diff --git a/changelog.d/fix-eternity-input-cache-preservation.fixed.md b/changelog.d/fix-eternity-input-cache-preservation.fixed.md
new file mode 100644
index 00000000..d9cae03d
--- /dev/null
+++ b/changelog.d/fix-eternity-input-cache-preservation.fixed.md
@@ -0,0 +1 @@
+Preserve user-provided ETERNITY inputs across cache invalidation when they were set for an ordinary period.
diff --git a/policyengine_core/simulations/simulation.py b/policyengine_core/simulations/simulation.py
index ba8e173a..e221c904 100644
--- a/policyengine_core/simulations/simulation.py
+++ b/policyengine_core/simulations/simulation.py
@@ -1,5 +1,6 @@
 import hashlib
 import tempfile
+from dataclasses import dataclass
 from typing import TYPE_CHECKING, Any, Dict, List, Type, Union
 
 import numpy as np
@@ -57,6 +58,14 @@ def _stable_hash_to_seed(value: str) -> int:
 )
 
 
+@dataclass(frozen=True)
+class PreservedUserInput:
+    variable_name: str
+    branch_name: str
+    period: Period
+    value: object
+
+
 class Simulation:
     """
     Represents a simulation, and handles the calculation logic
@@ -273,16 +282,23 @@ def _invalidate_all_caches(self) -> None:
         self._fast_cache = {}
         self.invalidated_caches = set()
         # Snapshot user-provided inputs before wiping so they can be
-        # replayed into the fresh storage. Storage keys each entry as
-        # f"{branch_name}:{period}"; preserve exactly those keys.
-        preserved: dict[str, dict[str, object]] = {}
+        # replayed into the fresh storage. Use the storage API instead of
+        # hand-building keys, since ETERNITY variables canonicalize every
+        # period to the single ETERNITY storage key.
+        preserved: list[PreservedUserInput] = []
         user_input_keys = getattr(self, "_user_input_keys", None) or set()
         for variable_name, branch_name, period in user_input_keys:
             holder = self.get_holder(variable_name)
-            storage_key = f"{branch_name}:{period}"
-            stored_value = holder._memory_storage._arrays.get(storage_key)
+            stored_value = holder._memory_storage.get(period, branch_name)
             if stored_value is not None:
-                preserved.setdefault(variable_name, {})[storage_key] = stored_value
+                preserved.append(
+                    PreservedUserInput(
+                        variable_name=variable_name,
+                        branch_name=branch_name,
+                        period=period,
+                        value=stored_value,
+                    )
+                )
         # Iterate only over holders that already exist on each population —
         # lazy-creating a holder for every variable in the tax-benefit
         # system (thousands in policyengine-us) inflated the cost of
@@ -295,9 +311,13 @@ def _invalidate_all_caches(self) -> None:
                 if holder._disk_storage is not None:
                     holder._disk_storage._files = {}
         # Replay preserved user inputs so ``calculate`` still sees them.
-        for variable_name, key_to_array in preserved.items():
-            holder = self.get_holder(variable_name)
-            holder._memory_storage._arrays.update(key_to_array)
+        for user_input in preserved:
+            holder = self.get_holder(user_input.variable_name)
+            holder._memory_storage.put(
+                user_input.value,
+                user_input.period,
+                user_input.branch_name,
+            )
         for branch in self.branches.values():
             branch._invalidate_all_caches()
 
diff --git a/tests/core/test_apply_reform_preserves_user_inputs.py b/tests/core/test_apply_reform_preserves_user_inputs.py
index 6cea6016..f11c5c57 100644
--- a/tests/core/test_apply_reform_preserves_user_inputs.py
+++ b/tests/core/test_apply_reform_preserves_user_inputs.py
@@ -89,6 +89,60 @@ def apply(self):
     assert sim.calculate("age", period=period)[0] == 27
 
 
+def test_apply_reform_preserves_eternity_inputs_set_for_a_period(
+    tax_benefit_system,
+):
+    """ETERNITY inputs set for ordinary periods must survive reform apply."""
+    sim = SimulationBuilder().build_from_entities(
+        tax_benefit_system, situation_examples.single
+    )
+    period = "2017"
+    expected_person_id = np.array([123], dtype=np.int32)
+
+    sim.set_input("person_id", period, expected_person_id)
+
+    class NoOpReform(Reform):
+        def apply(self):
+            pass
+
+    sim.apply_reform(NoOpReform)
+
+    result = sim.calculate("person_id", period=period)
+    assert np.array_equal(result, expected_person_id), (
+        "apply_reform lost an ETERNITY input set through Simulation.set_input "
+        f"for {period}; got {result} instead of {expected_person_id}."
+    )
+
+
+def test_apply_reform_preserves_eternity_inputs_set_through_holder(
+    tax_benefit_system,
+):
+    """ETERNITY preservation must also cover direct ``holder.set_input``."""
+    sim = SimulationBuilder().build_from_entities(
+        tax_benefit_system, situation_examples.single
+    )
+    period = "2017"
+    expected_person_id = np.array([456], dtype=np.int32)
+
+    sim.get_holder("person_id").set_input(
+        period,
+        expected_person_id,
+        sim.branch_name,
+    )
+
+    class NoOpReform(Reform):
+        def apply(self):
+            pass
+
+    sim.apply_reform(NoOpReform)
+
+    result = sim.calculate("person_id", period=period)
+    assert np.array_equal(result, expected_person_id), (
+        "apply_reform lost an ETERNITY input set through Holder.set_input "
+        f"for {period}; got {result} instead of {expected_person_id}."
+    )
+
+
 def test_apply_reform_preserves_situation_dict_inputs(tax_benefit_system):
     """Situation-dict inputs must survive ``apply_reform`` too.