diff --git a/dashboard/routes/batch.py b/dashboard/routes/batch.py
index 1452e9e..72fc94d 100644
--- a/dashboard/routes/batch.py
+++ b/dashboard/routes/batch.py
@@ -101,20 +101,27 @@ def get(batch_name: str, chart_index: int):
     Returns:
         Card: The chart.
     """
-    df = app.state.df_cache[batch_name]
-    metric_stats = app.state.stats_cache[batch_name]
+    # Read each cache once into a local. A concurrent toggle or refresh can
+    # clear these shared dicts mid-request, so rebuild on a miss instead of
+    # indexing the dict a second time.
+    df = app.state.df_cache.get(batch_name)
+    metric_stats = app.state.stats_cache.get(batch_name)
+    if df is None or metric_stats is None:
+        df = get_batch_data(batch_name)
+        app.state.df_cache[batch_name] = df
+        metric_stats = app.state.calculate_metric_stats(batch_name, df=df)
+
     metric_name = metric_stats[chart_index]["metric_name"]
     anomaly_rate = metric_stats[chart_index]["anomaly_rate"]
     avg_score = metric_stats[chart_index]["avg_score"]
 
-    if batch_name not in app.state.chart_cache:
-        app.state.chart_cache[batch_name] = {}
-
-    if chart_index not in app.state.chart_cache[batch_name]:
+    batch_charts = app.state.chart_cache.setdefault(batch_name, {})
+    fig = batch_charts.get(chart_index)
+    if fig is None:
         df_metric = df[df["metric_name"] == metric_name]
         df_metric = extract_metadata(df_metric, "anomaly_explanation")
         fig = ChartManager.create_chart(df_metric, chart_index)
-        app.state.chart_cache[batch_name][chart_index] = fig
+        batch_charts[chart_index] = fig
 
     modal_id = f"modal-{batch_name}-{chart_index}"
 
@@ -170,7 +177,7 @@ def get(batch_name: str, chart_index: int):
                 }
             """
             ),
-            Safe(app.state.chart_cache[batch_name][chart_index]),
+            Safe(fig),
             # Expand button overlay positioned on entire card
             Button(
                 UkIcon("expand", height=16, width=16),
diff --git a/dashboard/state.py b/dashboard/state.py
index 3c1b4c1..02f1661 100644
--- a/dashboard/state.py
+++ b/dashboard/state.py
@@ -171,11 +171,16 @@ def clear_batch_cache(self, batch_name):
         self.chart_cache.pop(batch_name, None)
         self.stats_cache.pop(batch_name, None)
 
-    def calculate_metric_stats(self, batch_name):
+    def calculate_metric_stats(self, batch_name, df=None):
         """
         Calculate the metric stats for a given batch name.
+
+        Reads the batch dataframe from the cache when ``df`` is not supplied.
+        Returns the metric stats so a caller can keep a local reference that a
+        concurrent cache clear cannot remove.
         """
-        df = self.df_cache[batch_name]
+        if df is None:
+            df = self.df_cache[batch_name]
         metric_stats = []
         for metric_name in df["metric_name"].unique():
             df_metric = df[df["metric_name"] == metric_name]
@@ -198,3 +203,4 @@ def calculate_metric_stats(self, batch_name):
             )
         metric_stats.sort(key=lambda x: (-x["anomaly_rate"], -x["avg_score"]))
         self.stats_cache[batch_name] = metric_stats
+        return metric_stats
diff --git a/tests/test_dashboard.py b/tests/test_dashboard.py
index 33dda9e..95275fa 100644
--- a/tests/test_dashboard.py
+++ b/tests/test_dashboard.py
@@ -129,3 +129,63 @@ def test_search_empty_and_invalid_patterns():
     assert "Invalid search pattern" in to_xml(metric_results(state, "demo"))
     state.search_term["demo"] = ""
     assert "/chart/0" in to_xml(metric_results(state, "demo"))
+
+
+def test_calculate_metric_stats_accepts_df_and_returns_stats():
+    state = AppState()
+    df = pd.DataFrame(
+        {
+            "metric_name": ["m1", "m1"],
+            "metric_alert": [1, 0],
+            "metric_score": [0.5, 0.7],
+            "thumbsup_sum": [1, 2],
+            "thumbsdown_sum": [0, 1],
+        }
+    )
+    # No df_cache entry: an explicit df must still produce and return the stats.
+    stats = state.calculate_metric_stats("b1", df=df)
+    assert stats == state.stats_cache["b1"]
+    assert stats[0]["metric_name"] == "m1"
+
+
+def test_chart_route_survives_cache_clear_during_render(monkeypatch):
+    """A toggle or refresh clearing the caches mid-request must not 500."""
+    import logging
+
+    from fasthtml.common import to_xml
+
+    # The batch route imports ``log`` and ``rt`` from dashboard.app; the stubbed
+    # module only supplies ``app``, so provide passthrough versions here.
+    stub_app.log = logging.getLogger("test")
+    stub_app.rt = lambda *args, **kwargs: lambda func: func
+
+    from dashboard.routes import batch as batch_route
+
+    state = AppState()
+    batch = "demo"
+    df = pd.DataFrame(
+        {
+            "metric_name": ["m1", "m1"],
+            "metric_alert": [1, 0],
+            "metric_score": [0.5, 0.7],
+            "thumbsup_sum": [1, 2],
+            "thumbsdown_sum": [0, 1],
+        }
+    )
+    state.df_cache[batch] = df
+    state.calculate_metric_stats(batch)
+    monkeypatch.setattr(batch_route.app, "state", state, raising=False)
+    monkeypatch.setattr(batch_route, "extract_metadata", lambda frame, column: frame)
+
+    def clear_everything_then_render(df_metric, chart_index):
+        # Emulate a concurrent toggle/refresh wiping the shared caches while
+        # this request is still building its chart.
+        state.df_cache.clear()
+        state.stats_cache.clear()
+        state.chart_cache.clear()
+        return "<div>chart-body</div>"
+
+    monkeypatch.setattr(batch_route.ChartManager, "create_chart", clear_everything_then_render)
+
+    html = to_xml(batch_route.get(batch, 0))
+    assert "chart-body" in html