diff --git a/examples/CMakeLists.txt b/examples/CMakeLists.txt
index 9a94c95e8..7ccb5f05d 100644
--- a/examples/CMakeLists.txt
+++ b/examples/CMakeLists.txt
@@ -35,6 +35,7 @@ CompileExample("t12_default_ports")
 CompileExample("t13_access_by_ref")
 CompileExample("t14_subtree_model")
 CompileExample("t15_nodes_mocking")
+CompileExample("t15_nodes_mocking_strict_failure")
 CompileExample("t16_global_blackboard")
 CompileExample("t17_blackboard_backup")
 CompileExample("t18_waypoints")
diff --git a/examples/t15_nodes_mocking.cpp b/examples/t15_nodes_mocking.cpp
index 805a46b95..6e6007066 100644
--- a/examples/t15_nodes_mocking.cpp
+++ b/examples/t15_nodes_mocking.cpp
@@ -39,7 +39,7 @@ const char* xml_text = R"(
 
 /**
  * @brief In this example we will see how we can substitute some nodes
- * in the Tree above with
+ * in the Tree above with mocks.
  * @param argc
  * @param argv
  * @return
diff --git a/examples/t15_nodes_mocking_strict_failure.cpp b/examples/t15_nodes_mocking_strict_failure.cpp
new file mode 100644
index 000000000..c3374ba5e
--- /dev/null
+++ b/examples/t15_nodes_mocking_strict_failure.cpp
@@ -0,0 +1,143 @@
+#include "dummy_nodes.h"
+
+#include "behaviortree_cpp/bt_factory.h"
+
+// clang-format off
+namespace
+{
+const char* xml_text = R"(
+<root BTCPP_format="4">
+
+  <BehaviorTree ID="MainTree">
+    <Sequence>
+      <SaySomething name="talk" message="hello world"/>
+
+      <SubTree ID="MySub" name="mysub"/>
+
+      <Script name="set_mock_flag" code="mock_should_fail:= false"/>
+      <Script name="set_message" code="msg:= 'the original message' "/>
+      <SaySomething message="{msg}"/>
+
+      <Sequence name="counting">
+        <SaySomething message="1"/>
+        <SaySomething message="2"/>
+        <SaySomething message="3"/>
+      </Sequence>
+    </Sequence>
+  </BehaviorTree>
+
+  <BehaviorTree ID="MySub">
+    <Sequence>
+      <AlwaysSuccess name="action_subA"/>
+      <AlwaysSuccess name="action_subB"/>
+    </Sequence>
+  </BehaviorTree>
+
+</root>
+ )";
+}  // namespace
+// clang-format on
+
+/**
+ * @brief Companion to tutorial 15 that preserves strict failure propagation.
+ *
+ * If the substituted TestNode resolves to FAILURE, the enclosing Sequence stops
+ * immediately. The final blackboard message is still printed by this executable,
+ * but the tree itself does not log the failure branch through a fallback.
+ */
+
+int main(int /*argc*/, char** /*argv*/)
+{
+  using namespace DummyNodes;
+  BT::BehaviorTreeFactory factory;
+  factory.registerNodeType<SaySomething>("SaySomething");
+  factory.registerBehaviorTreeFromText(xml_text);
+
+  {
+    auto tree = factory.createTree("MainTree");
+
+    std::cout << "----- Nodes fullPath() -------\n";
+    tree.applyVisitor(
+        [](BT::TreeNode* node) { std::cout << node->fullPath() << std::endl; });
+
+    std::cout << "\n------ Output (original) ------\n";
+    auto status = tree.tickWhileRunning();
+    std::cout << "Original status: " << BT::toStr(status, false) << std::endl;
+  }
+
+  factory.registerSimpleAction("DummyAction", [](BT::TreeNode& self) {
+    std::cout << "DummyAction substituting node with fullPath(): " << self.fullPath()
+              << std::endl;
+    return BT::NodeStatus::SUCCESS;
+  });
+
+  factory.registerSimpleAction("DummySaySomething", [](BT::TreeNode& self) {
+    auto msg = self.getInput<std::string>("message");
+    std::cout << "DummySaySomething: " << msg.value() << std::endl;
+    return BT::NodeStatus::SUCCESS;
+  });
+
+  BT::TestNodeConfig test_config;
+  test_config.return_status_script = "(mock_should_fail == true) ? FAILURE : SUCCESS";
+  test_config.async_delay = std::chrono::milliseconds(2000);
+  test_config.success_script = "msg := 'message SUBSTITUTED' ";
+  test_config.failure_script = "msg := 'message FAILURE branch' ";
+
+  BT::TestNodeConfig counting_config;
+  counting_config.return_status = BT::NodeStatus::SUCCESS;
+
+  factory.addSubstitutionRule("mysub/action_*", "DummyAction");
+  factory.addSubstitutionRule("talk", "DummySaySomething");
+  factory.addSubstitutionRule("set_message", test_config);
+  factory.addSubstitutionRule("counting", counting_config);
+
+  auto blackboard = BT::Blackboard::create();
+  auto tree = factory.createTree("MainTree", blackboard);
+  std::cout << "\n------ Output (substituted, strict failure) ------\n";
+  auto status = tree.tickWhileRunning();
+  std::cout << "Substituted status: " << BT::toStr(status, false) << std::endl;
+  if(blackboard->getEntry("msg"))
+  {
+    std::cout << "Substituted final msg: " << blackboard->get<std::string>("msg")
+              << std::endl;
+  }
+
+  return 0;
+}
+
+/* Expected output:
+
+----- Nodes fullPath() -------
+Sequence::1
+talk
+mysub
+mysub/Sequence::4
+mysub/action_subA
+mysub/action_subB
+set_mock_flag
+set_message
+SaySomething::9
+counting
+SaySomething::11
+SaySomething::12
+SaySomething::13
+
+------ Output (original) ------
+Robot says: hello world
+Robot says: the original message
+Robot says: 1
+Robot says: 2
+Robot says: 3
+Original status: SUCCESS
+
+------ Output (substituted, strict failure) ------
+DummySaySomething: hello world
+DummyAction substituting node with fullPath(): mysub/action_subA
+DummyAction substituting node with fullPath(): mysub/action_subB
+Substituted status: SUCCESS
+Substituted final msg: message SUBSTITUTED
+
+If you change mock_should_fail to true in the XML above, the substituted status
+becomes FAILURE and the final msg becomes "message FAILURE branch".
+
+*/
diff --git a/include/behaviortree_cpp/actions/test_node.h b/include/behaviortree_cpp/actions/test_node.h
index 17b91a23f..c675d2e4c 100644
--- a/include/behaviortree_cpp/actions/test_node.h
+++ b/include/behaviortree_cpp/actions/test_node.h
@@ -14,15 +14,19 @@
 #pragma once
 
 #include "behaviortree_cpp/action_node.h"
-#include "behaviortree_cpp/scripting/script_parser.hpp"
-#include "behaviortree_cpp/utils/timer_queue.h"
+
+#include <chrono>
+#include <functional>
+#include <memory>
+#include <string>
 
 namespace BT
 {
 
 struct TestNodeConfig
 {
-  /// status to return when the action is completed.
+  /// Status to return when the action is completed.
+  /// If return_status_script is also set, the script takes precedence.
   NodeStatus return_status = NodeStatus::SUCCESS;
 
   /// script to execute when complete_func() returns SUCCESS
@@ -38,14 +42,27 @@ struct TestNodeConfig
   std::chrono::milliseconds async_delay = std::chrono::milliseconds(0);
 
   /// Function invoked when the action is completed.
-  /// If not specified, the node will return [return_status]
+  /// If not specified, the node will use return_status_script when present,
+  /// otherwise it will return [return_status].
   std::function<NodeStatus(void)> complete_func;
+
+  /// Optional script to compute the completion status dynamically.
+  ///
+  /// This script is evaluated when the TestNode completes, after any
+  /// async_delay has elapsed, using the current blackboard state.
+  /// The result must resolve to the same set of statuses supported by
+  /// return_status, except IDLE which is always rejected.
+  /// When set, this takes precedence over return_status.
+  ///
+  /// NOTE: kept last to preserve the byte offsets of the fields above.
+  std::string return_status_script;
 };
 
 /**
  * @brief The TestNode is a Node that can be configure to:
  *
  * 1. Return a specific status (SUCCESS / FAILURE)
+ * 1.b Compute the returned status from a script evaluated at completion time
  * 2. Execute a post condition script (unless halted)
  * 3. Either complete immediately (synchronous action), or after a
  *    given period of time (asynchronous action)
@@ -73,26 +90,32 @@ class TestNode : public BT::StatefulActionNode
   TestNode(const std::string& name, const NodeConfig& config,
            std::shared_ptr<TestNodeConfig> test_config);
 
+  ~TestNode() override;
+
+  TestNode(const TestNode& other) = delete;
+  TestNode& operator=(const TestNode& other) = delete;
+
+  TestNode(TestNode&& other) = delete;
+  TestNode& operator=(TestNode&& other) = delete;
+
   static PortsList providedPorts()
   {
     return {};
   }
 
 protected:
-  virtual NodeStatus onStart() override;
+  NodeStatus onStart() override;
 
-  virtual NodeStatus onRunning() override;
+  NodeStatus onRunning() override;
 
-  virtual void onHalted() override;
+  void onHalted() override;
 
   NodeStatus onCompleted();
 
-  std::shared_ptr<TestNodeConfig> _config;
-  ScriptFunction _success_executor;
-  ScriptFunction _failure_executor;
-  ScriptFunction _post_executor;
-  TimerQueue<> _timer;
-  std::atomic_bool _completed = false;
+  // All state lives behind this pointer, so that adding or changing it does not
+  // alter the layout that consumers compile against.
+  struct PImpl;
+  std::unique_ptr<PImpl> _p;
 };
 
 }  // namespace BT
diff --git a/src/actions/test_node.cpp b/src/actions/test_node.cpp
index f2471c085..6990ffe14 100644
--- a/src/actions/test_node.cpp
+++ b/src/actions/test_node.cpp
@@ -1,8 +1,93 @@
 #include "behaviortree_cpp/actions/test_node.h"
 
+#include "behaviortree_cpp/scripting/script_parser.hpp"
+#include "behaviortree_cpp/utils/timer_queue.h"
+
+#include <atomic>
+
 namespace BT
 {
 
+namespace
+{
+
+bool isAllowedTestNodeStatus(NodeStatus status)
+{
+  switch(status)
+  {
+    case NodeStatus::RUNNING:
+    case NodeStatus::SUCCESS:
+    case NodeStatus::FAILURE:
+    case NodeStatus::SKIPPED:
+      return true;
+
+    case NodeStatus::IDLE:
+      return false;
+  }
+  return false;
+}
+
+void validateTestNodeStatus(NodeStatus status, StringView source)
+{
+  if(!isAllowedTestNodeStatus(status))
+  {
+    throw RuntimeError("TestNode ", std::string(source),
+                       " resolved to invalid status (integer value: ",
+                       std::to_string(static_cast<int>(status)), ")");
+  }
+}
+
+EnumsTablePtr createTestNodeEnums(const EnumsTablePtr& base_enums)
+{
+  auto enums = base_enums ? std::make_shared<EnumsTable>(*base_enums) :
+                            std::make_shared<EnumsTable>();
+
+  (*enums)["IDLE"] = static_cast<int>(NodeStatus::IDLE);
+  (*enums)["RUNNING"] = static_cast<int>(NodeStatus::RUNNING);
+  (*enums)["SUCCESS"] = static_cast<int>(NodeStatus::SUCCESS);
+  (*enums)["FAILURE"] = static_cast<int>(NodeStatus::FAILURE);
+  (*enums)["SKIPPED"] = static_cast<int>(NodeStatus::SKIPPED);
+  return enums;
+}
+
+NodeStatus convertScriptResultToStatus(const Any& result)
+{
+  if(result.empty())
+  {
+    throw RuntimeError("return_status_script returned an empty value");
+  }
+
+  if(result.isString())
+  {
+    auto status = convertFromString<NodeStatus>(result.cast<std::string>());
+    validateTestNodeStatus(status, "return_status_script");
+    return status;
+  }
+
+  if(auto status = result.tryCast<int>())
+  {
+    auto resolved = static_cast<NodeStatus>(status.value());
+    validateTestNodeStatus(resolved, "return_status_script");
+    return resolved;
+  }
+
+  throw RuntimeError("return_status_script must evaluate to a NodeStatus-compatible "
+                     "value");
+}
+
+}  // namespace
+
+struct TestNode::PImpl
+{
+  std::shared_ptr<TestNodeConfig> config;
+  ScriptFunction return_status_executor;
+  ScriptFunction success_executor;
+  ScriptFunction failure_executor;
+  ScriptFunction post_executor;
+  TimerQueue<> timer;
+  std::atomic_bool completed = false;
+};
+
 TestNode::TestNode(const std::string& name, const NodeConfig& config,
                    TestNodeConfig test_config)
   : TestNode(name, config, std::make_shared<TestNodeConfig>(std::move(test_config)))
@@ -10,13 +95,14 @@ TestNode::TestNode(const std::string& name, const NodeConfig& config,
 
 TestNode::TestNode(const std::string& name, const NodeConfig& config,
                    std::shared_ptr<TestNodeConfig> test_config)
-  : StatefulActionNode(name, config), _config(std::move(test_config))
+  : StatefulActionNode(name, config), _p(std::make_unique<PImpl>())
 {
+  _p->config = std::move(test_config);
   setRegistrationID("TestNode");
 
-  if(_config->return_status == NodeStatus::IDLE)
+  if(_p->config->return_status_script.empty())
   {
-    throw RuntimeError("TestNode can not return IDLE");
+    validateTestNodeStatus(_p->config->return_status, "return_status");
   }
 
   auto prepareScript = [](const std::string& script, auto& executor) {
@@ -30,29 +116,32 @@ TestNode::TestNode(const std::string& name, const NodeConfig& config,
       executor = result.value();
     }
   };
-  prepareScript(_config->success_script, _success_executor);
-  prepareScript(_config->failure_script, _failure_executor);
-  prepareScript(_config->post_script, _post_executor);
+  prepareScript(_p->config->return_status_script, _p->return_status_executor);
+  prepareScript(_p->config->success_script, _p->success_executor);
+  prepareScript(_p->config->failure_script, _p->failure_executor);
+  prepareScript(_p->config->post_script, _p->post_executor);
 }
 
+TestNode::~TestNode() = default;
+
 NodeStatus TestNode::onStart()
 {
-  if(_config->async_delay <= std::chrono::milliseconds(0))
+  if(_p->config->async_delay <= std::chrono::milliseconds(0))
   {
     return onCompleted();
   }
   // convert this in an asynchronous operation. Use another thread to count
   // a certain amount of time.
-  _completed = false;
-  _timer.add(std::chrono::milliseconds(_config->async_delay), [this](bool aborted) {
+  _p->completed = false;
+  _p->timer.add(std::chrono::milliseconds(_p->config->async_delay), [this](bool aborted) {
     if(!aborted)
     {
-      _completed.store(true);
+      _p->completed.store(true);
       this->emitWakeUpSignal();
     }
     else
     {
-      _completed.store(false);
+      _p->completed.store(false);
     }
   });
   return NodeStatus::RUNNING;
@@ -60,7 +149,7 @@ NodeStatus TestNode::onStart()
 
 NodeStatus TestNode::onRunning()
 {
-  if(_completed)
+  if(_p->completed)
   {
     return onCompleted();
   }
@@ -69,27 +158,48 @@ NodeStatus TestNode::onRunning()
 
 void TestNode::onHalted()
 {
-  _timer.cancelAll();
+  _p->timer.cancelAll();
 }
 
 NodeStatus TestNode::onCompleted()
 {
-  Ast::Environment env = { config().blackboard, config().enums };
+  NodeStatus status = NodeStatus::IDLE;
 
-  auto status =
-      (_config->complete_func) ? _config->complete_func() : _config->return_status;
+  if(_p->config->complete_func)
+  {
+    status = _p->config->complete_func();
+  }
+  else if(_p->return_status_executor)
+  {
+    // return_status_script may reference NodeStatus names (e.g. SUCCESS, FAILURE),
+    // so it is evaluated with an enum table augmented with those values. The
+    // post-execution scripts below deliberately do NOT see these names.
+    auto status_enums = createTestNodeEnums(config().enums);
+    Ast::Environment status_env = { config().blackboard, status_enums };
+    status = convertScriptResultToStatus(_p->return_status_executor(status_env));
+  }
+  else
+  {
+    status = _p->config->return_status;
+  }
+
+  validateTestNodeStatus(status, "completion");
+
+  // success/failure/post scripts use only the node's own enums, so that a
+  // blackboard entry named like a status (e.g. "SUCCESS") is not shadowed.
+  Ast::Environment env = { config().blackboard, config().enums };
 
-  if(status == NodeStatus::SUCCESS && _success_executor)
+  if(status == NodeStatus::SUCCESS && _p->success_executor)
   {
-    _success_executor(env);
+    _p->success_executor(env);
   }
-  else if(status == NodeStatus::FAILURE && _failure_executor)
+  else if(status == NodeStatus::FAILURE && _p->failure_executor)
   {
-    _failure_executor(env);
+    _p->failure_executor(env);
   }
-  if(_post_executor)
+  if(_p->post_executor)
   {
-    _post_executor(env);
+    _p->post_executor(env);
   }
   return status;
 }
diff --git a/src/bt_factory.cpp b/src/bt_factory.cpp
index acd907a31..a3ef80def 100644
--- a/src/bt_factory.cpp
+++ b/src/bt_factory.cpp
@@ -531,9 +531,19 @@ void BehaviorTreeFactory::loadSubstitutionRuleFromJSON(const std::string& json_t
     for(auto const& [name, test_config] : test_configs.items())
     {
       auto& config = configs[name];
+      bool has_return_status = false;
 
-      auto return_status = test_config.at("return_status").get<std::string>();
-      config.return_status = convertFromString<NodeStatus>(return_status);
+      if(test_config.contains("return_status"))
+      {
+        auto return_status = test_config.at("return_status").get<std::string>();
+        config.return_status = convertFromString<NodeStatus>(return_status);
+        has_return_status = true;
+      }
+      if(test_config.contains("return_status_script"))
+      {
+        config.return_status_script =
+            test_config["return_status_script"].get<std::string>();
+      }
       if(test_config.contains("async_delay"))
       {
         config.async_delay =
@@ -551,6 +561,12 @@ void BehaviorTreeFactory::loadSubstitutionRuleFromJSON(const std::string& json_t
       {
         config.failure_script = test_config["failure_script"].get<std::string>();
       }
+
+      if(!has_return_status && config.return_status_script.empty())
+      {
+        throw RuntimeError("TestNodeConfig [", name,
+                           "] must contain return_status or return_status_script");
+      }
     }
   }
 
diff --git a/tests/gtest_substitution.cpp b/tests/gtest_substitution.cpp
index fda7afdfc..c64f6b1b6 100644
--- a/tests/gtest_substitution.cpp
+++ b/tests/gtest_substitution.cpp
@@ -19,13 +19,18 @@ const char* json_text = R"(
     },
     "TestB": {
       "return_status": "FAILURE"
+    },
+    "TestScript": {
+      "return_status_script": "(mock_should_fail == true) ? FAILURE : SUCCESS",
+      "failure_script": "branch := 'failure'"
     }
   },
 
   "SubstitutionRules": {
     "actionA": "TestA",
     "actionB": "TestB",
-    "actionC": "NotAConfig"
+    "actionC": "NotAConfig",
+    "actionD": "TestScript"
   }
 }
  )";
@@ -48,10 +53,11 @@ TEST(Substitution, Parser)
 
   const auto& rules = factory.substitutionRules();
 
-  ASSERT_EQ(rules.size(), 3);
+  ASSERT_EQ(rules.size(), 4);
   ASSERT_EQ(rules.count("actionA"), 1);
   ASSERT_EQ(rules.count("actionB"), 1);
   ASSERT_EQ(rules.count("actionC"), 1);
+  ASSERT_EQ(rules.count("actionD"), 1);
 
   auto configA = std::get_if<TestNodeConfig>(&rules.at("actionA"));
   ASSERT_EQ(configA->return_status, NodeStatus::SUCCESS);
@@ -63,6 +69,12 @@ TEST(Substitution, Parser)
   ASSERT_EQ(configB->async_delay, std::chrono::milliseconds(0));
   ASSERT_TRUE(configB->post_script.empty());
 
+  auto configScript = std::get_if<TestNodeConfig>(&rules.at("actionD"));
+  ASSERT_FALSE(configScript->return_status_script.empty());
+  ASSERT_EQ(configScript->return_status_script, "(mock_should_fail == true) ? FAILURE : "
+                                                "SUCCESS");
+  ASSERT_EQ(configScript->failure_script, "branch := 'failure'");
+
   ASSERT_EQ(*std::get_if<std::string>(&rules.at("actionC")), "NotAConfig");
 }
 
@@ -102,6 +114,145 @@ TEST(Substitution, TestNodePostScriptReadsStatusNamedBlackboardKey)
   ASSERT_EQ(blackboard->get<int>("observed"), 99);
 }
 
+TEST(Substitution, ParserRejectsMissingReturnStatusAndScript)
+{
+  static const char* invalid_json = R"(
+  {
+    "TestNodeConfigs": {
+      "BrokenConfig": {
+        "post_script": "branch := 'unused'"
+      }
+    },
+    "SubstitutionRules": {
+      "actionA": "BrokenConfig"
+    }
+  }
+  )";
+
+  BehaviorTreeFactory factory;
+  ASSERT_THROW(factory.loadSubstitutionRuleFromJSON(invalid_json), RuntimeError);
+}
+
+TEST(Substitution, ScriptedReturnStatusFromJson)
+{
+  static const char* xml_text = R"(
+  <root BTCPP_format="4">
+    <BehaviorTree ID="MainTree">
+      <Sequence>
+        <Script code="mock_should_fail := true"/>
+        <AlwaysSuccess name="action_A"/>
+      </Sequence>
+    </BehaviorTree>
+  </root>
+  )";
+
+  static const char* json_rules = R"(
+  {
+    "TestNodeConfigs": {
+      "ConditionalMock": {
+        "return_status_script": "(mock_should_fail == true) ? FAILURE : SUCCESS",
+        "success_script": "branch := 'success'",
+        "failure_script": "branch := 'failure'"
+      }
+    },
+    "SubstitutionRules": {
+      "action_A": "ConditionalMock"
+    }
+  }
+  )";
+
+  BehaviorTreeFactory factory;
+  factory.loadSubstitutionRuleFromJSON(json_rules);
+  factory.registerBehaviorTreeFromText(xml_text);
+
+  auto blackboard = Blackboard::create();
+  auto tree = factory.createTree("MainTree", blackboard);
+
+  ASSERT_EQ(tree.tickWhileRunning(), NodeStatus::FAILURE);
+  ASSERT_EQ(blackboard->get<std::string>("branch"), "failure");
+}
+
+TEST(Substitution, ScriptedReturnStatusOverridesFixedStatus)
+{
+  static const char* xml_text = R"(
+  <root BTCPP_format="4">
+    <BehaviorTree ID="MainTree">
+      <Sequence>
+        <Script code="mock_should_fail := true"/>
+        <AlwaysSuccess name="action_A"/>
+      </Sequence>
+    </BehaviorTree>
+  </root>
+  )";
+
+  BehaviorTreeFactory factory;
+  factory.registerBehaviorTreeFromText(xml_text);
+
+  TestNodeConfig test_config;
+  test_config.return_status = NodeStatus::SUCCESS;
+  test_config.return_status_script = "(mock_should_fail == true) ? FAILURE : SUCCESS";
+  test_config.failure_script = "branch := 'failure'";
+  factory.addSubstitutionRule("action_A", test_config);
+
+  auto blackboard = Blackboard::create();
+  auto tree = factory.createTree("MainTree", blackboard);
+
+  ASSERT_EQ(tree.tickWhileRunning(), NodeStatus::FAILURE);
+  ASSERT_EQ(blackboard->get<std::string>("branch"), "failure");
+}
+
+TEST(Substitution, ScriptedReturnStatusAsyncSubstitution)
+{
+  static const char* xml_text = R"(
+  <root BTCPP_format="4">
+    <BehaviorTree ID="MainTree">
+      <Sequence>
+        <Script code="mock_should_fail := false"/>
+        <AlwaysSuccess name="action_A"/>
+      </Sequence>
+    </BehaviorTree>
+  </root>
+  )";
+
+  BehaviorTreeFactory factory;
+  factory.registerBehaviorTreeFromText(xml_text);
+
+  TestNodeConfig test_config;
+  test_config.return_status_script = "(mock_should_fail == true) ? FAILURE : SUCCESS";
+  test_config.async_delay = std::chrono::milliseconds(50);
+  factory.addSubstitutionRule("action_A", test_config);
+
+  auto tree = factory.createTree("MainTree");
+  auto future =
+      std::async(std::launch::async, [&tree]() { return tree.tickWhileRunning(); });
+
+  auto status = future.wait_for(std::chrono::seconds(5));
+  ASSERT_NE(status, std::future_status::timeout) << "Tree hung! tickWhileRunning did not "
+                                                    "complete within 5 seconds";
+  ASSERT_EQ(future.get(), NodeStatus::SUCCESS);
+}
+
+TEST(Substitution, ScriptedReturnStatusRejectsIdle)
+{
+  static const char* xml_text = R"(
+  <root BTCPP_format="4">
+    <BehaviorTree ID="MainTree">
+      <AlwaysSuccess name="action_A"/>
+    </BehaviorTree>
+  </root>
+  )";
+
+  BehaviorTreeFactory factory;
+  factory.registerBehaviorTreeFromText(xml_text);
+
+  TestNodeConfig test_config;
+  test_config.return_status_script = "IDLE";
+  factory.addSubstitutionRule("action_A", test_config);
+
+  auto tree = factory.createTree("MainTree");
+  ASSERT_THROW(tree.tickWhileRunning(), RuntimeError);
+}
+
 // Regression test for issue #934: segfault when substituting a SubTree node
 TEST(Substitution, SubTreeNodeSubstitution)
 {