diff --git a/CLAUDE.md b/CLAUDE.md
index 3ced319..88ef953 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -45,7 +45,7 @@ Coverage gate in CI is 80% minimum; this repo targets 100% in practice — don't
 ## Architecture
 
 ### Temporal layering
-- **Worker** (`model_manager/worker/worker.py`): entrypoint. Builds config from env, connects Postgres/MLFlow/MinIO/Plugin Store, installs the `RUNTIME` model runtime via `PluginStore.install_runtime`, starts two Temporal workers (one per task queue) plus Prometheus metrics server and the cleanup schedule. All heavy imports are wrapped in `workflow.unsafe.imports_passed_through()` — required because Temporal workflow sandboxing forbids non-deterministic imports at module scope outside that block.
+- **Worker** (`model_manager/worker/worker.py`): entrypoint. Builds config from env, connects Postgres/MLFlow/MinIO/Plugin Store, installs the `RUNTIME` model runtime via `PluginStore.install_runtime` (boot fails if that changes the version of an already-imported package, so the `requirements.txt` pins must match the factory runtimes; `app_up` goes to 1 only after this check), starts two Temporal workers (one per task queue) plus Prometheus metrics server and the cleanup schedule. All heavy imports are wrapped in `workflow.unsafe.imports_passed_through()` — required because Temporal workflow sandboxing forbids non-deterministic imports at module scope outside that block.
 - **Workflows** (`model_manager/workflows/`): pure orchestration, no I/O beyond `execute_activity_method` calls.
   - `train_model.py` (`TrainModel`, workflow name `train_model`): validate `experiment_run_id` → `load_model_metadata` → `validate_train_params` → update DB status `ORCHESTRATOR_WAITING_PROC` → `train_model` activity → update DB status `TRAINING_SUCCESS`/`TRAINING_ERROR` → `cleanup_resources` (always runs in `finally` if a `run_dir` exists, even on failure). Uses distinct `RetryPolicy`s per operation class: `network_retry_policy` (transient I/O), `no_retry_policy` (training/validation — data errors are permanent, and retrying would duplicate MLflow writes), `database_retry_policy` (Postgres). Timeouts are env-driven (`TIMEOUT_*`), extend `TIMEOUT_TRAIN_MODEL` instead of adding retries for slow training.
   - `cleanup_files.py` (`CleanupFiles`): scheduled (`cleanup_schedule.py`, default daily at midnight UTC via `CLEANUP_CRON`) deletion of stale local temp dirs matching `name_YYYYMMDD_HHMMSS_microseconds`, older than `CLEANUP_RETENTION_HOURS`.
diff --git a/README.md b/README.md
index 4483917..e53e7f0 100644
--- a/README.md
+++ b/README.md
@@ -149,6 +149,7 @@ The Model Manager system uses a Temporal-based workflow architecture with clear
   - Prometheus metrics server initialization
   - Notification handler setup and configuration
   - **Runtime Installation**: Automatically installs the required model runtime from the Plugin Store
+  - **Runtime version guard**: boot fails if the install changes the version of a package the worker already imported, so the `requirements.txt` pins must match `model-plugin-factory` `runtime/basic.yaml` and `runtime/xgboost.yaml`; `app_up` goes to 1 only after this check
 - **Key Features**:
   - Automatic scaling with `PollerBehaviorAutoscaling`
   - Health check endpoints for Kubernetes liveness/readiness probes
@@ -1249,6 +1250,10 @@ sientia-dataops-model-manager/
    - Check training parameter validation errors
    - Verify input data format and required fields
 
+5. **Worker exits at boot with `Runtime ... changed packages already imported by the worker`**
+   - A runtime pin in `model-plugin-factory` `runtime/*.yaml` no longer matches `requirements.txt`; the message lists each package as `old -> new`
+   - Set the runtime version in `requirements.txt` and `requirements-local.txt` here and in `sientia-dataops-laborious`, then redeploy
+
 ### Debug Mode
 
 Enable debug logging by setting the log level:
diff --git a/model_manager/worker/worker.py b/model_manager/worker/worker.py
index 70bdb32..8e94429 100644
--- a/model_manager/worker/worker.py
+++ b/model_manager/worker/worker.py
@@ -79,7 +79,8 @@ def _get_runtime(runtime: str | None) -> str:
 
 def _loaded_distribution_versions() -> dict[str, str]:
     """Version on disk of every installed distribution whose top-level module is imported."""
-    loaded = {name.partition('.')[0] for name in sys.modules}
+    # list() copies the keys: another thread importing while we iterate would raise RuntimeError.
+    loaded = {name.partition('.')[0] for name in list(sys.modules)}
     return {
         dist: importlib.metadata.version(dist)
         for module, dists in importlib.metadata.packages_distributions().items()
@@ -114,7 +115,8 @@ async def main():
     1. Sets up logging and metadata
     2. Starts Prometheus metrics server
     3. Initializes notification handler, then installs the runtime; exits if the install
-       changed the version of a package the worker already imported
+       changed the version of a package the worker already imported, and only then sets
+       ``app_up`` (what the probes check) to 1
     4. Creates and configures activities
     5. Starts Temporal client and workers
     6. Manages worker lifecycle and graceful shutdown
@@ -188,6 +190,10 @@ async def main():
         metrics.APP_UP.labels(pod_id=POD_ID).set(0)
         sys.exit(1)
 
+    # The probes only check app_up, so it goes to 1 after the guard: a pod that would fail it
+    # never turns Ready, and a rollout keeps the old pod.
+    metrics.APP_UP.labels(pod_id=POD_ID).set(1)
+
     activities = Activities(
         postgres_config=build_postgres_config(),
         mlflow_config=build_mlflow_config(),
@@ -274,8 +280,8 @@ def start_prometheus_server(logger: SientiaLogger, metadata: dict[str, str | Non
     """
     Starts the Prometheus metrics server for monitoring and observability.
 
-    This function initializes the Prometheus HTTP server on the configured port
-    and sets the application health metric to indicate the service is running.
+    This function initializes the Prometheus HTTP server on the configured port.
+    ``main`` sets ``app_up`` to 1 later, once the runtime install passed the version guard.
 
     The server exposes metrics that can be scraped by Prometheus for monitoring
     the health and performance of the Model Manager worker.
@@ -291,7 +297,6 @@ def start_prometheus_server(logger: SientiaLogger, metadata: dict[str, str | Non
         port = int(os.getenv('HTTP_METRICS_PORT', 9090))
         start_http_server(port)
         logger.custom_info(f'Prometheus server initialized on port {port}.', metadata)
-        metrics.APP_UP.labels(pod_id=POD_ID).set(1)  # Mark app as UP
     except Exception as e:  # noqa: BLE001
         logger.custom_critical(f'Failed to start Prometheus server: {e}', metadata)
         os._exit(1)
diff --git a/requirements-local.txt b/requirements-local.txt
index d517cec..4e3af07 100644
--- a/requirements-local.txt
+++ b/requirements-local.txt
@@ -8,4 +8,15 @@ git+ssh://git@github.com/Aignosi/sientia-model-library.git@0.14.0
 prometheus-client==0.23.1
 beautifulsoup4==4.12.3
 evidently==0.6.7
-jsonschema==4.26.0
\ No newline at end of file
+jsonschema==4.26.0
+# Same versions as the plugin runtimes (model-plugin-factory runtime/basic.yaml and xgboost.yaml).
+# The worker imports these before install_runtime; if the runtime installed a different version,
+# the old code would stay in memory, so the worker refuses to boot (SIENTIAPDE-2214).
+# Changing a runtime pin means changing it here and in requirements.txt too.
+mlflow==3.8.1
+pathspec==1.0.4
+pandas==2.3.3
+cloudpickle==3.1.2
+scikit-learn==1.8.0
+joblib==1.5.3
+rich==14.3.2
diff --git a/requirements.txt b/requirements.txt
index 9ae8fa6..ace4c48 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -4,7 +4,7 @@ sqlalchemy==2.0.50
 boto3==1.42.70
 botocore==1.42.70
 sientia_do>=1.12.2
-sientia_model>=0.8.1
+sientia_model>=0.14.0
 prometheus-client==0.23.1
 beautifulsoup4==4.12.3
 evidently==0.6.7
@@ -12,7 +12,7 @@ jsonschema==4.26.0
 # Same versions as the plugin runtimes (model-plugin-factory runtime/basic.yaml and xgboost.yaml).
 # The worker imports these before install_runtime; if the runtime installed a different version,
 # the old code would stay in memory, so the worker refuses to boot (SIENTIAPDE-2214).
-# Changing a runtime pin means changing it here too.
+# Changing a runtime pin means changing it here and in requirements-local.txt too.
 mlflow==3.8.1
 pathspec==1.0.4
 pandas==2.3.3
diff --git a/tests/worker/test_worker.py b/tests/worker/test_worker.py
index 9fb43ff..07fd18c 100644
--- a/tests/worker/test_worker.py
+++ b/tests/worker/test_worker.py
@@ -132,9 +132,8 @@ def test_start_prometheus_server_success(
     # Verify HTTP server started
     mock_start_http_server.assert_called_once_with(9090)
 
-    # Verify APP_UP metric was set to 1
-    mock_metrics.APP_UP.labels.assert_called_once_with(pod_id='test-pod-123')
-    mock_app_up.set.assert_called_once_with(1)
+    # app_up goes to 1 in main(), after the runtime version guard.
+    mock_app_up.set.assert_not_called()
     mock_logger.custom_info.assert_called_once()
 
 
@@ -284,7 +283,8 @@ async def test_main_successful_startup(
     # Verify cleanup was performed
     mock_notification_handler.shutdown.assert_called_once()
     mock_activities.shutdown.assert_called_once()
-    mock_app_up.set.assert_called_with(0)
+    # Ready after the guard, down again on shutdown.
+    assert [c.args[0] for c in mock_app_up.set.call_args_list] == [1, 0]
 
 
 @pytest.mark.asyncio
@@ -941,22 +941,30 @@ async def test_main_runtime_changed_loaded_package_exits(mock_env_vars, mock_log
 
     plugin_store = Mock()
     plugin_store.install_runtime = AsyncMock(return_value=None)
+    before = {'scikit-learn': '1.9.1'}
+
+    def snapshot():
+        # Taken after install_runtime, it would already see 1.8.0 and the guard would pass.
+        assert plugin_store.install_runtime.await_count == 0
+        return before
+
     mongo = {'connection_string': 'mongodb://test', 'database_name': 'test_db', 'uri': 'x'}
     with (
         patch.object(worker_module, 'RUNTIME', 'basic'),
         patch.object(worker_module, 'ensure_runtime_directories'),
-        patch.object(worker_module, 'metrics'),
+        patch.object(worker_module, 'metrics') as mock_metrics,
         patch.object(worker_module, 'start_prometheus_server'),
         patch.object(worker_module, 'get_logger', return_value=mock_logger),
         patch.object(worker_module, 'build_mongodb_config', return_value=mongo),
         patch.object(worker_module, 'NotificationHandler'),
         patch.object(worker_module, 'build_plugin_store_config', return_value=MagicMock()),
         patch.object(worker_module, 'PluginStore', return_value=plugin_store),
+        patch.object(worker_module, '_loaded_distribution_versions', side_effect=snapshot),
         patch.object(
             worker_module,
             '_runtime_changed_loaded_packages',
             return_value={'scikit-learn': '1.9.1 -> 1.8.0'},
-        ),
+        ) as mock_changed,
         patch.object(worker_module, 'Activities') as mock_activities_class,
         patch.object(worker_module.sys, 'exit', side_effect=SystemExit(1)) as mock_exit,
     ):
@@ -965,5 +973,35 @@ async def test_main_runtime_changed_loaded_package_exits(mock_env_vars, mock_log
 
     mock_exit.assert_called_once_with(1)
     mock_activities_class.assert_not_called()
+    plugin_store.install_runtime.assert_awaited_once()
+    mock_changed.assert_called_once_with(before)
+    mock_metrics.APP_UP.labels.return_value.set.assert_called_once_with(0)
     message = mock_logger.custom_critical.call_args.args[0]
     assert "'scikit-learn': '1.9.1 -> 1.8.0'" in message
+
+
+def test_guard_detects_version_change_on_disk(tmp_path, monkeypatch):
+    """The real importlib.metadata path: a package swapped on disk after import is reported."""
+    import types
+
+    from model_manager.worker.worker import (
+        _loaded_distribution_versions,
+        _runtime_changed_loaded_packages,
+    )
+
+    info = tmp_path / 'fakepkg_2214-1.0.dist-info'
+    info.mkdir()
+    (info / 'METADATA').write_text('Metadata-Version: 2.1\nName: fakepkg-2214\nVersion: 1.0\n')
+    (info / 'top_level.txt').write_text('fakepkg_2214\n')
+    monkeypatch.syspath_prepend(str(tmp_path))
+    # The guard only reads sys.modules keys; setitem removes the entry again on teardown.
+    monkeypatch.setitem(sys.modules, 'fakepkg_2214', types.ModuleType('fakepkg_2214'))
+
+    before = _loaded_distribution_versions()
+    assert before['fakepkg-2214'] == '1.0'
+
+    # What pip does on an upgrade: the old dist-info goes away and a new one takes its place.
+    new_info = info.rename(tmp_path / 'fakepkg_2214-2.0.dist-info')
+    (new_info / 'METADATA').write_text('Metadata-Version: 2.1\nName: fakepkg-2214\nVersion: 2.0\n')
+
+    assert _runtime_changed_loaded_packages(before) == {'fakepkg-2214': '1.0 -> 2.0'}