diff --git a/features/steps/tdd_langgraph_execute_closed_state_steps.py b/features/steps/tdd_langgraph_execute_closed_state_steps.py
new file mode 100644
index 000000000..ab710b1ac
--- /dev/null
+++ b/features/steps/tdd_langgraph_execute_closed_state_steps.py
@@ -0,0 +1,95 @@
+"""Behave step definitions for TDD Issue #9994.
+
+Verifies that LangGraph.execute() raises RuntimeError when StateManager is closed,
+preventing silent state corruption after StateManager.close() has been called.
+"""
+
+from __future__ import annotations
+
+import asyncio
+from unittest.mock import MagicMock
+
+from behave import given, then, when
+from behave.runner import Context
+
+from cleveragents.langgraph.graph import GraphConfig, LangGraph
+from cleveragents.langgraph.state import GraphState
+
+
+def _make_graph(name: str = "test-graph") -> LangGraph:
+    """Create a minimal LangGraph for testing."""
+    config = GraphConfig(name=name)
+    graph = LangGraph(config=config)
+    graph.is_running = True
+    graph.stream_router.send_message = MagicMock()
+    return graph
+
+
+@given("a LangGraph instance in running state with a closed StateManager")
+def step_graph_running_with_closed_state_manager(context: Context) -> None:
+    context.graph = _make_graph("closed-graph")
+    context.graph.state_manager.close()
+
+
+@when("I attempt to execute the graph after close")
+def step_execute_after_close(context: Context) -> None:
+    context.execute_error = None
+    loop = asyncio.new_event_loop()
+    asyncio.set_event_loop(loop)
+    try:
+        loop.run_until_complete(context.graph.execute({"messages": []}))
+    except RuntimeError as exc:
+        context.execute_error = exc
+    finally:
+        loop.close()
+        replacement_loop = asyncio.new_event_loop()
+        asyncio.set_event_loop(replacement_loop)
+
+
+@then('a RuntimeError should be raised with message "StateManager is closed"')
+def step_assert_runtime_error(context: Context) -> None:
+    assert context.execute_error is not None, (
+        "Expected RuntimeError but no error was raised"
+    )
+    assert isinstance(context.execute_error, RuntimeError), (
+        f"Expected RuntimeError, got {type(context.execute_error).__name__}"
+    )
+    assert "StateManager is closed" in str(context.execute_error), (
+        f'Expected "StateManager is closed" in error message, '
+        f"got: {context.execute_error}"
+    )
+
+
+@given("a LangGraph instance in running state with an open StateManager")
+def step_graph_running_with_open_state_manager(context: Context) -> None:
+    context.graph = _make_graph("open-graph")
+    assert not context.graph.state_manager.is_closed
+
+
+@when("I execute the graph with valid input")
+def step_execute_with_valid_input(context: Context) -> None:
+    context.execute_result = None
+    context.execute_error = None
+    loop = asyncio.new_event_loop()
+    asyncio.set_event_loop(loop)
+    try:
+        context.execute_result = loop.run_until_complete(
+            context.graph.execute({"messages": []})
+        )
+    except Exception as exc:  # pylint: disable=broad-except
+        context.execute_error = exc
+    finally:
+        loop.close()
+        replacement_loop = asyncio.new_event_loop()
+        asyncio.set_event_loop(replacement_loop)
+
+
+@then("the execution should succeed and return a GraphState")
+def step_assert_execution_success(context: Context) -> None:
+    assert context.execute_error is None, (
+        f"Expected no error but got: {context.execute_error}"
+    )
+    assert context.execute_result is not None, "Expected a GraphState result"
+    assert isinstance(context.execute_result, GraphState), (
+        f"Expected GraphState, got {type(context.execute_result).__name__}"
+    )
diff --git a/features/tdd_langgraph_execute_closed_state.feature b/features/tdd_langgraph_execute_closed_state.feature
new file mode 100644
index 000000000..94cff58c4
--- /dev/null
+++ b/features/tdd_langgraph_execute_closed_state.feature
@@ -0,0 +1,18 @@
+@tdd_issue @tdd_issue_9994
+Feature: TDD Issue #9994 — LangGraph.execute() bypasses is_closed guard
+  As a developer using LangGraph
+  I want execute() to raise RuntimeError when StateManager is closed
+  So that state corruption after close() is prevented
+
+  Background:
+    Given the state management system is available
+
+  Scenario: execute() raises RuntimeError after StateManager.close()
+    Given a LangGraph instance in running state with a closed StateManager
+    When I attempt to execute the graph after close
+    Then a RuntimeError should be raised with message "StateManager is closed"
+
+  Scenario: execute() succeeds when StateManager is open
+    Given a LangGraph instance in running state with an open StateManager
+    When I execute the graph with valid input
+    Then the execution should succeed and return a GraphState
diff --git a/src/cleveragents/langgraph/graph.py b/src/cleveragents/langgraph/graph.py
index a1b36ccb3..17fc46583 100644
--- a/src/cleveragents/langgraph/graph.py
+++ b/src/cleveragents/langgraph/graph.py
@@ -108,7 +108,9 @@ class LangGraph:  # pylint: disable=too-many-instance-attributes
             if isinstance(input_data, GraphState)
             else GraphState.from_dict(input_data)
         )
-        # Replace state manager state for a fresh execution context
+        # Use replace_state() so the is_closed guard is enforced and state stream
+        # subscribers are notified through the proper StateManager API, preventing
+        # silent state corruption after StateManager.close() has been called.
         self.state_manager.replace_state(state)
         start_stream = f"__{self.name}_node_start__"
         if start_stream in self.stream_router.streams:
diff --git a/src/cleveragents/langgraph/state.py b/src/cleveragents/langgraph/state.py
index 6d8650e09..ecc7a7605 100644
--- a/src/cleveragents/langgraph/state.py
+++ b/src/cleveragents/langgraph/state.py
@@ -237,12 +237,16 @@ class StateManager:  # pylint: disable=too-many-instance-attributes
     def replace_state(self, new_state: GraphState) -> None:
         """Atomically replace the internal state and notify subscribers.
 
+        Raises :class:`RuntimeError` if the manager has been closed.
+
         .. note::
 
             See :meth:`update_state` for a note on emission ordering
             under concurrent access.
         """
         with self._lock:
+            if self.is_closed:
+                raise RuntimeError("StateManager is closed")
             self.state = new_state.model_copy(deep=True)
             state_copy = new_state.model_copy(deep=True)
         self.state_stream.on_next(state_copy)