diff --git a/keras_tuner/engine/metrics_tracking.py b/keras_tuner/engine/metrics_tracking.py
index 7c612a6aa..eb181297f 100644
--- a/keras_tuner/engine/metrics_tracking.py
+++ b/keras_tuner/engine/metrics_tracking.py
@@ -404,23 +404,21 @@ def to_proto(self) -> object:
         """Create proto from MetricsTracker instance."""
         Mt = proto.MetricsTracker  # type:ignore  # noqa: PGH003
         return Mt(
-            to_register=[
-                {
-                    name: self.metrics[name].to_proto()
-                    for name in list(self.metrics.keys())
-                }
-            ]
+            metrics={
+                name: self.metrics[name].to_proto()
+                for name in list(self.metrics.keys())
+            }
         )
 
     @classmethod
     def from_proto(cls, proto: "MetricsTracker") -> "MetricsTracker":
         """Create a MetricsTracker instance from a proto."""
-        metrics: _MetricNameToHistory = {
-            name: MetricHistory.from_proto(proto.metrics[name])
+        metrics: _MetricsHistoriesInput = [
+            {name: MetricHistory.from_proto(proto.metrics[name])}
             for name in list(proto.metrics.keys())
-        }
+        ]
 
-        return cls(to_register=[metrics])
+        return cls(to_register=metrics)
 
     def _assert_exists(self, name: str) -> None:
         """Ensure that name is a metric."""
diff --git a/keras_tuner/engine/metrics_tracking_test.py b/keras_tuner/engine/metrics_tracking_test.py
index c140550fb..eb15a7a94 100644
--- a/keras_tuner/engine/metrics_tracking_test.py
+++ b/keras_tuner/engine/metrics_tracking_test.py
@@ -229,17 +229,16 @@ def test_metric_history_proto():
     ]
 
 
-def test_metricstracker_proto():
+def test_metrics_tracker_proto():
     tracker = metrics_tracking.MetricsTracker()
     tracker.register("score", direction="max")
-    tracker.append_execution_value("score", value=10)
-    tracker.append_execution_value("score", value=20)
+    tracker.append_execution_value("score", value=[10, 20])
     tracker.append_execution_value("score", value=30)
 
     proto = tracker.to_proto()
-    obs = proto.metrics["score"].get_history()
-    assert obs[0].value == [10, 20]
-    assert obs[1].value == [30]
+    executions = proto.metrics["score"].executions
+    assert executions[0].value == [10, 20]
+    assert executions[1].value == [30]
     assert proto.metrics["score"].direction
 
     new_tracker = metrics_tracking.MetricsTracker.from_proto(proto)