From d7f069b7c58af9311fd913e3709ca20279bef753 Mon Sep 17 00:00:00 2001
From: Khalim Conn-Kowlessar <kconnkowlessar@gmail.com>
Date: Wed, 1 Jul 2026 13:05:47 +0000
Subject: [PATCH] =?UTF-8?q?Deterministically=20guard=20structured=20biomas?=
 =?UTF-8?q?s=20and=20immersion=20water-heating=20descriptions=20?=
 =?UTF-8?q?=F0=9F=9F=A5?=
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Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
---
 .../property_overrides/water_heating_guard.py |  9 +++
 tests/domain/epc/test_water_heating_guard.py  | 80 +++++++++++++++++++
 2 files changed, 89 insertions(+)
 create mode 100644 domain/epc/property_overrides/water_heating_guard.py
 create mode 100644 tests/domain/epc/test_water_heating_guard.py

diff --git a/domain/epc/property_overrides/water_heating_guard.py b/domain/epc/property_overrides/water_heating_guard.py
new file mode 100644
index 000000000..4b447ff6e
--- /dev/null
+++ b/domain/epc/property_overrides/water_heating_guard.py
@@ -0,0 +1,9 @@
+from __future__ import annotations
+
+from typing import Optional
+
+from domain.epc.property_overrides.water_heating_type import WaterHeatingType
+
+
+def water_heating_guard(description: str) -> Optional[WaterHeatingType]:
+    raise NotImplementedError
diff --git a/tests/domain/epc/test_water_heating_guard.py b/tests/domain/epc/test_water_heating_guard.py
new file mode 100644
index 000000000..227ed3f4e
--- /dev/null
+++ b/tests/domain/epc/test_water_heating_guard.py
@@ -0,0 +1,80 @@
+from __future__ import annotations
+
+import pytest
+
+from domain.epc.property_overrides.water_heating_guard import water_heating_guard
+from domain.epc.property_overrides.water_heating_type import WaterHeatingType
+
+
+@pytest.mark.parametrize(
+    ("description", "expected"),
+    [
+        # The biomass/wood family the LLM funnelled into "house coal" (#1376).
+        (
+            "From main heating system: Wood Logs",
+            WaterHeatingType.FROM_MAIN_WOOD_LOGS,
+        ),
+        (
+            "From main heating system: Dual Fuel: Mineral and Wood",
+            WaterHeatingType.FROM_MAIN_DUAL_FUEL_MINERAL_WOOD,
+        ),
+        # "Hot-water only community scheme" flattens to "from main", per the
+        # existing convention (community mains-gas already resolves that way).
+        (
+            "From main heating system: Biomass (Community)",
+            WaterHeatingType.FROM_MAIN_BIOMASS_COMMUNITY,
+        ),
+        (
+            "Hot-water only community scheme - boilers: Biomass (Community)",
+            WaterHeatingType.FROM_MAIN_BIOMASS_COMMUNITY,
+        ),
+        (
+            "Hot-water only community scheme - boilers: Heat from Boilers "
+            "using Biodiesel from any source (Community)",
+            WaterHeatingType.FROM_MAIN_BIODIESEL_COMMUNITY,
+        ),
+        # "No Heating or Hot Water" under "electric immersion assumed" is the
+        # assessment's own immersion assumption — honour it (ADR-0043).
+        (
+            "No hot water system present - electric immersion assumed: "
+            "No Heating or Hot Water",
+            WaterHeatingType.ELECTRIC_IMMERSION,
+        ),
+        # A genuinely coal DHW stays house coal.
+        (
+            "From main heating system: House Coal",
+            WaterHeatingType.FROM_MAIN_HOUSE_COAL,
+        ),
+    ],
+)
+def test_guard_resolves_the_structured_biomass_and_immersion_descriptions(
+    description: str, expected: WaterHeatingType
+) -> None:
+    # Act
+    result = water_heating_guard(description)
+
+    # Assert
+    assert result is expected
+
+
+@pytest.mark.parametrize(
+    "description",
+    [
+        # Fuels the LLM already classifies correctly — the guard must not claim
+        # them (it only rescues the previously-misclassified structured cases).
+        "From main heating system: Mains Gas",
+        "Electric immersion: Electricity",
+        "From main heating system: Oil",
+        # A varied / unstructured phrasing is the LLM's job.
+        "hot water from the wood burner",
+        "",
+    ],
+)
+def test_guard_defers_recognised_and_unstructured_descriptions_to_the_llm(
+    description: str,
+) -> None:
+    # Act
+    result = water_heating_guard(description)
+
+    # Assert
+    assert result is None