diff --git a/tests/backend/app/modelling/test_trigger_run_router.py b/tests/backend/app/modelling/test_trigger_run_router.py
index 9da1405f2..30ebb17b7 100644
--- a/tests/backend/app/modelling/test_trigger_run_router.py
+++ b/tests/backend/app/modelling/test_trigger_run_router.py
@@ -131,6 +131,22 @@ def test_trigger_run_rejects_filters_that_match_no_properties(api: Api) -> None:
     assert api.subtasks_for(task) == []
 
 
+def test_trigger_run_rejects_scenarios_outside_the_portfolio(api: Api) -> None:
+    # arrange — one valid scenario, one belonging to a different portfolio
+    task = api.seed_task()
+    ours = api.seed_scenario()
+    theirs = api.seed_scenario(portfolio_id=999)
+    api.seed_property()
+
+    # act
+    response = _trigger(api, task, [ours, theirs])
+
+    # assert — refused outright; nothing partially distributed
+    assert response.status_code == 400
+    assert api.sent_bodies == []
+    assert api.subtasks_for(task) == []
+
+
 def test_trigger_run_fans_out_one_subtask_and_message_per_scenario_batch(
     api: Api,
 ) -> None: