Preserve masked integer dtypes in GroupBy cumsum/cumprod

python/cudf/cudf/core/groupby/groupby.py

@@ -1273,6 +1273,17 @@ def agg(self, func=None, *args, engine=None, engine_kwargs=None, **kwargs):
                         )
                 elif agg_kind == "NUNIQUE":
                     cast_dtype = np.dtype(np.int64)
+                elif (
+                    agg_name in {"cumsum", "cumprod"}
+                    and is_pandas_nullable_extension_dtype(orig_dtype)
+                    and orig_dtype.kind in {"i", "u"}
+                ):
+                    # libcudf's SUM/PRODUCT scans promote narrow integers
+                    # to 64-bit. pandas does the same for numpy dtypes
+                    # (int8 -> int64, GH#37493) but preserves masked
+                    # extension dtypes (Int16 stays Int16, GH#58811),
+                    # wrapping on overflow.
+                    cast_dtype = orig_dtype
                 elif (
                     (
                         isinstance(agg_name, str)

python/cudf/cudf/pandas/scripts/pandas-testing-plugin.py

@@ -1838,20 +1838,13 @@ def pytest_unconfigure(config):
     "tests/groupby/test_groupby.py::test_groupby_cumsum_mask[Float32-False-val1]": "TODO: Add a reason for failure",
     "tests/groupby/test_groupby.py::test_groupby_cumsum_mask[Float64-False-val1]": "TODO: Add a reason for failure",
     "tests/groupby/test_groupby.py::test_groupby_cumsum_mask[Int16-False-val1]": "TODO: Add a reason for failure",
-    "tests/groupby/test_groupby.py::test_groupby_cumsum_mask[Int16-True-3]": "TODO: Add a reason for failure",
     "tests/groupby/test_groupby.py::test_groupby_cumsum_mask[Int32-False-val1]": "TODO: Add a reason for failure",
-    "tests/groupby/test_groupby.py::test_groupby_cumsum_mask[Int32-True-3]": "TODO: Add a reason for failure",
     "tests/groupby/test_groupby.py::test_groupby_cumsum_mask[Int64-False-val1]": "TODO: Add a reason for failure",
     "tests/groupby/test_groupby.py::test_groupby_cumsum_mask[Int8-False-val1]": "TODO: Add a reason for failure",
-    "tests/groupby/test_groupby.py::test_groupby_cumsum_mask[Int8-True-3]": "TODO: Add a reason for failure",
     "tests/groupby/test_groupby.py::test_groupby_cumsum_mask[UInt16-False-val1]": "TODO: Add a reason for failure",
-    "tests/groupby/test_groupby.py::test_groupby_cumsum_mask[UInt16-True-3]": "TODO: Add a reason for failure",
     "tests/groupby/test_groupby.py::test_groupby_cumsum_mask[UInt32-False-val1]": "TODO: Add a reason for failure",
-    "tests/groupby/test_groupby.py::test_groupby_cumsum_mask[UInt32-True-3]": "TODO: Add a reason for failure",
     "tests/groupby/test_groupby.py::test_groupby_cumsum_mask[UInt64-False-val1]": "TODO: Add a reason for failure",
-    "tests/groupby/test_groupby.py::test_groupby_cumsum_mask[UInt64-True-3]": "TODO: Add a reason for failure",
     "tests/groupby/test_groupby.py::test_groupby_cumsum_mask[UInt8-False-val1]": "TODO: Add a reason for failure",
-    "tests/groupby/test_groupby.py::test_groupby_cumsum_mask[UInt8-True-3]": "TODO: Add a reason for failure",
     "tests/groupby/test_groupby.py::test_groupby_cumsum_skipna_false": "TODO: Add a reason for failure",
     "tests/groupby/test_groupby.py::test_groupby_cumsum_timedelta64": "TODO: Add a reason for failure",
     "tests/groupby/test_groupby.py::test_groupby_groups_in_BaseGrouper": "TODO: Add a reason for failure",
@@ -1918,7 +1911,6 @@ def pytest_unconfigure(config):
     "tests/groupby/test_reductions.py::test_sum_skipna_object[False]": "Inherent cudf.pandas None-vs-NaN difference for object-dtype null (skipna logic is correct)",
     "tests/groupby/test_timegrouper.py::TestGroupBy::test_groupby_with_timegrouper": "TODO: Add a reason for failure",
     "tests/groupby/test_timegrouper.py::TestGroupBy::test_scalar_call_versus_list_call": "TODO: Add a reason for failure",
-    "tests/groupby/transform/test_transform.py::test_nan_in_cumsum_group_label": "AssertionError: Attributes of Series are different",
     "tests/indexes/base_class/test_reshape.py::TestReshape::test_insert_missing[Decimal]": "TODO: Add a reason for failure",
     "tests/indexes/categorical/test_astype.py::TestAstype::test_categorical_date_roundtrip[False]": "TODO: Add a reason for failure",
     "tests/indexes/categorical/test_astype.py::TestAstype::test_categorical_date_roundtrip[True]": "TODO: Add a reason for failure",

python/cudf/cudf/tests/groupby/test_cummulative.py

@@ -1,4 +1,4 @@
-# SPDX-FileCopyrightText: Copyright (c) 2025, NVIDIA CORPORATION.
+# SPDX-FileCopyrightText: Copyright (c) 2025-2026, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
 import numpy as np
 import pandas as pd
@@ -105,3 +105,27 @@ def test_scan_int_null_pandas_compatible(op):
     with cudf.option_context("mode.pandas_compatible", True):
         result = getattr(df_cudf.groupby("b")["a"], op)()
     assert_eq(result, expected)
+
+
+@pytest.mark.parametrize("op", ["cumsum", "cumprod"])
+def test_groupby_cumscan_masked_dtype_preserved(op):
+    # pandas preserves masked extension dtypes for groupby cum-scans
+    # (Int16 stays Int16, GH#58811) while numpy ints promote to 64-bit
+    pdf = pd.DataFrame({"a": [1, 1, 2], "b": [1, pd.NA, 2]}, dtype="Int16")
+    gdf = cudf.DataFrame(pdf)
+
+    expected = getattr(pdf.groupby("a")["b"], op)()
+    result = getattr(gdf.groupby("a")["b"], op)()
+
+    assert_eq(expected, result)
+
+
+def test_groupby_cumsum_numpy_dtype_promotes():
+    # numpy int8 promotes to int64 (pandas GH#37493)
+    pdf = pd.DataFrame({"a": [1, 1], "b": [111, 111]}, dtype="int8")
+    gdf = cudf.DataFrame(pdf)
+
+    expected = pdf.groupby("a").cumsum()
+    result = gdf.groupby("a").cumsum()
+
+    assert_eq(expected, result)