diff --git a/tests/test_optional_dependencies.py b/tests/test_optional_dependencies.py
index f8491046..610f7a0d 100644
--- a/tests/test_optional_dependencies.py
+++ b/tests/test_optional_dependencies.py
@@ -52,5 +52,58 @@ class SuiteWithoutJsonschema(unittest.TestCase):
         self.assertEqual(completed.returncode, 0, completed.stdout + completed.stderr[-2000:])
 
 
+class ScriptsNeedOnlyPyyaml(unittest.TestCase):
+    """The build scripts need the standard library and pyyaml; jsonschema is
+    validate_schemas.py's alone. CI installs jsonschema with its whole tree
+    (attrs, referencing, rpds-py), so an import of any of them would pass
+    there and fail for a contributor following the documented setup."""
+
+    def test_no_script_imports_another_package(self):
+        import ast  # noqa: PLC0415
+
+        scripts = Path(__file__).resolve().parents[1] / "scripts"
+        local = {p.stem for p in scripts.rglob("*.py")} | {
+            p.name for p in scripts.iterdir() if p.is_dir()
+        } | {"scripts", "__future__"}
+        allowed = {"yaml"}
+        found = []
+        for path in sorted(scripts.rglob("*.py")):
+            for node in ast.walk(ast.parse(path.read_text(encoding="utf-8"))):
+                if isinstance(node, ast.Import):
+                    names = [a.name.split(".")[0] for a in node.names]
+                elif isinstance(node, ast.ImportFrom) and node.level == 0 and node.module:
+                    names = [node.module.split(".")[0]]
+                else:
+                    continue
+                for name in names:
+                    if name in sys.stdlib_module_names or name in local or name in allowed:
+                        continue
+                    if name == "jsonschema" and path.name == "validate_schemas.py":
+                        continue
+                    found.append(f"{path.relative_to(scripts)}: {name}")
+        self.assertEqual(found, [])
+
+
+class TheDownloaderRunsWithoutPyyaml(unittest.TestCase):
+    """download.py is the documented Windows downloader, "stdlib only". It
+    stays so only because common.py imports yaml optionally: made a plain
+    import, every CI job (which installs pyyaml) would stay green."""
+
+    def test_help_runs_with_yaml_unimportable(self):
+        import subprocess  # noqa: PLC0415
+
+        root = Path(__file__).resolve().parents[1]
+        code = (
+            "import sys, runpy; sys.modules['yaml'] = None; "
+            "sys.path.insert(0, 'scripts'); sys.argv = ['download.py', '--help']; "
+            "runpy.run_path('scripts/download.py', run_name='__main__')"
+        )
+        completed = subprocess.run(
+            [sys.executable, "-I", "-c", code], cwd=root, capture_output=True,
+            text=True, timeout=120, check=False,
+        )
+        self.assertEqual(completed.returncode, 0, completed.stderr[-1500:])
+
+
 if __name__ == "__main__":
     unittest.main()