From 705c86d5e6d281199387b6c0725fa7af6cdb0e8a Mon Sep 17 00:00:00 2001
From: Khalim Conn-Kowlessar <kconnkowlessar@gmail.com>
Date: Thu, 9 Jul 2026 11:40:29 +0000
Subject: [PATCH] =?UTF-8?q?Phase-2=20candidates=20are=20valued=20against?=
 =?UTF-8?q?=20the=20post-fabric=20dwelling=20=F0=9F=9F=A5?=
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Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
---
 .../modelling/test_optimiser_fabric_first.py  | 65 +++++++++++++++++++
 1 file changed, 65 insertions(+)

diff --git a/tests/domain/modelling/test_optimiser_fabric_first.py b/tests/domain/modelling/test_optimiser_fabric_first.py
index d1011ed73..f345d6e75 100644
--- a/tests/domain/modelling/test_optimiser_fabric_first.py
+++ b/tests/domain/modelling/test_optimiser_fabric_first.py
@@ -44,6 +44,12 @@ _ROOF_OVERLAY = EpcSimulation(
     }
 )
 _HEATING_OVERLAY = EpcSimulation(heating=HeatingOverlay(sap_main_heating_code=201))
+_BOILER_OVERLAY = EpcSimulation(heating=HeatingOverlay(sap_main_heating_code=201))
+_ASHP_OVERLAY = EpcSimulation(
+    heating=HeatingOverlay(main_heating_index_number=13000)
+)
+
+
 def _scored(
     measure_type: str, *, gain: float, cost: float, overlay: EpcSimulation
 ) -> ScoredOption:
@@ -144,3 +150,62 @@ def test_fabric_short_of_target_is_topped_up_with_non_fabric_measures() -> None:
     assert abs(package.score.sap_continuous - 85.0) <= 1e-9
 
 
+class _InteractionScorer:
+    """A stub whose boiler gain collapses once the wall is insulated (+10 raw,
+    +3 post-fabric) while the heat pump's holds (+8 either way) — so a phase 2
+    that keeps valuing candidates against the raw baseline picks the wrong
+    heating system."""
+
+    def score(
+        self, baseline: EpcPropertyData, simulations: Sequence[EpcSimulation]
+    ) -> Score:
+        wall_present = any(
+            part.wall_insulation_type is not None
+            for sim in simulations
+            for part in sim.building_parts.values()
+        )
+        sap = 60.0 + (5.0 if wall_present else 0.0)
+        for sim in simulations:
+            if sim.heating is None:
+                continue
+            if sim.heating.sap_main_heating_code is not None:
+                sap += 3.0 if wall_present else 10.0
+            if sim.heating.main_heating_index_number is not None:
+                sap += 8.0
+        return Score(
+            sap_continuous=sap, co2_kg_per_yr=0.0, primary_energy_kwh_per_yr=0.0
+        )
+
+
+def test_phase_two_values_candidates_against_the_post_fabric_dwelling() -> None:
+    # Arrange — one heating Recommendation, two Options. The boiler's role-1
+    # signal (vs the raw baseline, +10) beats the heat pump's (+8) and it is
+    # cheaper — but on the insulated dwelling the boiler is only worth +3.
+    # Only a heat pump gets the fabric-applied dwelling to the target.
+    groups: list[list[ScoredOption]] = [
+        [_scored("cavity_wall_insulation", gain=5.0, cost=1000.0, overlay=_WALL_OVERLAY)],
+        [
+            _scored("gas_boiler_upgrade", gain=10.0, cost=2000.0, overlay=_BOILER_OVERLAY),
+            _scored("air_source_heat_pump", gain=8.0, cost=6000.0, overlay=_ASHP_OVERLAY),
+        ],
+    ]
+    scorer = _InteractionScorer()
+
+    # Act — target 73: wall (65) + boiler-post-fabric (+3) = 68 misses; wall +
+    # heat pump (+8) = 73 reaches. The heating group is consumed by whichever
+    # option phase 2 warm-starts with, so the choice must be made on
+    # post-fabric values, not raw-baseline signals.
+    package: OptimisedPackage = optimise_package_fabric_first(
+        groups=groups,
+        scorer=scorer,
+        baseline_epc=build_epc(),
+        budget=20000.0,
+        target_sap=73.0,
+    )
+
+    # Assert
+    assert _selected_types(package) == {
+        "cavity_wall_insulation",
+        "air_source_heat_pump",
+    }
+    assert abs(package.score.sap_continuous - 73.0) <= 1e-9