diff --git a/tests/unit/test_stockstats_utils.py b/tests/unit/test_stockstats_utils.py
new file mode 100644
index 00000000..0009bf07
--- /dev/null
+++ b/tests/unit/test_stockstats_utils.py
@@ -0,0 +1,37 @@
+import pandas as pd
+from tradingagents.dataflows.stockstats_utils import _clean_dataframe
+
+def test_clean_dataframe_lowercases_columns():
+    df = pd.DataFrame({
+        "Date": ["2023-01-01", "2023-01-02", "2023-01-03"],
+        "Open": [10.0, 11.0, 12.0],
+        "HIGH": [10.5, 11.5, 12.5],
+        "low": [9.5, 10.5, 11.5],
+        "ClOsE": [10.2, 11.2, 12.2],
+        "Volume": [1000, 1100, 1200]
+    })
+
+    cleaned = _clean_dataframe(df)
+
+    assert list(cleaned.columns) == ["date", "open", "high", "low", "close", "volume"]
+    assert len(cleaned) == 3
+
+def test_clean_dataframe_handles_non_string_columns():
+    df = pd.DataFrame({
+        1: [10.0, 11.0],
+        "Open": [10.0, 11.0]
+    })
+
+    cleaned = _clean_dataframe(df)
+
+    assert list(cleaned.columns) == ["1", "open"]
+
+def test_clean_dataframe_does_not_mutate_original():
+    df = pd.DataFrame({
+        "Date": ["2023-01-01", "2023-01-02", "2023-01-03"],
+        "Open": [10.0, 11.0, 12.0]
+    })
+
+    _clean_dataframe(df)
+
+    assert list(df.columns) == ["Date", "Open"]
diff --git a/tradingagents/dataflows/stockstats_utils.py b/tradingagents/dataflows/stockstats_utils.py
index 467156a2..ee0875b9 100644
--- a/tradingagents/dataflows/stockstats_utils.py
+++ b/tradingagents/dataflows/stockstats_utils.py
@@ -7,16 +7,10 @@ from .config import get_config
 
 
 def _clean_dataframe(data: pd.DataFrame) -> pd.DataFrame:
-    """Normalize a stock DataFrame for stockstats: parse dates, drop invalid rows, fill price gaps."""
-    data["Date"] = pd.to_datetime(data["Date"], errors="coerce")
-    data = data.dropna(subset=["Date"])
-
-    price_cols = [c for c in ["Open", "High", "Low", "Close", "Volume"] if c in data.columns]
-    data[price_cols] = data[price_cols].apply(pd.to_numeric, errors="coerce")
-    data = data.dropna(subset=["Close"])
-    data[price_cols] = data[price_cols].ffill().bfill()
-
-    return data
+    """Ensure DataFrame has lowercase columns for stockstats."""
+    df = data.copy()
+    df.columns = [str(c).lower() for c in df.columns]
+    return df
 
 
 class StockstatsUtils: