diff --git a/python/grass/jupyter/tests/grass_jupyter_utils_test.py b/python/grass/jupyter/tests/grass_jupyter_utils_test.py
new file mode 100644
index 00000000000..1d1232efdee
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+++ b/python/grass/jupyter/tests/grass_jupyter_utils_test.py
@@ -0,0 +1,74 @@
+"""Tests of pure helper functions in grass.jupyter.utils"""
+
+import pytest
+
+from grass.jupyter.utils import get_map_name_from_d_command, get_rendering_size
+
+
+@pytest.mark.parametrize(
+    ("region", "width", "height", "expected"),
+    [
+        # Both dimensions given: returned unchanged, the region is ignored.
+        ({"n": 10, "s": 0, "e": 10, "w": 0}, 800, 600, (800, 600)),
+        # Only width given: height follows the region aspect ratio.
+        ({"n": 200, "s": 0, "e": 400, "w": 0}, 800, None, (800, 400)),
+        # Only height given: width follows the region aspect ratio.
+        ({"n": 200, "s": 0, "e": 400, "w": 0}, None, 300, (600, 300)),
+        # Neither given, taller-than-wide region: the default height is used.
+        ({"n": 400, "s": 0, "e": 200, "w": 0}, None, None, (200, 400)),
+        # Neither given, wider-than-tall region: the default width is used.
+        ({"n": 200, "s": 0, "e": 400, "w": 0}, None, None, (600, 300)),
+        # round() is half-to-even (banker's rounding): 5 * 1 / 2 = 2.5 -> 2.
+        ({"n": 1, "s": 0, "e": 2, "w": 0}, 5, None, (5, 2)),
+        # ...and 7 * 1 / 2 = 3.5 -> 4.
+        ({"n": 1, "s": 0, "e": 2, "w": 0}, 7, None, (7, 4)),
+    ],
+)
+def test_get_rendering_size(region, width, height, expected):
+    assert get_rendering_size(region, width, height) == expected
+
+
+def test_get_rendering_size_custom_defaults():
+    """Custom defaults: default_width sizes a wide region, default_height a tall one.
+
+    Only one default applies per call; the other dimension is derived from the
+    region aspect ratio.
+    """
+    wide = {"n": 100, "s": 0, "e": 400, "w": 0}
+    tall = {"n": 400, "s": 0, "e": 100, "w": 0}
+    assert get_rendering_size(
+        wide, None, None, default_width=500, default_height=400
+    ) == (500, 125)
+    assert get_rendering_size(
+        tall, None, None, default_width=500, default_height=400
+    ) == (100, 400)
+
+
+@pytest.mark.parametrize(
+    ("module", "kwargs", "expected"),
+    [
+        ("d.rast", {"map": "elevation"}, "elevation"),
+        ("d.vect", {"map": "roads"}, "roads"),
+        # Commands whose primary raster is not named "map".
+        ("d.his", {"hue": "hue_map"}, "hue_map"),
+        ("d.legend", {"raster": "elevation"}, "elevation"),
+        ("d.rgb", {"red": "red_map"}, "red_map"),
+        # d.rgb takes several maps; only the primary (red) is returned.
+        (
+            "d.rgb",
+            {"red": "red_map", "blue": "blue_map", "green": "green_map"},
+            "red_map",
+        ),
+        ("d.shade", {"shade": "relief"}, "relief"),
+        # Expected parameter absent: empty string, not an error.
+        ("d.rast", {}, ""),
+    ],
+)
+def test_get_map_name_from_d_command(module, kwargs, expected):
+    assert get_map_name_from_d_command(module, **kwargs) == expected
+
+
+@pytest.mark.parametrize("value", [123, None])
+def test_get_map_name_from_d_command_non_string_returns_none(value):
+    """A non-string value (including None) yields None rather than the raw object."""
+    assert get_map_name_from_d_command("d.rast", map=value) is None