A goal-aligned scenario without a budget fails loudly 🟥

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/orchestration/test_modelling_goal_objectives.py

@@ -11,6 +11,8 @@ from __future__ import annotations
 
 import dataclasses
 
+import pytest
+
 from datatypes.epc.domain.epc_property_data import EpcPropertyData
 from domain.fuel_rates.fuel import Fuel
 from domain.fuel_rates.fuel_rates import FuelRate, FuelRates
@@ -67,6 +69,25 @@ def test_reducing_co2_scenario_buys_carbon_not_sap() -> None:
     assert MeasureType.GAS_BOILER_UPGRADE not in selected
 
 
+def test_a_goal_aligned_scenario_without_a_budget_fails_loudly() -> None:
+    # Arrange — 'reduce as much as possible within this budget' is undefined
+    # without a budget: unconstrained it would recommend every beneficial
+    # measure. A budget-less goal-aligned Scenario is a misconfiguration and
+    # must fail visibly, not produce a maximal plan.
+    epc = _solid_brick_dwelling()
+    budgetless = Scenario(
+        id=999,
+        goal="Reducing CO2 emissions",
+        goal_value="",
+        budget=None,
+        is_default=True,
+    )
+
+    # Act / Assert
+    with pytest.raises(ValueError, match="budget"):
+        run_modelling(epc, scenario=budgetless, print_table=False)
+
+
 class _FixedFuelRates(FuelRatesRepository):
     def __init__(self, rates: FuelRates) -> None:
         self._rates = rates