diff --git a/solvis_graphql_api/color_scale/compute.py b/solvis_graphql_api/color_scale/compute.py
index 8dc3b80..c27970f 100644
--- a/solvis_graphql_api/color_scale/compute.py
+++ b/solvis_graphql_api/color_scale/compute.py
@@ -64,7 +64,9 @@ def slim(new_intervals, max):
         while len(new_intervals) > MAX_LEN:
             new_intervals = new_intervals[::2]
         new_intervals = sorted(new_intervals)
-        if new_intervals[-1] < max:
+        # unreachable: the sole caller passes `max=intervals[0]` (the minimum), so the largest
+        # element is never < it. Faithfully ported dead branch from the legacy color_scale.py.
+        if new_intervals[-1] < max:  # pragma: no cover
             new_intervals.append(max)
         return new_intervals
 
diff --git a/tests/test_compute.py b/tests/test_compute.py
new file mode 100644
index 0000000..c54ac36
--- /dev/null
+++ b/tests/test_compute.py
@@ -0,0 +1,62 @@
+"""Unit tests for the graphene-free colour-scale compute (extracted at the de-graphene cleanup).
+
+Covers the maths and the error/edge branches directly, so the new `color_scale/compute.py`
+module is exercised without going through the schema.
+"""
+
+import pytest
+
+from solvis_graphql_api.color_scale import compute as C
+
+
+def test_get_normaliser_log_lin_and_error():
+    assert C.get_normaliser(10.0, 1.0, "log") is not None
+    assert C.get_normaliser(10.0, 0.0, "lin") is not None
+    with pytest.raises(RuntimeError):
+        C.get_normaliser(10.0, 1.0, "bogus")
+
+
+@pytest.mark.parametrize(
+    "vmin, vmax",
+    [
+        (1.0e-6, 1.0),  # wide multi-decade span -> slim() path
+        (0.0042171, 0.00967),  # narrow span -> interpolate() path
+        (3.2937376598254063e-18, 0.0008977324469015002),  # very wide -> slim + ensure_max
+        (1.0e-20, 1.0e-11),  # slim path where ensure_max appends the boundary
+    ],
+)
+def test_log_intervals_lengths(vmin, vmax):
+    iv = C.log_intervals(vmin, vmax)
+    assert 4 <= len(iv) <= 10
+    assert iv == sorted(iv)
+
+
+def test_log_intervals_same_decade():
+    # min_exponent == max_exponent branch (max_exponent += 1)
+    iv = C.log_intervals(1.2, 3.4)
+    assert len(iv) >= 4
+
+
+def test_compute_colour_scale_log():
+    res = C.compute_colour_scale("inferno", "log", 1.0, 1.0e-3)
+    assert res.normalisation == "log"
+    assert len(res.levels) == len(res.hexrgbs) >= 4
+    assert all(h.startswith("#") for h in res.hexrgbs)
+
+
+def test_compute_colour_scale_lin():
+    res = C.compute_colour_scale("inferno", "lin", 10.0, 0.0)
+    assert res.normalisation == "lin"
+    assert len(res.levels) == len(res.hexrgbs)
+    assert all(h.startswith("#") for h in res.hexrgbs)
+
+
+def test_compute_colour_scale_unknown_normalisation():
+    with pytest.raises(RuntimeError):
+        C.compute_colour_scale("inferno", "bogus", 10.0, 1.0)
+
+
+def test_get_colour_values_includes_none_sentinel():
+    vals = list(C.get_colour_values("inferno", 1.0, 1.0e-3, "log", (1.0e-3, 1.0e-2, None)))
+    assert vals[-1] == "x000000"  # None maps to the sentinel, not a hex
+    assert all(v.startswith("#") for v in vals[:2])
diff --git a/tests/test_schema_helpers.py b/tests/test_schema_helpers.py
new file mode 100644
index 0000000..d8cb4c4
--- /dev/null
+++ b/tests/test_schema_helpers.py
@@ -0,0 +1,37 @@
+"""Unit coverage for small Strawberry-schema helper branches.
+
+The trivial empty/None branches of the input-coercion helpers and the null-sortby skip aren't
+reached by the parity/behavioural queries (which always supply styles, filter_set_options and
+non-null sortby items), so exercise them directly.
+"""
+
+from solvis_graphql_api.schema import _fso_dict, _style_dict, schema
+
+
+def test_style_dict_none_returns_empty():
+    assert _style_dict(None) == {}
+
+
+def test_fso_dict_falsy_returns_empty():
+    assert _fso_dict(None) == {}
+
+
+_FILTER_RUPTURES_NULL_SORTBY = """
+query ($model_id: String! $location_ids: [String]! $fault_system: String!) {
+  filter_ruptures(first: 1 sortby: [null] filter: {
+      model_id: $model_id fault_system: $fault_system location_ids: $location_ids
+      radius_km: 5 minimum_mag: 8.3 minimum_rate: 1.0e-6}) {
+    total_count
+  }
+}
+"""
+
+
+def test_filter_ruptures_skips_null_sortby_item(archive_fixture):
+    # a null element in the sortby list must be skipped (not crash the resolver)
+    result = schema.execute_sync(
+        _FILTER_RUPTURES_NULL_SORTBY,
+        variable_values={"model_id": "NSHM_v1.0.0", "fault_system": "HIK", "location_ids": ["WLG"]},
+    )
+    assert not result.errors
+    assert result.data["filter_ruptures"]["total_count"] >= 0