diff --git a/backend/app/services/simulation_workflow.py b/backend/app/services/simulation_workflow.py
index 2c8ddd7..2df406d 100644
--- a/backend/app/services/simulation_workflow.py
+++ b/backend/app/services/simulation_workflow.py
@@ -55,6 +55,9 @@ def _resolve_technique_ids(
     """
     from backend.app.services import mitre as mitre_svc
 
+    if not mitre_svc.mitre_loaded:
+        return None, (jsonify({"error": "mitre bundle not loaded"}), 503)
+
     # Dedup, preserve order.
     seen: dict[str, None] = dict.fromkeys(technique_ids)
     resolved: list[dict[str, str]] = []
diff --git a/backend/tests/test_simulations_techniques.py b/backend/tests/test_simulations_techniques.py
index 5212577..f515965 100644
--- a/backend/tests/test_simulations_techniques.py
+++ b/backend/tests/test_simulations_techniques.py
@@ -305,6 +305,27 @@ def test_technique_ids_empty_does_not_trigger_auto_transition(
     assert resp.get_json()["status"] == "pending"
 
 
+# ---------------------------------------------------------------------------
+# Bundle not loaded — 503 on technique_ids PATCH
+# ---------------------------------------------------------------------------
+
+
+def test_patch_technique_ids_bundle_not_loaded_returns_503(
+    client: FlaskClient, redteam_token: str
+) -> None:
+    """When MITRE bundle is absent, PATCH with technique_ids must return 503."""
+    mitre_svc.mitre_loaded = False
+    mitre_svc._index = []
+    mitre_svc._name_by_id = {}
+
+    eng = _make_engagement(client, redteam_token)
+    sim = _make_sim(client, redteam_token, eng["id"])
+
+    resp = _patch(client, redteam_token, sim["id"], {"technique_ids": ["T1059"]})
+    assert resp.status_code == 503
+    assert resp.get_json()["error"] == "mitre bundle not loaded"
+
+
 # ---------------------------------------------------------------------------
 # SOC cannot patch technique_ids (it's a redteam field)
 # ---------------------------------------------------------------------------