diff --git a/tests/domain/epc_prediction/test_historic_conditioning.py b/tests/domain/epc_prediction/test_historic_conditioning.py
index 8f26febb1..886319b64 100644
--- a/tests/domain/epc_prediction/test_historic_conditioning.py
+++ b/tests/domain/epc_prediction/test_historic_conditioning.py
@@ -47,6 +47,25 @@ def test_resolves_stable_attributes_into_cohort_code_spaces():
     assert conditioning.total_floor_area_m2 == 84.0
 
 
+def test_legacy_register_fuel_descriptions_resolve():
+    # Arrange / Act / Assert — the old register lodged pre-RdSAP-17 fuels with
+    # a "backwards compatibility" rider or a SAP-style prefix; they name the
+    # same physical fuels (dominant in the pre-2012 dump: a 65-shard scan found
+    # 636/663 unresolved values were these variants).
+    cases = {
+        "mains gas - this is for backwards compatibility only and should not be used": 26,
+        "electricity - this is for backwards compatibility only and should not be used": 29,
+        "LPG - this is for backwards compatibility only and should not be used": 27,
+        "oil - this is for backwards compatibility only and should not be used": 28,
+        "Gas: mains gas": 26,
+        "Electricity: electricity, unspecified tariff": 29,
+        "dual fuel - mineral + wood": 10,
+    }
+    for text, code in cases.items():
+        record = _hist(main_fuel=text)
+        assert conditioning_from_historic(record).main_fuel == code, text
+
+
 def test_unresolvable_values_degrade_to_none():
     # Arrange — junk and blanks must never guess a code.
     record = _hist(