diff --git a/src/quant_engine/alpha_factors.py b/src/quant_engine/alpha_factors.py
index eaa15ea..2079417 100644
--- a/src/quant_engine/alpha_factors.py
+++ b/src/quant_engine/alpha_factors.py
@@ -216,6 +216,8 @@ def indneutralize(series: pd.Series, groups: pd.Series) -> pd.Series:
 # orchestration.  The full alpha158 formula catalogue can continue to grow,
 # while callers use one validated dispatch entry point for the first ten
 # deterministic building blocks.
+ALPHA158_PHASE1_MAX_WINDOW = 252
+
 ALPHA158_PHASE1_OPERATOR_SPECS: dict[str, dict[str, Any]] = {
     "rank": {
         "name": "rank",
@@ -316,13 +318,13 @@ def evaluate_phase1_operator(
         raise KeyError(f"operator {name!r} not registered")
 
     is_windowed = bool(ALPHA158_PHASE1_OPERATOR_SPECS[name]["windowed"])
-    if is_windowed and (
-        window is None
-        or isinstance(window, bool)
-        or not isinstance(window, int)
-        or window <= 0
-    ):
-        raise ValueError(f"window must be a positive integer for {name}")
+    if is_windowed:
+        if isinstance(window, bool) or not isinstance(window, int) or window <= 0:
+            raise ValueError(f"window must be a positive integer for {name}")
+        if window > ALPHA158_PHASE1_MAX_WINDOW:
+            raise ValueError(
+                f"window exceeds maximum supported value {ALPHA158_PHASE1_MAX_WINDOW} for {name}"
+            )
     if not is_windowed and window is not None:
         raise ValueError(f"window is not supported for {name}")
 
@@ -2888,6 +2890,7 @@ __all__ = [
     "max_pair",
     "min_pair",
     "indneutralize",
+    "ALPHA158_PHASE1_MAX_WINDOW",
     "ALPHA158_PHASE1_OPERATOR_SPECS",
     "list_phase1_operators",
     "evaluate_phase1_operator",
diff --git a/tests/test_alpha_factors.py b/tests/test_alpha_factors.py
index 78cfe2a..6c3fc09 100644
--- a/tests/test_alpha_factors.py
+++ b/tests/test_alpha_factors.py
@@ -1321,3 +1321,10 @@ def test_phase1_dispatch_rejects_unknown_or_unused_arguments():
         evaluate_phase1_operator("rank", values, window=2)
     with pytest.raises(ValueError, match="secondary"):
         evaluate_phase1_operator("rank", values, values)
+
+
+def test_phase1_dispatch_rejects_window_above_supported_limit():
+    values = pd.Series([1.0, 2.0, 3.0])
+
+    with pytest.raises(ValueError, match="maximum"):
+        evaluate_phase1_operator("ts_mean", values, window=2**63)