diff --git a/domain/property_baseline/calculator_rebaseliner.py b/domain/property_baseline/calculator_rebaseliner.py
index b2443784..cbfaace7 100644
--- a/domain/property_baseline/calculator_rebaseliner.py
+++ b/domain/property_baseline/calculator_rebaseliner.py
@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 import logging
-from typing import TYPE_CHECKING, Optional, Protocol
+from typing import TYPE_CHECKING, Optional
 
 from datatypes.epc.domain.epc import Epc
 from domain.property_baseline.performance import Performance
@@ -9,7 +9,7 @@ from domain.property_baseline.rebaseliner import Rebaseliner, RebaselineReason
 
 if TYPE_CHECKING:
     from datatypes.epc.domain.epc_property_data import EpcPropertyData
-    from domain.sap10_calculator.calculator import SapResult
+    from domain.sap10_calculator.calculator import SapCalculator, SapResult
 
 logger = logging.getLogger(__name__)
 
@@ -22,13 +22,6 @@ _REL_TOL = 0.01
 _KG_PER_TONNE = 1000.0
 
 
-class Calculator(Protocol):
-    """The slice of `Sap10Calculator` the rebaseliner needs — `Sap10Calculator`
-    satisfies it structurally, so this module does not import the calculator."""
-
-    def calculate(self, epc: "EpcPropertyData") -> "SapResult": ...
-
-
 def performance_from_sap_result(result: "SapResult") -> Performance:
     """The four rated quantities, read off a `SapResult`: band derived from the
     score, CO2 converted kg→tonnes, PEUI rounded to the lodged integer scale."""
@@ -58,7 +51,7 @@ class CalculatorRebaseliner(Rebaseliner):
     is fixed immediately.
     """
 
-    def __init__(self, calculator: Calculator) -> None:
+    def __init__(self, calculator: "SapCalculator") -> None:
         self._calculator = calculator
 
     def rebaseline(
diff --git a/domain/sap10_calculator/calculator.py b/domain/sap10_calculator/calculator.py
index 47366741..43226da1 100644
--- a/domain/sap10_calculator/calculator.py
+++ b/domain/sap10_calculator/calculator.py
@@ -41,6 +41,7 @@ Appendix L + U. RdSAP10 Table 32 (p.95) for fuel prices/CO2/PE factors.
 
 from __future__ import annotations
 
+from abc import ABC, abstractmethod
 from dataclasses import dataclass, field
 from typing import Final, Optional, TYPE_CHECKING
 
@@ -751,7 +752,21 @@ def calculate_sap_from_inputs(inputs: CalculatorInputs) -> SapResult:
     )
 
 
-class Sap10Calculator:
+class SapCalculator(ABC):
+    """The contract a SAP calculator satisfies: an `EpcPropertyData` in, a
+    typed `SapResult` out. `Sap10Calculator` is the SAP 10.2 implementation;
+    a future methodology (e.g. SAP 10.3 / a successor) is another subclass.
+
+    Consumers (e.g. `CalculatorRebaseliner`) depend on this abstraction, not
+    on a concrete calculator — so the engine can be swapped without touching
+    them.
+    """
+
+    @abstractmethod
+    def calculate(self, epc: "EpcPropertyData") -> SapResult: ...
+
+
+class Sap10Calculator(SapCalculator):
     """Deterministic SAP 10.2 calculator entry point. Maps an
     `EpcPropertyData` to typed `CalculatorInputs` via the RdSAP-driven
     `cert_to_inputs` mapper and runs the 12-month worksheet loop.
diff --git a/tests/domain/property_baseline/test_calculator_rebaseliner.py b/tests/domain/property_baseline/test_calculator_rebaseliner.py
index ea1230fc..f22e152f 100644
--- a/tests/domain/property_baseline/test_calculator_rebaseliner.py
+++ b/tests/domain/property_baseline/test_calculator_rebaseliner.py
@@ -9,7 +9,7 @@ from datatypes.epc.domain.epc import Epc
 from datatypes.epc.domain.epc_property_data import EpcPropertyData
 from domain.property_baseline.calculator_rebaseliner import CalculatorRebaseliner
 from domain.property_baseline.performance import Performance
-from domain.sap10_calculator.calculator import SapResult
+from domain.sap10_calculator.calculator import SapCalculator, SapResult
 from domain.sap10_calculator.exceptions import UnmappedSapCode
 
 
@@ -54,7 +54,7 @@ def _sap_result(
     )
 
 
-class _StubCalculator:
+class _StubCalculator(SapCalculator):
     def __init__(self, result: SapResult) -> None:
         self._result = result
 
@@ -122,7 +122,7 @@ def test_a_10_2_cert_logs_divergence_when_the_calculator_disagrees(
 
 def test_a_calculator_raise_propagates_and_aborts() -> None:
     # Arrange — the calculator is load-bearing, so a raise is not swallowed.
-    class _Raising:
+    class _Raising(SapCalculator):
         def calculate(self, epc: EpcPropertyData) -> SapResult:
             raise UnmappedSapCode("heat_emitter_type", 99)