diff --git a/e2e/conftest.py b/e2e/conftest.py
index d54bb69..807a5ce 100644
--- a/e2e/conftest.py
+++ b/e2e/conftest.py
@@ -29,6 +29,7 @@ from laborious.workflows.sub_workflows.prediction_process import PredictionProce
 from sientia_do.notifications.handlers import CoreNotificationHandler
 from sientia_do.observability.logger import Logger
 from sientia_do.observability.metrics_controller import MetricsController
+from sientia_do.repository.mongodb_repository_sync import MongoDBRepository
 
 # Single source of truth for the test database schema. Mirrors the production
 # DDL for ``sientia_data`` so any production change can be pasted directly into
@@ -141,6 +142,24 @@ def notification_inserts(notification_handler):
         collection.insert_one = original_insert_one
 
 
+@pytest_asyncio.fixture
+def mongo_repository(mock_logger, notification_handler, metrics_controller, mongo_container):
+    """Real MongoDBRepository for gate metadata (prediction_metadata), same testcontainer as
+    ``notification_handler`` above."""
+    mongo_port = mongo_container.get_exposed_port(27017)
+    repository = MongoDBRepository(
+        connection_string=f'mongodb://localhost:{mongo_port}',
+        database_name='test_db',
+        logger=mock_logger,
+        notification_handler=notification_handler,
+        metrics_controller=metrics_controller,
+    )
+    try:
+        yield repository
+    finally:
+        repository.close()
+
+
 class _FakeModelWrapper:
     """External MLflow wrapper double used by repository stub."""
 
@@ -233,6 +252,7 @@ async def test_activities(
     plugin_store_stub,
     pi_web_api_client_stub,
     opc_repository_stub,
+    mongo_repository,
 ):
     """Activities with real infra and external-system stubs only."""
     minio_client = minio_container.get_client()
@@ -269,6 +289,7 @@ async def test_activities(
         notification_handler=notification_handler,
         metrics_controller=metrics_controller,
         mlflow_repository=mlflow_repository_stub,
+        mongo_repository=mongo_repository,
     )
     activities.pi_web_api_client = pi_web_api_client_stub
     activities.opc_repository = {'1': opc_repository_stub}
@@ -289,6 +310,7 @@ async def test_activities_real_minio(
     plugin_store_stub,
     pi_web_api_client_stub,
     opc_repository_stub,
+    mongo_repository,
 ):
     """Compatibility alias for offload tests."""
     minio_client = minio_container.get_client()
@@ -325,6 +347,7 @@ async def test_activities_real_minio(
         notification_handler=notification_handler,
         metrics_controller=metrics_controller,
         mlflow_repository=mlflow_repository_stub,
+        mongo_repository=mongo_repository,
     )
     activities.pi_web_api_client = pi_web_api_client_stub
     activities.opc_repository = {'1': opc_repository_stub}
@@ -535,6 +558,7 @@ async def test_activities_real_opc(
     plugin_store_stub,
     pi_web_api_client_stub,
     opc_e2e_server: OpcE2ETestServer,
+    mongo_repository,
 ):
     """Activities with real OpcRepository connected to the in-process OPC UA server."""
     minio_client = minio_container.get_client()
@@ -571,6 +595,7 @@ async def test_activities_real_opc(
         notification_handler=notification_handler,
         metrics_controller=metrics_controller,
         mlflow_repository=mlflow_repository_stub,
+        mongo_repository=mongo_repository,
     )
     activities.pi_web_api_client = pi_web_api_client_stub
     await asyncio.to_thread(_connect_activities_to_opc_server, activities, opc_e2e_server.url)
diff --git a/laborious/activities/activities.py b/laborious/activities/activities.py
index 45e9541..320c97c 100644
--- a/laborious/activities/activities.py
+++ b/laborious/activities/activities.py
@@ -7,6 +7,8 @@ with workflow.unsafe.imports_passed_through():
     from sientia_do.notifications.handlers import CoreNotificationHandler as NotificationHandler
     from sientia_do.observability.logger import Logger
     from sientia_do.repository.minio_repository_sync import MinioRepository
+    from sientia_do.repository.mongodb_repository_sync import MongoDBRepository
+    from sientia_do.utils.connectors_config import build_mongodb_config
     from sientia_model.model_repository.mlflow_repository import SientiaMLflowRepository
     from sientia_model.model_repository.plugin_store import PluginStore
 
@@ -48,6 +50,7 @@ class Activities(Storage, MLFlow, Gates, OPC, ModelMetrics, API):
         notification_handler: NotificationHandler,
         metrics_controller: MetricsController | None = None,
         mlflow_repository: SientiaMLflowRepository | None = None,
+        mongo_repository: MongoDBRepository | None = None,
     ):
         """
         Wire Postgres, MinIO, MLflow, OPC, gates, metrics, and PI Web API into a single object.
@@ -67,6 +70,9 @@ class Activities(Storage, MLFlow, Gates, OPC, ModelMetrics, API):
          - metrics_controller: Optional shared controller; if ``None``, a new one is created.
          - mlflow_repository: Optional ``SientiaMLflowRepository`` for unit/e2e tests; in production
            leave unset so the repository is built from environment via ``build_mlflow_config()``.
+         - mongo_repository: Optional ``MongoDBRepository`` (gate metadata, best-effort) for
+           unit/e2e tests; in production leave unset so the repository is built from environment
+           via ``build_mongodb_config()``, same pattern as ``mlflow_repository`` above.
 
         Raises:
             Exception: If any parent ``__init__`` fails (e.g. invalid config keys).
@@ -89,6 +95,16 @@ class Activities(Storage, MLFlow, Gates, OPC, ModelMetrics, API):
                 metrics_controller=mc,
             )
 
+        if mongo_repository is None:
+            mongo_cfg = build_mongodb_config()
+            mongo_repository = MongoDBRepository(
+                connection_string=mongo_cfg['connection_string'],
+                database_name=mongo_cfg['database_name'],
+                logger=logger,
+                notification_handler=notification_handler,
+                metrics_controller=mc,
+            )
+
         minio_repository = MinioRepository(
             endpoint=minio_config['endpoint_url'],
             access_key=minio_config['access_key'],
@@ -129,6 +145,7 @@ class Activities(Storage, MLFlow, Gates, OPC, ModelMetrics, API):
         Gates.__init__(
             self,
             minio_repository=minio_repository,
+            mongo_repository=mongo_repository,
             logger=logger,
             notification_handler=notification_handler,
             metrics_controller=mc,
diff --git a/laborious/activities/gates.py b/laborious/activities/gates.py
index f72bd2e..71d19cc 100644
--- a/laborious/activities/gates.py
+++ b/laborious/activities/gates.py
@@ -5,6 +5,7 @@ with workflow.unsafe.imports_passed_through():
     from collections.abc import Callable, Mapping
     from typing import Any
 
+    from bson import ObjectId
     from pandas import DataFrame
     from sientia_do.notifications.handlers import CoreNotificationHandler as NotificationHandler
     from sientia_do.notifications.models import NotificationLevel
@@ -12,6 +13,7 @@ with workflow.unsafe.imports_passed_through():
     from sientia_do.observability.metrics_controller import MetricsController
     from sientia_do.observability.sientia_monitoring import SientiaMonitoring
     from sientia_do.repository.minio_repository_sync import MinioRepository
+    from sientia_do.repository.mongodb_repository_sync import MongoDBRepository
     from sientia_do.utils.formatters import create_sample_dict
 
     from laborious import metrics
@@ -86,11 +88,14 @@ class Gates(SientiaMonitoring):
     """
 
     minio_repository: MinioRepository | None = None
+    mongo_repository: MongoDBRepository | None = None
     _MAX_DEBUG_DATAFRAME_ROWS = 100
+    _PREDICTION_METADATA_COLLECTION = 'prediction_metadata'
 
     def __init__(
         self,
         minio_repository: MinioRepository | None = None,
+        mongo_repository: MongoDBRepository | None = None,
         logger: Logger | None = None,
         notification_handler: NotificationHandler | None = None,
         metrics_controller: MetricsController | None = None,
@@ -101,11 +106,14 @@ class Gates(SientiaMonitoring):
         Args:
             logger: Logger instance for observability and debugging
             notification_handler: Notification handler for alerts and monitoring
+            mongo_repository: MongoDB, where gate metadata (triggered filter, policy, counter) is
+                written best-effort. `None` is a valid state (see `_write_prediction_metadata`).
 
         Raises:
             Exception: If BaseActivity initialization fails
         """
         self.minio_repository = minio_repository
+        self.mongo_repository = mongo_repository
         SientiaMonitoring.__init__(
             self,
             logger=logger,
@@ -123,6 +131,11 @@ class Gates(SientiaMonitoring):
                 self.minio_repository.close()
             finally:
                 self.minio_repository = None
+        if self.mongo_repository is not None:
+            try:
+                self.mongo_repository.close()
+            finally:
+                self.mongo_repository = None
         SientiaMonitoring.shutdown(self)
 
     def __del__(self):
@@ -146,6 +159,56 @@ class Gates(SientiaMonitoring):
             metadata,
         )
 
+    def _write_prediction_metadata(
+        self, document: dict[str, Any] | None, metadata: dict[str, Any]
+    ) -> str | None:
+        """
+        Best-effort write of gate metadata (triggered filter, policy, counter) to Mongo.
+
+        Implements both business rules for this feature in one place:
+        - The prediction write must never be blocked or failed by a metadata write. A failed
+          `insert(...)` is swallowed, never re-raised, logged here with gate-level context (which
+          `mongo_repository.insert(...)` itself can't know) — matching how `storage.py`'s
+          `cleanup_minio_objects_expired` also logs again at the call site even though the
+          repository already logs internally and emits its own Prometheus metric
+          (`MONGODB_WRITE_ERROR_COUNT`); the generic library log is for the metric/trace, this
+          one is for whoever is reading logs for this specific gate.
+        - Nothing is written when there's nothing relevant to report. A falsy `document` (e.g.
+          no filter triggered) skips Mongo entirely, so callers can call this unconditionally
+          and get `None` back for "no metadata this time" instead of deciding upfront whether
+          there's something worth saving.
+
+        `insert(...)` doesn't hand back the inserted document's `_id`, so it is generated here,
+        client-side, before the insert, as a plain string (not a BSON ObjectId) — matching this
+        codebase's convention of string `_id`s (e.g. the `models` collection) and the 24-hex-char
+        size of the `predictions.metadata_id` Postgres column that will reference it.
+
+        Args:
+            document: Metadata payload (filter, policy, counter, etc.); `None`/empty skips the
+                write entirely. No `_id` key expected.
+            metadata: Workflow metadata for contextual logging.
+
+        Return:
+            The generated id, or `None` when there's nothing to write, Mongo isn't configured, or
+            the insert failed.
+        """
+        if not document or self.mongo_repository is None:
+            return None
+
+        generated_id = str(ObjectId())
+
+        try:
+            self.mongo_repository.insert(
+                self._PREDICTION_METADATA_COLLECTION,
+                {**document, '_id': generated_id},
+                metadata,
+            )
+        except Exception as e:
+            self.error(f'Failed to write prediction metadata: {e}', metadata)
+            return None
+
+        return generated_id
+
     @staticmethod
     def _read_filter_entry(config: dict[str, Any]) -> tuple[str, dict[str, Any]]:
         """
diff --git a/tests/laborious/activities/test_activities.py b/tests/laborious/activities/test_activities.py
index 4f39f9b..d861f76 100644
--- a/tests/laborious/activities/test_activities.py
+++ b/tests/laborious/activities/test_activities.py
@@ -47,6 +47,7 @@ def test___init__(
     }
 
     mlflow_repository = MagicMock()
+    mongo_repository = MagicMock()
     plugin_store = MagicMock()
 
     opc_config = {
@@ -73,6 +74,7 @@ def test___init__(
         logger=logger,
         notification_handler=notification_handler,
         mlflow_repository=mlflow_repository,
+        mongo_repository=mongo_repository,
     )
 
     assert isinstance(activities, Activities)
@@ -120,6 +122,7 @@ def test___init__(
     mock_gates_init.assert_called_once_with(
         ANY,
         minio_repository=mock_minio_repository.return_value,
+        mongo_repository=mongo_repository,
         logger=logger,
         notification_handler=notification_handler,
         metrics_controller=mock_metrics_controller.return_value,
@@ -217,6 +220,7 @@ def test_shutdown(
         logger=logger,
         notification_handler=notification_handler,
         mlflow_repository=mlflow_repository,
+        mongo_repository=MagicMock(),
     )
 
     activities.shutdown()
@@ -286,6 +290,7 @@ def test___init___builds_mlflow_repository_when_not_provided(
         pi_web_api_config=pi_web_api_config,
         logger=logger,
         notification_handler=notification_handler,
+        mongo_repository=MagicMock(),
     )
 
     mock_build_mlflow_config.assert_called_once()
@@ -297,3 +302,83 @@ def test___init___builds_mlflow_repository_when_not_provided(
         notification_handler=notification_handler,
         metrics_controller=mock_metrics_controller.return_value,
     )
+
+
+@patch('laborious.activities.activities.MongoDBRepository')
+@patch('laborious.activities.activities.build_mongodb_config')
+@patch('laborious.activities.activities.Storage.__init__')
+@patch('laborious.activities.activities.MLFlow.__init__')
+@patch('laborious.activities.activities.OPC.__init__')
+@patch('laborious.activities.activities.Gates.__init__')
+@patch('laborious.activities.activities.ModelMetrics.__init__')
+@patch('laborious.activities.activities.API.__init__')
+@patch('laborious.activities.activities.MinioRepository')
+@patch('laborious.activities.activities.MetricsController')
+def test___init___builds_mongo_repository_when_not_provided(
+    mock_metrics_controller,
+    mock_minio_repository,
+    _mock_api_init,
+    _mock_model_metrics_init,
+    mock_gates_init,
+    _mock_opc_init,
+    _mock_mlflow_init,
+    _mock_storage_init,
+    mock_build_mongodb_config,
+    mock_mongo_repository_cls,
+):
+    postgres_config = {
+        'host': 'localhost',
+        'port': 5432,
+        'user': 'postgres',
+        'password': 'postgres',
+        'dbname': 'postgres',
+        'min_connections': 1,
+        'max_connections': 10,
+    }
+    minio_config = {
+        'endpoint_url': 'localhost:9000',
+        'access_key': 'minio',
+        'secret_key': 'minio123',
+        'default_bucket': 'test',
+        'retention_hours': 24,
+        'secure': False,
+    }
+    opc_config = {'bootstrap_servers': 'localhost:9092', 'polling_time': 1000, 'group_id': 'test'}
+    pi_web_api_config = {'base_url': 'https://pi', 'auth_type': 'bearer', 'auth_token': 'token'}
+    logger = MagicMock()
+    notification_handler = MagicMock()
+    plugin_store = MagicMock()
+    mlflow_repository = MagicMock()
+    mock_build_mongodb_config.return_value = {
+        'connection_string': 'mongodb://root:pass@localhost:27018',
+        'database_name': 'sientia',
+    }
+
+    Activities(
+        postgres_config=postgres_config,
+        plugin_store=plugin_store,
+        minio_config=minio_config,
+        opc_config=opc_config,
+        pi_web_api_config=pi_web_api_config,
+        logger=logger,
+        notification_handler=notification_handler,
+        mlflow_repository=mlflow_repository,
+    )
+
+    mock_build_mongodb_config.assert_called_once()
+    mock_mongo_repository_cls.assert_called_once_with(
+        connection_string='mongodb://root:pass@localhost:27018',
+        database_name='sientia',
+        logger=logger,
+        notification_handler=notification_handler,
+        metrics_controller=mock_metrics_controller.return_value,
+    )
+    # The same repository built here is the one handed to Gates, not a second instance.
+    mock_gates_init.assert_called_once_with(
+        ANY,
+        minio_repository=mock_minio_repository.return_value,
+        mongo_repository=mock_mongo_repository_cls.return_value,
+        logger=logger,
+        notification_handler=notification_handler,
+        metrics_controller=mock_metrics_controller.return_value,
+    )
diff --git a/tests/laborious/activities/test_gates.py b/tests/laborious/activities/test_gates.py
index 5f28509..80fc28f 100644
--- a/tests/laborious/activities/test_gates.py
+++ b/tests/laborious/activities/test_gates.py
@@ -35,8 +35,14 @@ def _minio_payload(retrieve_return, status=None):
 
 
 @fixture
-def gates_activity():
+def mongo() -> MagicMock:
+    return MagicMock()
+
+
+@fixture
+def gates_activity(mongo: MagicMock):
     gates = Gates(
+        mongo_repository=mongo,
         logger=MagicMock(),
         notification_handler=MagicMock(),
         metrics_controller=MagicMock(spec=MetricsController),
@@ -61,6 +67,75 @@ metadata = {
 }
 
 
+def test_write_prediction_metadata_no_document_skips_mongo(gates_activity, mongo: MagicMock):
+    # Act
+    result = gates_activity._write_prediction_metadata(None, metadata)
+
+    # Assert
+    assert result is None
+    mongo.insert.assert_not_called()
+
+
+def test_write_prediction_metadata_empty_document_skips_mongo(gates_activity, mongo: MagicMock):
+    # Act
+    result = gates_activity._write_prediction_metadata({}, metadata)
+
+    # Assert
+    assert result is None
+    mongo.insert.assert_not_called()
+
+
+def test_write_prediction_metadata_not_configured_returns_none():
+    # Arrange
+    gates = Gates(
+        mongo_repository=None,
+        logger=MagicMock(),
+        notification_handler=MagicMock(),
+        metrics_controller=MagicMock(spec=MetricsController),
+    )
+    gates.error = MagicMock()
+
+    # Act
+    result = gates._write_prediction_metadata({'filter_triggered': 'EMPTY_DATA'}, metadata)
+
+    # Assert
+    assert result is None
+    gates.error.assert_not_called()
+
+
+def test_write_prediction_metadata_success_generates_id_and_inserts(
+    gates_activity, mongo: MagicMock
+):
+    # Arrange
+    document = {'filter_triggered': 'EMPTY_DATA', 'policy': 'STOP', 'counter': 3}
+
+    # Act
+    result = gates_activity._write_prediction_metadata(document, metadata)
+
+    # Assert: a plain 24-hex-char string id (matches predictions.metadata_id's varchar(24)),
+    # not a BSON ObjectId — `insert(...)` never hands one back, so it's generated client-side.
+    assert result is not None
+    assert len(result) == 24
+    int(result, 16)  # raises ValueError if not hex
+
+    collection, inserted_document, inserted_metadata = mongo.insert.call_args.args
+    assert collection == Gates._PREDICTION_METADATA_COLLECTION
+    assert inserted_document == {**document, '_id': result}
+    assert inserted_metadata == metadata
+
+
+def test_write_prediction_metadata_insert_failure_swallowed(gates_activity, mongo: MagicMock):
+    # Arrange
+    mongo.insert.side_effect = RuntimeError('connection reset')
+
+    # Act
+    result = gates_activity._write_prediction_metadata({'filter_triggered': 'EMPTY_DATA'}, metadata)
+
+    # Assert: the prediction write must never be blocked or failed by this
+    assert result is None
+    gates_activity.error.assert_called_once()
+
+
 def test_input_gate_invalid_filter(gates_activity):
     # Arrange
     input_data = {