diff --git a/macros.aaa-pyproject-srpm b/macros.aaa-pyproject-srpm
index ee98cf3..5795c7b 100644
--- a/macros.aaa-pyproject-srpm
+++ b/macros.aaa-pyproject-srpm
@@ -20,6 +20,9 @@
 %pyproject_patch_dependency() %{expand:\\\
 %{!?1:%{error:%%pyproject_patch_dependency requires an argument}}\\\
 %{?2:%{error:%%pyproject_patch_dependency accepts exactly one argument per call}}\\\
+if [ -f %{__python3} ] && [ -f %{_rpmconfigdir}/redhat/pyproject_dependency_overrides.py ]; then
+  %{__python3} -Bs %{_rpmconfigdir}/redhat/pyproject_dependency_overrides.py '%1'
+fi
 echo '%1' >> %{_pyproject_dep_overrides}
 }
 
diff --git a/pyproject-rpm-macros.spec b/pyproject-rpm-macros.spec
index 8ef8905..3d1f51e 100644
--- a/pyproject-rpm-macros.spec
+++ b/pyproject-rpm-macros.spec
@@ -183,6 +183,7 @@ export HOSTNAME="rpmbuild"  # to speedup tox in network-less mock, see rhbz#1856
 - Add long option support for all public parametric macros
 - E.g. %%pyproject_buildrequires --no-runtime is equivalent to %%pyproject_buildrequires -R
 - %%pyproject_save_files: Allow to use --auto instead of +auto
+- %%pyproject_patch_dependency: Validate arguments early, in %%prep
 
 * Wed Apr 29 2026 Tomáš Hrnčiar <thrnciar@redhat.com> - 1.21.0-1
 - Implement extras validation
diff --git a/pyproject_dependency_overrides.py b/pyproject_dependency_overrides.py
index dbe3f03..33e0afc 100644
--- a/pyproject_dependency_overrides.py
+++ b/pyproject_dependency_overrides.py
@@ -135,3 +135,13 @@ def apply_overrides_to_specifiers(specifiers, overrides, package_name=None,
             new_specifiers.append(Specifier(f'>={value}'))
 
     return new_specifiers
+
+
+if __name__ == '__main__':
+    import sys
+    override_str = sys.argv[1].removesuffix(':br_only')
+    try:
+        parse_override_string(override_str)
+    except ValueError as e:
+        print(e, file=sys.stderr)
+        sys.exit(1)
diff --git a/test_dependency_overrides.py b/test_dependency_overrides.py
index 93b65d9..be041cf 100644
--- a/test_dependency_overrides.py
+++ b/test_dependency_overrides.py
@@ -1,5 +1,8 @@
 """Unit tests for dependency override functionality."""
 
+import subprocess
+import sys
+
 import pytest
 
 from packaging.requirements import Requirement
@@ -47,6 +50,56 @@ class TestParseOverrideString:
             parse_override_string('pkg:set_upper:not_a_version!!!')
 
 
+# ---- CLI validation (pyproject_dependency_overrides.py __main__) ----
+
+SCRIPT = 'pyproject_dependency_overrides.py'
+
+
+class TestCLIValidation:
+    def _run(self, arg):
+        return subprocess.run(
+            [sys.executable, '-Bs', SCRIPT, arg],
+            capture_output=True, text=True,
+        )
+
+    def test_valid_drop_upper(self):
+        assert self._run('numpy:drop_upper').returncode == 0
+
+    def test_valid_set_upper(self):
+        assert self._run('numpy:set_upper:2.0').returncode == 0
+
+    def test_valid_ignore_br_only(self):
+        assert self._run('pkg:ignore:br_only').returncode == 0
+
+    def test_valid_set_lower_br_only(self):
+        assert self._run('pkg:set_lower:1.0:br_only').returncode == 0
+
+    def test_invalid_no_colon(self):
+        r = self._run('justpackage')
+        assert r.returncode == 1
+        assert 'Invalid dependency override format' in r.stderr
+
+    def test_invalid_action(self):
+        r = self._run('pkg:bogus')
+        assert r.returncode == 1
+        assert 'Invalid dependency override action' in r.stderr
+
+    def test_set_upper_missing_value(self):
+        r = self._run('pkg:set_upper')
+        assert r.returncode == 1
+        assert 'requires a value' in r.stderr
+
+    def test_drop_upper_rejects_value(self):
+        r = self._run('pkg:drop_upper:2.0')
+        assert r.returncode == 1
+        assert 'does not accept a value' in r.stderr
+
+    def test_invalid_version(self):
+        r = self._run('pkg:set_upper:not_a_version!!!')
+        assert r.returncode == 1
+        assert 'Invalid version' in r.stderr
+
+
 # ---- apply_overrides_to_specifiers (shared module) ----
 
 class TestApplyOverridesToSpecifiers: