diff --git a/tests/test_shadow_invariant.py b/tests/test_shadow_invariant.py
new file mode 100644
index 0000000..33c8d41
--- /dev/null
+++ b/tests/test_shadow_invariant.py
@@ -0,0 +1,59 @@
+"""Static tripwire for the observation-only rule (AGENTS.md safety rule 1).
+
+The HA suite rejects service calls at runtime; this check also covers code paths
+no test reaches. A failure here means actuator code was added: that needs the
+separately approved control milestone, not an edit to this list.
+"""
+import ast
+import unittest
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parents[1] / 'custom_components' / 'heating_mpc'
+
+# Calling, registering or forwarding HA services, and entity write methods.
+FORBIDDEN_ATTRIBUTES = {
+    'services', 'async_call', 'async_register', 'call_service',
+    'async_turn_on', 'async_turn_off', 'async_toggle', 'turn_on', 'turn_off', 'toggle',
+    'async_set_temperature', 'async_set_hvac_mode', 'async_set_preset_mode',
+    'set_temperature', 'set_hvac_mode', 'set_preset_mode',
+}
+FORBIDDEN_STRINGS = {
+    'turn_on', 'turn_off', 'toggle', 'set_temperature', 'set_hvac_mode',
+    'set_preset_mode', 'call_service',
+}
+READ_ONLY_PLATFORMS = {'SENSOR', 'BINARY_SENSOR'}
+
+
+class ShadowOnlyInvariant(unittest.TestCase):
+    def test_no_actuator_code_in_integration(self):
+        problems = []
+        for path in sorted(ROOT.glob('*.py')):
+            for node in ast.walk(ast.parse(path.read_text(), str(path))):
+                where = f'{path.name}:{getattr(node, "lineno", "?")}'
+                if isinstance(node, ast.Attribute) and node.attr in FORBIDDEN_ATTRIBUTES:
+                    problems.append(f'{where} uses .{node.attr}')
+                elif isinstance(node, (ast.FunctionDef, ast.AsyncFunctionDef)) and node.name in FORBIDDEN_ATTRIBUTES:
+                    problems.append(f'{where} defines {node.name}()')
+                elif isinstance(node, ast.Constant) and node.value in FORBIDDEN_STRINGS:
+                    problems.append(f'{where} names service {node.value!r}')
+                elif (isinstance(node, ast.Attribute) and isinstance(node.value, ast.Name)
+                        and node.value.id == 'Platform' and node.attr not in READ_ONLY_PLATFORMS):
+                    problems.append(f'{where} forwards writable platform {node.attr}')
+        self.assertEqual(problems, [])
+
+    def test_tripwire_detects_a_service_call(self):
+        tree = ast.parse('await hass.services.async_call("switch", "turn_on", {})')
+        attrs = {n.attr for n in ast.walk(tree) if isinstance(n, ast.Attribute)}
+        strings = {n.value for n in ast.walk(tree) if isinstance(n, ast.Constant)}
+        self.assertTrue(attrs & FORBIDDEN_ATTRIBUTES)
+        self.assertTrue(strings & FORBIDDEN_STRINGS)
+
+    def test_only_read_only_entity_platform_files(self):
+        platforms = {'sensor', 'binary_sensor', 'switch', 'climate', 'number', 'select',
+                     'button', 'water_heater', 'valve', 'fan', 'light'}
+        present = {p.stem for p in ROOT.glob('*.py')} & platforms
+        self.assertEqual(present, {'sensor', 'binary_sensor'})
+
+
+if __name__ == '__main__':
+    unittest.main()