQ(item1.get(), 1);
+}
+
+} // namespace pytree
+} // namespace executor
+} // namespace torch
diff --git a/runtime/core/function_ref.h b/runtime/core/function_ref.h
index 92171134291..a07f6151f10 100644
--- a/runtime/core/function_ref.h
+++ b/runtime/core/function_ref.h
@@ -59,9 +59,7 @@ class FunctionRef;
 
 template <typename Ret, typename... Params>
 class FunctionRef<Ret(Params...)> {
-  Ret (*callback_)(const void* memory, Params... params) = nullptr;
   union Storage {
-    void* callable;
     Ret (*function)(Params...);
   } storage_;
 
@@ -70,57 +68,18 @@ class FunctionRef<Ret(Params...)> {
   explicit FunctionRef(std::nullptr_t) {}
 
   /**
-   * Case 1: A callable object passed by lvalue reference.
-   * Taking rvalue reference is error prone because the object will be always
-   * be destroyed immediately.
-   */
-  template <
-      typename Callable,
-      // This is not the copy-constructor.
-      typename std::enable_if<
-          !std::is_same<remove_cvref_t<Callable>, FunctionRef>::value,
-          int32_t>::type = 0,
-      // Avoid lvalue reference to non-capturing lambda.
-      typename std::enable_if<
-          !std::is_convertible<Callable, Ret (*)(Params...)>::value,
-          int32_t>::type = 0,
-      // Functor must be callable and return a suitable type.
-      // To make this container type safe, we need to ensure either:
-      // 1. The return type is void.
-      // 2. Or the resulting type from calling the callable is convertible to
-      // the declared return type.
-      typename std::enable_if<
-          std::is_void<Ret>::value ||
-              std::is_convertible<
-                  decltype(std::declval<Callable>()(std::declval<Params>()...)),
-                  Ret>::value,
-          int32_t>::type = 0>
-  explicit FunctionRef(Callable& callable)
-      : callback_([](const void* memory, Params... params) {
-          auto& storage = *static_cast<const Storage*>(memory);
-          auto& callable = *static_cast<Callable*>(storage.callable);
-          return static_cast<Ret>(callable(std::forward<Params>(params)...));
-        }) {
-    storage_.callable = &callable;
-  }
-
-  /**
-   * Case 2: A plain function pointer.
+   * Case 1: A plain function pointer.
    * Instead of storing an opaque pointer to underlying callable object,
    * store a function pointer directly.
    * Note that in the future a variant which coerces compatible function
    * pointers could be implemented by erasing the storage type.
    */
-  /* implicit */ FunctionRef(Ret (*ptr)(Params...))
-      : callback_([](const void* memory, Params... params) {
-          auto& storage = *static_cast<const Storage*>(memory);
-          return storage.function(std::forward<Params>(params)...);
-        }) {
+  /* implicit */ FunctionRef(Ret (*ptr)(Params...)) {
     storage_.function = ptr;
   }
 
   /**
-   * Case 3: Implicit conversion from lambda to FunctionRef.
+   * Case 2: Implicit conversion from lambda to FunctionRef.
    * A common use pattern is like:
    * void foo(FunctionRef<...>) {...}
    * foo([](...){...})
@@ -144,11 +103,11 @@ class FunctionRef<Ret(Params...)> {
       : FunctionRef(static_cast<Ret (*)(Params...)>(function)) {}
 
   Ret operator()(Params... params) const {
-    return callback_(&storage_, std::forward<Params>(params)...);
+    return storage_.function(std::forward<Params>(params)...);
   }
 
   explicit operator bool() const {
-    return callback_;
+    return storage_.function;
   }
 };
 
diff --git a/runtime/core/test/function_ref_test.cpp b/runtime/core/test/function_ref_test.cpp
index 69442a8ba4e..1ec8faccf50 100644
--- a/runtime/core/test/function_ref_test.cpp
+++ b/runtime/core/test/function_ref_test.cpp
@@ -37,17 +37,7 @@ void one(int32_t& i) {
 
 } // namespace
 
-TEST(FunctionRefTest, CapturingLambda) {
-  auto one = 1;
-  auto f = [&](int32_t& i) { i = one; };
-  Item item(0, FunctionRef<void(int32_t&)>{f});
-  EXPECT_EQ(item.get(), 1);
-  // ERROR:
-  // Item item1(0, f);
-  // Item item2(0, [&](int32_t& i) { i = 2; });
-  // FunctionRef<void(int32_t&)> ref([&](int32_t&){});
-}
-
+// Only non-capturing lambdas can be used to initialize a function reference.
 TEST(FunctionRefTest, NonCapturingLambda) {
   int32_t val = 0;
   FunctionRef<void(int32_t&)> ref([](int32_t& i) { i = 1; });
diff --git a/runtime/kernel/operator_registry.h b/runtime/kernel/operator_registry.h
index 7f9ef5b6244..55cb4164715 100644
--- a/runtime/kernel/operator_registry.h
+++ b/runtime/kernel/operator_registry.h
@@ -42,7 +42,9 @@ namespace executor {
 
 class KernelRuntimeContext; // Forward declaration
 using RuntimeContext = KernelRuntimeContext; // TODO(T147221312): Remove
-using OpFunction = FunctionRef<void(KernelRuntimeContext&, EValue**)>;
+using OpFunction =
+    FunctionRef<void(KernelRuntimeContext&, EValue**)>; // TODO(T165139545):
+                                                        // Remove FunctionRef
 
 /**
  * Dtype and dim order metadata for a Tensor argument to an operator.