diff --git a/scripts/tests/test-prompt-budget.py b/scripts/tests/test-prompt-budget.py
new file mode 100644
index 0000000..480de75
--- /dev/null
+++ b/scripts/tests/test-prompt-budget.py
@@ -0,0 +1,50 @@
+import runpy
+import unittest
+from pathlib import Path
+
+
+MODULE = runpy.run_path(str(Path(__file__).resolve().parents[1] / 'lib' / 'prompt-budget.py'))
+
+
+class PromptBudgetTests(unittest.TestCase):
+    def setUp(self):
+        self.baseline = {
+            'regression_percent': 10,
+            'permanent_context_soft_target_tokens': 4000,
+            'permanent_context_hard_limit_tokens': 6000,
+            'metrics': {
+                metric: {'claude': 100, 'codex': 100}
+                for metric in MODULE['METRICS']
+            },
+        }
+        self.current = {
+            client: {metric: 110 for metric in MODULE['METRICS']}
+            for client in MODULE['CLIENTS']
+        }
+
+    def test_ten_percent_growth_passes(self):
+        failures, warnings = MODULE['evaluate'](self.current, self.baseline)
+        self.assertEqual(failures, [])
+        self.assertEqual(warnings, [])
+
+    def test_growth_above_ten_percent_fails(self):
+        self.current['claude']['permanent_context_tokens'] = 111
+        failures, _ = MODULE['evaluate'](self.current, self.baseline)
+        self.assertEqual(len(failures), 1)
+        self.assertIn('permanent_context_tokens claude', failures[0])
+
+    def test_permanent_context_hard_limit_fails(self):
+        self.current['codex']['permanent_context_tokens'] = 6001
+        failures, _ = MODULE['evaluate'](self.current, self.baseline)
+        self.assertTrue(any('permanent_context_tokens codex' in failure for failure in failures))
+
+    def test_soft_target_warns_in_tokens(self):
+        self.baseline['metrics']['permanent_context_tokens']['claude'] = 4000
+        self.current['claude']['permanent_context_tokens'] = 4001
+        failures, warnings = MODULE['evaluate'](self.current, self.baseline)
+        self.assertEqual(failures, [])
+        self.assertTrue(any('permanent_context_tokens claude' in warning for warning in warnings))
+
+
+if __name__ == '__main__':
+    unittest.main()