diff --git a/tools/ci/version_support.py b/tools/ci/version_support.py
index 9f47e70a9..d2a77b002 100644
--- a/tools/ci/version_support.py
+++ b/tools/ci/version_support.py
@@ -38,6 +38,11 @@ def _expand_half_open_minor_range(spec: str) -> list[str]:
         return []
     major_min, minor_min = map(int, min_match.group(1).split("."))
     major_max, minor_max = map(int, max_match.group(1).split("."))
+    if major_min != major_max:
+        raise ValueError(
+            f"Version constraint '{spec}' spans major versions "
+            f"({major_min} -> {major_max}); only single-major ranges are supported."
+        )
     versions = []
     major, minor = major_min, minor_min
     while (major, minor) < (major_max, minor_max):
diff --git a/ultraplot/tests/test_core_versions.py b/ultraplot/tests/test_core_versions.py
index 352c71d8c..95e0e3d09 100644
--- a/ultraplot/tests/test_core_versions.py
+++ b/ultraplot/tests/test_core_versions.py
@@ -4,6 +4,8 @@
 import re
 from pathlib import Path
 
+import pytest
+
 ROOT = Path(__file__).resolve().parents[2]
 PYPROJECT = ROOT / "pyproject.toml"
 MAIN_WORKFLOW = ROOT / ".github" / "workflows" / "main.yml"
@@ -67,3 +69,12 @@ def test_publish_workflow_python_is_supported():
     match = re.search(r'python-version:\s*"(\d+\.\d+)"', text)
     assert match is not None
     assert match.group(1) in supported
+
+
+def test_expand_half_open_minor_range_rejects_cross_major_constraints():
+    """
+    Cross-major constraints (e.g. ">=3.10,<4.0") must raise ValueError immediately.
+    """
+    version_support = _load_version_support()
+    with pytest.raises(ValueError, match=r"spans major versions \(3 -> 4\)"):
+        version_support._expand_half_open_minor_range(">=3.10,<4.0")