diff --git a/agent_assembly/adapters/openai_agents/patch.py b/agent_assembly/adapters/openai_agents/patch.py
index 3f3e5249..41c3f269 100644
--- a/agent_assembly/adapters/openai_agents/patch.py
+++ b/agent_assembly/adapters/openai_agents/patch.py
@@ -435,8 +435,14 @@ async def _record_async_tool_result(
 
 
 def _is_governance_error(error: Exception) -> bool:
-    del error
-    return True
+    """Check if error is a governance-specific error that should be handled.
+
+    Non-governance errors (network, malformed response, etc.) should propagate
+    to maintain fail-closed security posture.
+    """
+    from agent_assembly.exceptions import AssemblyError
+
+    return isinstance(error, AssemblyError)
 
 
 def _apply_function_tool_patch(function_tool_cls: type[Any], callback_handler: Any) -> None:
diff --git a/test/unit/adapters/openai_agents/test_patch.py b/test/unit/adapters/openai_agents/test_patch.py
index bc7bb100..e85ef64a 100644
--- a/test/unit/adapters/openai_agents/test_patch.py
+++ b/test/unit/adapters/openai_agents/test_patch.py
@@ -280,9 +280,15 @@ async def record_result(self, **kwargs: object) -> None:
 
 
 @pytest.mark.asyncio
-async def test_gateway_error_uses_fail_open_and_executes_original_tool(
+async def test_non_governance_error_propagates_fail_closed(
     monkeypatch: pytest.MonkeyPatch,
 ) -> None:
+    """Non-governance errors (RuntimeError, ValueError, etc.) must propagate.
+
+    This ensures fail-closed security posture: if an unexpected error occurs
+    during governance checks, the tool call is blocked rather than allowed
+    through. See AAASM-4252.
+    """
     function_tool_cls = _install_fake_openai_agents_module(monkeypatch)
 
     class Interceptor:
@@ -293,6 +299,34 @@ async def check_tool_start(self, **kwargs: object) -> dict[str, str]:
     patcher = openai_patch.OpenAIAgentsPatch(callback_handler=Interceptor())
     assert patcher.apply() is True
 
+    tool = function_tool_cls(name="fail_closed_tool")
+    with pytest.raises(RuntimeError, match="gateway unavailable"):
+        await tool.on_invoke_tool(SimpleNamespace(agent_id="agent-fail-closed"), '{"x": 2}')
+
+
+@pytest.mark.asyncio
+async def test_governance_error_allows_fail_open(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """AssemblyError subclasses are handled gracefully (fail-open).
+
+    Governance-specific errors (e.g., transient gateway issues represented as
+    GatewayError) are caught and the tool executes. This is intentional: if
+    the governance system itself has a known error, we fail-open rather than
+    blocking all tool calls. See AAASM-4252.
+    """
+    from agent_assembly.exceptions import GatewayError
+
+    function_tool_cls = _install_fake_openai_agents_module(monkeypatch)
+
+    class Interceptor:
+        async def check_tool_start(self, **kwargs: object) -> dict[str, str]:
+            del kwargs
+            raise GatewayError("gateway temporarily unavailable")
+
+    patcher = openai_patch.OpenAIAgentsPatch(callback_handler=Interceptor())
+    assert patcher.apply() is True
+
     tool = function_tool_cls(name="fail_open_tool")
     result = await tool.on_invoke_tool(SimpleNamespace(agent_id="agent-fail-open"), '{"x": 2}')
 
@@ -300,9 +334,10 @@ async def check_tool_start(self, **kwargs: object) -> dict[str, str]:
 
 
 @pytest.mark.asyncio
-async def test_non_governance_error_is_reraised(
+async def test_value_error_propagates_fail_closed(
     monkeypatch: pytest.MonkeyPatch,
 ) -> None:
+    """ValueError (non-governance) must propagate for fail-closed posture."""
     function_tool_cls = _install_fake_openai_agents_module(monkeypatch)
 
     class Interceptor:
@@ -310,8 +345,6 @@ async def check_tool_start(self, **kwargs: object) -> dict[str, str]:
             del kwargs
             raise ValueError("unexpected")
 
-    monkeypatch.setattr(openai_patch, "_is_governance_error", lambda _error: False)
-
     patcher = openai_patch.OpenAIAgentsPatch(callback_handler=Interceptor())
     assert patcher.apply() is True