diff --git a/README.md b/README.md
index 68c6eef..35bf1c4 100644
--- a/README.md
+++ b/README.md
@@ -1490,10 +1490,9 @@ A subsequent solve uses the time limit in its own search parameters.
 Native improvement-rate stopping is available through search parameters:
 
 ```ruby
-search_parameters.improvement_limit_parameters = {
-  improvement_rate_coefficient: 0.01,
-  improvement_rate_solutions_distance: 5
-}
+limits = search_parameters.improvement_limit_parameters
+limits.improvement_rate_coefficient = 0.01
+limits.improvement_rate_solutions_distance = 5
 ```
 
 Piecewise linear costs can be applied to dimension cumul variables:
diff --git a/ext/or-tools/routing.cpp b/ext/or-tools/routing.cpp
index de960d8..73ec71d 100644
--- a/ext/or-tools/routing.cpp
+++ b/ext/or-tools/routing.cpp
@@ -354,6 +354,12 @@ void init_routing(Rice::Module& m) {
 
   m.define_singleton_function("default_routing_search_parameters", &DefaultRoutingSearchParameters);
 
+  Rice::define_class_under<RoutingSearchParameters::ImprovementSearchLimitParameters>(rb_cRoutingSearchParameters, "ImprovementSearchLimitParameters")
+    .define_method("improvement_rate_coefficient", &RoutingSearchParameters::ImprovementSearchLimitParameters::improvement_rate_coefficient)
+    .define_method("improvement_rate_coefficient=", &RoutingSearchParameters::ImprovementSearchLimitParameters::set_improvement_rate_coefficient)
+    .define_method("improvement_rate_solutions_distance", &RoutingSearchParameters::ImprovementSearchLimitParameters::improvement_rate_solutions_distance)
+    .define_method("improvement_rate_solutions_distance=", &RoutingSearchParameters::ImprovementSearchLimitParameters::set_improvement_rate_solutions_distance);
+
   rb_cRoutingSearchParameters
     .define_method("christofides_use_minimum_matching", &RoutingSearchParameters::christofides_use_minimum_matching)
     .define_method("christofides_use_minimum_matching=", &RoutingSearchParameters::set_christofides_use_minimum_matching)
@@ -475,6 +481,7 @@ void init_routing(Rice::Module& m) {
 
         return self.set_local_search_metaheuristic(v);
       })
+    .define_method("improvement_limit_parameters", &RoutingSearchParameters::mutable_improvement_limit_parameters, Rice::Return().keepAlive())
     .define_method(
       "time_limit=",
       [](RoutingSearchParameters& self, int64_t value) {
@@ -484,32 +491,6 @@ void init_routing(Rice::Module& m) {
       "lns_time_limit=",
       [](RoutingSearchParameters& self, int64_t value) {
         self.mutable_lns_time_limit()->set_seconds(value);
-      })
-    .define_method(
-      "improvement_limit_parameters",
-      [](RoutingSearchParameters& self) -> Object {
-        if (!self.has_improvement_limit_parameters()) {
-          return Object(Qnil);
-        }
-
-        const auto& parameters = self.improvement_limit_parameters();
-        Hash result;
-        result[Symbol("improvement_rate_coefficient")] =
-          parameters.improvement_rate_coefficient();
-        result[Symbol("improvement_rate_solutions_distance")] =
-          parameters.improvement_rate_solutions_distance();
-        return result;
-      })
-    .define_method(
-      "improvement_limit_parameters=",
-      [](RoutingSearchParameters& self, Hash value) {
-        const double coefficient =
-          value.get<double>(Symbol("improvement_rate_coefficient"));
-        const int distance =
-          value.get<int>(Symbol("improvement_rate_solutions_distance"));
-        auto* parameters = self.mutable_improvement_limit_parameters();
-        parameters->set_improvement_rate_coefficient(coefficient);
-        parameters->set_improvement_rate_solutions_distance(distance);
       });
 
   Rice::define_class_under<RoutingIndexManager>(m, "RoutingIndexManager")
diff --git a/test/routing_test.rb b/test/routing_test.rb
index 9a7074e..94431ad 100644
--- a/test/routing_test.rb
+++ b/test/routing_test.rb
@@ -1,4 +1,5 @@
 require_relative "test_helper"
+require "weakref"
 
 class RoutingTest < Minitest::Test
   # https://developers.google.com/optimization/routing/tsp
@@ -907,20 +908,28 @@ def test_search_parameters
     search_parameters.first_solution_strategy = :path_cheapest_arc
     search_parameters.local_search_metaheuristic = :guided_local_search
     search_parameters.log_search = true
-    assert_nil search_parameters.improvement_limit_parameters
-
-    search_parameters.improvement_limit_parameters = {
-      improvement_rate_coefficient: 0.01,
-      improvement_rate_solutions_distance: 5
-    }
-
-    assert_equal(
-      {
-        improvement_rate_coefficient: 0.01,
-        improvement_rate_solutions_distance: 5
-      },
-      search_parameters.improvement_limit_parameters
-    )
+
+    assert_equal 0, search_parameters.improvement_limit_parameters.improvement_rate_coefficient
+    search_parameters.improvement_limit_parameters.improvement_rate_coefficient = 0.5
+    assert_equal 0.5, search_parameters.improvement_limit_parameters.improvement_rate_coefficient
+
+    assert_equal 0, search_parameters.improvement_limit_parameters.improvement_rate_solutions_distance
+    search_parameters.improvement_limit_parameters.improvement_rate_solutions_distance = 1
+    assert_equal 1, search_parameters.improvement_limit_parameters.improvement_rate_solutions_distance
+  end
+
+  def test_improvement_parameters_retain_search_parameters
+    search_parameters = ORTools.default_routing_search_parameters
+    parent = WeakRef.new(search_parameters)
+    limits = search_parameters.improvement_limit_parameters
+    search_parameters = nil
+    3.times { GC.start }
+
+    assert parent.weakref_alive?
+    limits.improvement_rate_coefficient = 0.5
+    limits.improvement_rate_solutions_distance = 1
+    assert_equal 0.5, limits.improvement_rate_coefficient
+    assert_equal 1, limits.improvement_rate_solutions_distance
   end
 
   def test_set_allowed_vehicles_for_index