diff --git a/include/pybind11/cast.h b/include/pybind11/cast.h
index 283ca36f..7d485f63 100644
--- a/include/pybind11/cast.h
+++ b/include/pybind11/cast.h
@@ -998,7 +998,7 @@ struct copyable_holder_caster<
             pybind11_fail("Passing `std::shared_ptr<T> *` from Python to C++ is not supported "
                           "(inherently unsafe).");
         }
-        return std::addressof(shared_ptr_holder);
+        return std::addressof(shared_ptr_storage);
     }
 
     explicit operator std::shared_ptr<type> &() {
@@ -1011,9 +1011,9 @@ struct copyable_holder_caster<
                 throw cast_error("Unowned pointer from a void pointer capsule cannot be converted "
                                  "to a std::shared_ptr.");
             }
-            shared_ptr_holder = sh_load_helper.load_as_shared_ptr(value);
+            shared_ptr_storage = sh_load_helper.load_as_shared_ptr(value);
         }
-        return shared_ptr_holder;
+        return shared_ptr_storage;
     }
 
     static handle
@@ -1059,7 +1059,7 @@ struct copyable_holder_caster<
         }
         if (v_h.holder_constructed()) {
             value = v_h.value_ptr();
-            shared_ptr_holder = v_h.template holder<std::shared_ptr<type>>();
+            shared_ptr_storage = v_h.template holder<std::shared_ptr<type>>();
             return;
         }
         throw cast_error("Unable to cast from non-held to held instance (T& to Holder<T>) "
@@ -1088,8 +1088,8 @@ struct copyable_holder_caster<
                 if (typeinfo->holder_enum_v == detail::holder_enum_t::smart_holder) {
                     sh_load_helper.loaded_v_h = sub_caster.sh_load_helper.loaded_v_h;
                 } else {
-                    shared_ptr_holder
-                        = std::shared_ptr<type>(sub_caster.shared_ptr_holder, (type *) value);
+                    shared_ptr_storage
+                        = std::shared_ptr<type>(sub_caster.shared_ptr_storage, (type *) value);
                 }
                 return true;
             }
@@ -1113,8 +1113,8 @@ struct copyable_holder_caster<
         return false;
     }
 
-    std::shared_ptr<type> shared_ptr_holder;
     smart_holder_type_caster_support::load_helper<remove_cv_t<type>> sh_load_helper; // Const2Mutbl
+    std::shared_ptr<type> shared_ptr_storage;
     bool from_direct_conversion = false;
     bool from_as_void_ptr_capsule = false;
 };
@@ -1191,7 +1191,7 @@ struct move_only_holder_caster<
             policy = return_value_policy::reference_internal;
         }
         if (policy != return_value_policy::reference_internal) {
-            throw cast_error("Invalid return_value_policy for unique_ptr&");
+            throw cast_error("Invalid return_value_policy for const unique_ptr&");
         }
         return type_caster_base<type>::cast(src.get(), policy, parent);
     }
@@ -1218,8 +1218,14 @@ struct move_only_holder_caster<
             + clean_type_id(typeinfo->cpptype->name()) + ")");
     }
 
-    template <typename>
-    using cast_op_type = std::unique_ptr<type, deleter>;
+    template <typename T_>
+    using cast_op_type
+        = conditional_t<std::is_same<typename std::remove_volatile<T_>::type,
+                                     const std::unique_ptr<type, deleter> &>::value
+                            || std::is_same<typename std::remove_volatile<T_>::type,
+                                            const std::unique_ptr<const type, deleter> &>::value,
+                        const std::unique_ptr<type, deleter> &,
+                        std::unique_ptr<type, deleter>>;
 
     explicit operator std::unique_ptr<type, deleter>() {
         if (typeinfo->holder_enum_v == detail::holder_enum_t::smart_holder) {
@@ -1236,6 +1242,28 @@ struct move_only_holder_caster<
         pybind11_fail("Expected to be UNREACHABLE: " __FILE__ ":" PYBIND11_TOSTRING(__LINE__));
     }
 
+    explicit operator const std::unique_ptr<type, deleter> &() {
+        if (typeinfo->holder_enum_v == detail::holder_enum_t::smart_holder) {
+            // Get shared_ptr to ensure that the Python object is not disowned elsewhere.
+            shared_ptr_storage = sh_load_helper.load_as_shared_ptr(value);
+            // Build a temporary unique_ptr that is meant to never expire.
+            unique_ptr_storage = std::shared_ptr<std::unique_ptr<type, deleter>>(
+                new std::unique_ptr<type, deleter>{
+                    sh_load_helper.template load_as_const_unique_ptr<deleter>(
+                        shared_ptr_storage.get())},
+                [](std::unique_ptr<type, deleter> *ptr) {
+                    if (!ptr) {
+                        pybind11_fail("FATAL: `const std::unique_ptr<T, D> &` was disowned "
+                                      "(EXPECT UNDEFINED BEHAVIOR).");
+                    }
+                    (void) ptr->release();
+                    delete ptr;
+                });
+            return *unique_ptr_storage;
+        }
+        pybind11_fail("Expected to be UNREACHABLE: " __FILE__ ":" PYBIND11_TOSTRING(__LINE__));
+    }
+
     bool try_implicit_casts(handle src, bool convert) {
         for (auto &cast : typeinfo->implicit_casts) {
             move_only_holder_caster sub_caster(*cast.first);
@@ -1270,6 +1298,8 @@ struct move_only_holder_caster<
     }
 
     smart_holder_type_caster_support::load_helper<remove_cv_t<type>> sh_load_helper; // Const2Mutbl
+    std::shared_ptr<type> shared_ptr_storage; // Serves as a pseudo lock.
+    std::shared_ptr<std::unique_ptr<type, deleter>> unique_ptr_storage;
     bool from_direct_conversion = false;
     bool from_as_void_ptr_capsule = false;
 };
diff --git a/include/pybind11/detail/struct_smart_holder.h b/include/pybind11/detail/struct_smart_holder.h
index ccd28971..b1e24d7b 100644
--- a/include/pybind11/detail/struct_smart_holder.h
+++ b/include/pybind11/detail/struct_smart_holder.h
@@ -231,6 +231,24 @@ struct smart_holder {
         vptr_del_ptr->armed_flag = armed_flag;
     }
 
+    // Caller is responsible for precondition: ensure_compatible_rtti_uqp_del<T, D>() must succeed.
+    template <typename T, typename D>
+    std::unique_ptr<D> extract_deleter(const char *context) const {
+        const auto *gd = std::get_deleter<guarded_delete>(vptr);
+        if (gd && gd->use_del_fun) {
+            const auto &custom_deleter_ptr = gd->del_fun.template target<custom_deleter<T, D>>();
+            if (custom_deleter_ptr == nullptr) {
+                throw std::runtime_error(
+                    std::string("smart_holder::extract_deleter() precondition failure (") + context
+                    + ").");
+            }
+            static_assert(std::is_copy_constructible<D>::value,
+                          "Required for compatibility with smart_holder functionality.");
+            return std::unique_ptr<D>(new D(custom_deleter_ptr->deleter));
+        }
+        return nullptr;
+    }
+
     static smart_holder from_raw_ptr_unowned(void *raw_ptr) {
         smart_holder hld;
         hld.vptr.reset(raw_ptr, [](void *) {});
diff --git a/include/pybind11/detail/type_caster_base.h b/include/pybind11/detail/type_caster_base.h
index af92dc11..db6ad8f6 100644
--- a/include/pybind11/detail/type_caster_base.h
+++ b/include/pybind11/detail/type_caster_base.h
@@ -797,22 +797,7 @@ struct load_helper : value_and_holder_helper {
                               "instance cannot safely be transferred to C++.");
         }
 
-        // Temporary variable to store the extracted deleter in.
-        std::unique_ptr<D> extracted_deleter;
-
-        auto *gd = std::get_deleter<pybindit::memory::guarded_delete>(holder().vptr);
-        if (gd && gd->use_del_fun) { // Note the ensure_compatible_rtti_uqp_del<T, D>() call above.
-            // In struct_smart_holder, a custom  deleter is always stored in a guarded delete.
-            // The guarded delete's std::function<void(void*)> actually points at the
-            // custom_deleter type, so we can verify it is of the custom deleter type and
-            // finally extract its deleter.
-            using custom_deleter_D = pybindit::memory::custom_deleter<T, D>;
-            const auto &custom_deleter_ptr = gd->del_fun.template target<custom_deleter_D>();
-            assert(custom_deleter_ptr != nullptr);
-            // Now that we have confirmed the type of the deleter matches the desired return
-            // value we can extract the function.
-            extracted_deleter = std::unique_ptr<D>(new D(std::move(custom_deleter_ptr->deleter)));
-        }
+        std::unique_ptr<D> extracted_deleter = holder().template extract_deleter<T, D>(context);
 
         // Critical transfer-of-ownership section. This must stay together.
         if (self_life_support != nullptr) {
@@ -832,6 +817,19 @@ struct load_helper : value_and_holder_helper {
 
         return result;
     }
+
+    // This assumes load_as_shared_ptr succeeded(), and the returned shared_ptr is still alive.
+    // The returned unique_ptr is meant to never expire (the behavior is undefined otherwise).
+    template <typename D>
+    std::unique_ptr<T, D>
+    load_as_const_unique_ptr(T *raw_type_ptr, const char *context = "load_as_const_unique_ptr") {
+        if (!have_holder()) {
+            return unique_with_deleter<T, D>(nullptr, std::unique_ptr<D>());
+        }
+        holder().template ensure_compatible_rtti_uqp_del<T, D>(context);
+        return unique_with_deleter<T, D>(
+            raw_type_ptr, std::move(holder().template extract_deleter<T, D>(context)));
+    }
 };
 
 PYBIND11_NAMESPACE_END(smart_holder_type_caster_support)
diff --git a/tests/test_class_sh_basic.cpp b/tests/test_class_sh_basic.cpp
index 9602387b..ac19a750 100644
--- a/tests/test_class_sh_basic.cpp
+++ b/tests/test_class_sh_basic.cpp
@@ -120,6 +120,17 @@ std::string get_mtxt(atyp const &obj) { return obj.mtxt; }
 std::ptrdiff_t get_ptr(atyp const &obj) { return reinterpret_cast<std::ptrdiff_t>(&obj); }
 
 std::unique_ptr<atyp> unique_ptr_roundtrip(std::unique_ptr<atyp> obj) { return obj; }
+
+std::string pass_unique_ptr_cref(const std::unique_ptr<atyp> &obj) { return obj->mtxt; }
+
+const std::unique_ptr<atyp> &rtrn_unique_ptr_cref(const std::string &mtxt) {
+    static std::unique_ptr<atyp> obj{new atyp{"static_ctor_arg"}};
+    if (!mtxt.empty()) {
+        obj->mtxt = mtxt;
+    }
+    return obj;
+}
+
 const std::unique_ptr<atyp> &unique_ptr_cref_roundtrip(const std::unique_ptr<atyp> &obj) {
     return obj;
 }
@@ -217,6 +228,9 @@ TEST_SUBMODULE(class_sh_basic, m) {
     m.def("get_ptr", get_ptr);   // pass_cref
 
     m.def("unique_ptr_roundtrip", unique_ptr_roundtrip); // pass_uqmp, rtrn_uqmp
+
+    m.def("pass_unique_ptr_cref", pass_unique_ptr_cref);
+    m.def("rtrn_unique_ptr_cref", rtrn_unique_ptr_cref);
     m.def("unique_ptr_cref_roundtrip", unique_ptr_cref_roundtrip);
 
     py::classh<SharedPtrStash>(m, "SharedPtrStash")
diff --git a/tests/test_class_sh_basic.py b/tests/test_class_sh_basic.py
index 87f1f8f0..7db7d31b 100644
--- a/tests/test_class_sh_basic.py
+++ b/tests/test_class_sh_basic.py
@@ -151,19 +151,31 @@ def test_unique_ptr_roundtrip(num_round_trips=1000):
         id_orig = id_rtrn
 
 
-# This currently fails, because a unique_ptr is always loaded by value
-# due to pybind11/detail/smart_holder_type_casters.h:689
-# I think, we need to provide more cast operators.
-@pytest.mark.skip()
-def test_unique_ptr_cref_roundtrip():
+def test_pass_unique_ptr_cref():
+    obj = m.atyp("ctor_arg")
+    assert re.match("ctor_arg(_MvCtor)*_MvCtor", m.get_mtxt(obj))
+    assert re.match("ctor_arg(_MvCtor)*_MvCtor", m.pass_unique_ptr_cref(obj))
+    assert re.match("ctor_arg(_MvCtor)*_MvCtor", m.get_mtxt(obj))
+
+
+def test_rtrn_unique_ptr_cref():
+    obj0 = m.rtrn_unique_ptr_cref("")
+    assert m.get_mtxt(obj0) == "static_ctor_arg"
+    obj1 = m.rtrn_unique_ptr_cref("passed_mtxt_1")
+    assert m.get_mtxt(obj1) == "passed_mtxt_1"
+    assert m.get_mtxt(obj0) == "passed_mtxt_1"
+    assert obj0 is obj1
+
+
+def test_unique_ptr_cref_roundtrip(num_round_trips=1000):
+    # Multiple roundtrips to stress-test implementation.
     orig = m.atyp("passenger")
-    id_orig = id(orig)
     mtxt_orig = m.get_mtxt(orig)
-
-    recycled = m.unique_ptr_cref_roundtrip(orig)
-    assert m.get_mtxt(orig) == mtxt_orig
-    assert m.get_mtxt(recycled) == mtxt_orig
-    assert id(recycled) == id_orig
+    recycled = orig
+    for _ in range(num_round_trips):
+        recycled = m.unique_ptr_cref_roundtrip(recycled)
+        assert recycled is orig
+        assert m.get_mtxt(recycled) == mtxt_orig
 
 
 @pytest.mark.parametrize(
diff --git a/tests/test_class_sh_trampoline_shared_from_this.cpp b/tests/test_class_sh_trampoline_shared_from_this.cpp
index e5d688af..9c2e4ec7 100644
--- a/tests/test_class_sh_trampoline_shared_from_this.cpp
+++ b/tests/test_class_sh_trampoline_shared_from_this.cpp
@@ -87,7 +87,12 @@ long pass_shared_ptr(const std::shared_ptr<Sft> &obj) {
     return sft.use_count();
 }
 
-void pass_unique_ptr(const std::unique_ptr<Sft> &) {}
+std::string pass_unique_ptr_cref(const std::unique_ptr<Sft> &obj) {
+    return obj ? obj->history : "<NULLPTR>";
+}
+void pass_unique_ptr_rref(std::unique_ptr<Sft> &&) {
+    throw std::runtime_error("Expected to not be reached.");
+}
 
 Sft *make_pure_cpp_sft_raw_ptr(const std::string &history_seed) { return new Sft{history_seed}; }
 
@@ -135,7 +140,8 @@ TEST_SUBMODULE(class_sh_trampoline_shared_from_this, m) {
 
     m.def("use_count", use_count);
     m.def("pass_shared_ptr", pass_shared_ptr);
-    m.def("pass_unique_ptr", pass_unique_ptr);
+    m.def("pass_unique_ptr_cref", pass_unique_ptr_cref);
+    m.def("pass_unique_ptr_rref", pass_unique_ptr_rref);
     m.def("make_pure_cpp_sft_raw_ptr", make_pure_cpp_sft_raw_ptr);
     m.def("make_pure_cpp_sft_unq_ptr", make_pure_cpp_sft_unq_ptr);
     m.def("make_pure_cpp_sft_shd_ptr", make_pure_cpp_sft_shd_ptr);
diff --git a/tests/test_class_sh_trampoline_shared_from_this.py b/tests/test_class_sh_trampoline_shared_from_this.py
index a074b20a..7c5ee0e8 100644
--- a/tests/test_class_sh_trampoline_shared_from_this.py
+++ b/tests/test_class_sh_trampoline_shared_from_this.py
@@ -137,11 +137,14 @@ def test_pass_released_shared_ptr_as_unique_ptr():
     obj = PySft("PySft")
     stash1 = m.SftSharedPtrStash(1)
     stash1.Add(obj)  # Releases shared_ptr to C++.
+    assert m.pass_unique_ptr_cref(obj) == "PySft_Stash1Add"
+    assert obj.history == "PySft_Stash1Add"
     with pytest.raises(ValueError) as exc_info:
-        m.pass_unique_ptr(obj)
+        m.pass_unique_ptr_rref(obj)
     assert str(exc_info.value) == (
         "Python instance is currently owned by a std::shared_ptr."
     )
+    assert obj.history == "PySft_Stash1Add"
 
 
 @pytest.mark.parametrize(