diff --git a/openhands/events/event.py b/openhands/events/event.py
index 9d7af19160ca6..751ef8fc66126 100644
--- a/openhands/events/event.py
+++ b/openhands/events/event.py
@@ -107,3 +107,13 @@ def tool_call_metadata(self) -> ToolCallMetadata | None:
     @tool_call_metadata.setter
     def tool_call_metadata(self, value: ToolCallMetadata) -> None:
         self._tool_call_metadata = value
+
+    @property
+    def hidden(self) -> bool:
+        if hasattr(self, '_hidden'):
+            return self._hidden  # type: ignore[attr-defined]
+        return False
+
+    @hidden.setter
+    def hidden(self, value: bool) -> None:
+        self._hidden = value
diff --git a/openhands/events/stream.py b/openhands/events/stream.py
index ca1025d0f1636..c2831e0719822 100644
--- a/openhands/events/stream.py
+++ b/openhands/events/stream.py
@@ -257,7 +257,7 @@ def unsubscribe(self, subscriber_id: EventStreamSubscriber, callback_id: str):
 
         self._clean_up_subscriber(subscriber_id, callback_id)
 
-    def add_event(self, event: Event, source: EventSource):
+    def add_event(self, event: Event, source: EventSource, hidden: bool = False):
         if hasattr(event, '_id') and event.id is not None:
             raise ValueError(
                 f'Event already has an ID:{event.id}. It was probably added back to the EventStream from inside a handler, triggering a loop.'
@@ -268,6 +268,7 @@ def add_event(self, event: Event, source: EventSource):
         logger.debug(f'Adding {type(event).__name__} id={event.id} from {source.name}')
         event._timestamp = datetime.now().isoformat()
         event._source = source  # type: ignore [attr-defined]
+        event.hidden = hidden
         data = event_to_dict(event)
         data = self._replace_secrets(data)
         event = event_from_dict(data)
diff --git a/tests/unit/test_trajectory.py b/tests/unit/test_trajectory.py
new file mode 100644
index 0000000000000..7153ad5e3fe98
--- /dev/null
+++ b/tests/unit/test_trajectory.py
@@ -0,0 +1,88 @@
+import pytest
+from fastapi import Request
+from fastapi.responses import JSONResponse
+
+from openhands.events import EventSource, EventStream
+from openhands.events.action import NullAction
+from openhands.events.observation import NullObservation
+from openhands.storage import get_file_store
+from openhands.server.routes.trajectory import get_trajectory
+
+pytestmark = pytest.mark.asyncio
+
+
+@pytest.fixture
+def temp_dir(tmp_path_factory: pytest.TempPathFactory) -> str:
+    return str(tmp_path_factory.mktemp("test_trajectory"))
+
+
+@pytest.fixture
+def event_stream(temp_dir: str):
+    file_store = get_file_store("local", temp_dir)
+    stream = EventStream("test_conversation", file_store)
+    # Add a mix of hidden and non-hidden events
+    stream.add_event(NullAction(), EventSource.AGENT, hidden=True)
+    stream.add_event(NullObservation("visible1"), EventSource.AGENT, hidden=False)
+    stream.add_event(NullAction(), EventSource.AGENT, hidden=True)
+    stream.add_event(NullObservation("visible2"), EventSource.AGENT, hidden=False)
+    return stream
+
+
+@pytest.fixture
+def mock_request(event_stream):
+    class MockRequest:
+        def __init__(self):
+            self.state = type(
+                "State",
+                (),
+                {
+                    "conversation": type(
+                        "Conversation", (), {"event_stream": event_stream}
+                    )()
+                },
+            )
+
+    return MockRequest()
+
+
+async def test_get_trajectory_filter_hidden(mock_request):
+    # Test with filter_hidden=True (default)
+    response = await get_trajectory(mock_request)
+    assert isinstance(response, JSONResponse)
+    assert response.status_code == 200
+
+    content = response.body.decode()
+    assert "visible1" in content
+    assert "visible2" in content
+    # Hidden events should not be in the response
+    assert (
+        len(response.body.decode().split("NullAction")) == 1
+    )  # Only in the class name
+
+
+async def test_get_trajectory_show_hidden(mock_request):
+    # Test with filter_hidden=False
+    response = await get_trajectory(mock_request, filter_hidden=False)
+    assert isinstance(response, JSONResponse)
+    assert response.status_code == 200
+
+    content = response.body.decode()
+    assert "visible1" in content
+    assert "visible2" in content
+    # Hidden events should be in the response
+    # Count the number of "action":"null" occurrences which represent NullAction events
+    assert (
+        len(content.split('"action":"null"')) > 2
+    )  # More occurrences due to hidden events
+
+
+async def test_get_trajectory_error_handling():
+    # Test error handling with a broken request
+    class BrokenRequest:
+        def __init__(self):
+            self.state = None
+
+    response = await get_trajectory(BrokenRequest())
+    assert isinstance(response, JSONResponse)
+    assert response.status_code == 500
+    assert "error" in response.body.decode()