diff --git a/ortools/sat/python/CMakeLists.txt b/ortools/sat/python/CMakeLists.txt
index c42f3e58446..e0d889a4785 100644
--- a/ortools/sat/python/CMakeLists.txt
+++ b/ortools/sat/python/CMakeLists.txt
@@ -59,6 +59,7 @@ add_custom_command(
      OUTPUT proto_builder_pybind11.h
      COMMAND gen_proto_builder_pybind11 > proto_builder_pybind11.h
      COMMENT "Generate C++ proto_builder_pybind11.h"
+     DEPENDS ${CMAKE_RUNTIME_OUTPUT_DIRECTORY}/gen_proto_builder_pybind11
      VERBATIM)
 
 pybind11_add_module(cp_model_helper_pybind11 MODULE cp_model_helper.cc proto_builder_pybind11.h)
diff --git a/ortools/sat/python/cp_model_helper.cc b/ortools/sat/python/cp_model_helper.cc
index 822d6f7392d..b442ebb9a27 100644
--- a/ortools/sat/python/cp_model_helper.cc
+++ b/ortools/sat/python/cp_model_helper.cc
@@ -1754,8 +1754,8 @@ PYBIND11_MODULE(cp_model_helper, m) {
       .def(py::init<std::shared_ptr<CpModelProto>, int>())
       .def(py::init<std::shared_ptr<CpModelProto>>())  // new variable.
       .def_property_readonly(
-          "proto", &IntVar::proto, py::return_value_policy::reference,
-          py::keep_alive<1, 0>(),
+          "proto", py::cpp_function(&IntVar::proto, py::return_value_policy::reference,
+          py::keep_alive<1, 0>()),
           "Returns the IntegerVariableProto of this variable.")
       .def_property_readonly("model_proto", &IntVar::model_proto,
                              "Returns the CP model protobuf")
@@ -2000,9 +2000,9 @@ PYBIND11_MODULE(cp_model_helper, m) {
           "Returns the index of the constraint in the model protobuf.")
       .def_property_readonly("model_proto", &Constraint::model_proto,
                              "Returns the model protobuf.")
-      .def_property_readonly("proto", &Constraint::proto,
+      .def_property_readonly("proto", py::cpp_function(&Constraint::proto,
                              py::return_value_policy::reference,
-                             py::keep_alive<1, 0>(),
+                             py::keep_alive<1, 0>()),
                              "Returns the ConstraintProto of this constraint.")
       .def_property("name", &Constraint::name, &Constraint::SetName,
                     "The name of the constraint.")
@@ -2118,8 +2118,8 @@ intervals into the schedule.
       .def_property_readonly("model_proto", &IntervalVar::model_proto,
                              "Returns the model protobuf.")
       .def_property_readonly(
-          "proto", &IntervalVar::proto, py::return_value_policy::reference,
-          py::keep_alive<1, 0>(), "Returns the interval constraint protobuf.")
+          "proto", py::cpp_function(&IntervalVar::proto, py::return_value_policy::reference,
+          py::keep_alive<1, 0>()), "Returns the interval constraint protobuf.")
       .def_property("name", &IntervalVar::name, &IntervalVar::SetName,
                     "The name of the interval variable.")
       .def(
diff --git a/ortools/sat/python/wrappers.cc b/ortools/sat/python/wrappers.cc
index 4e2acfbf417..351dc289187 100644
--- a/ortools/sat/python/wrappers.cc
+++ b/ortools/sat/python/wrappers.cc
@@ -344,7 +344,7 @@ class Generator {
     .def_property_readonly(
         "$0",
         []($1 self) { return self->mutable_$2(); },
-        py::return_value_policy::reference, py::keep_alive<0, 1>()))",
+        py::return_value_policy::reference_internal))",
           field.name(), current_context_.self_mutable_name, field.name());
       // We'll need to generate the wrapping for
       // `google::protobuf::RepeatedPtrField<$3>`.
@@ -357,7 +357,7 @@ class Generator {
     .def_property_readonly(
         "$0",
         []($1 self) { return self->mutable_$0(); },
-        py::return_value_policy::reference, py::keep_alive<0, 1>()))",
+        py::return_value_policy::reference_internal))",
                                 field.name(),
                                 current_context_.self_mutable_name);
       // We'll need to generate the wrapping for