diff --git a/scripts/lib/semantic-retrieval.py b/scripts/lib/semantic-retrieval.py
index 5bd30e0..e45bfac 100644
--- a/scripts/lib/semantic-retrieval.py
+++ b/scripts/lib/semantic-retrieval.py
@@ -3,6 +3,7 @@ import hashlib
 import json
 import os
 from pathlib import Path
+import re
 import shutil
 import subprocess
 import sys
@@ -89,10 +90,15 @@ def check_runtime(target, state):
         path = target / name
         if path.is_symlink() or not path.is_file() or digest(path) != expected:
             raise ValueError(f'Owned semantic retrieval file changed: {name}')
-    allowed = set(state['files']) | {'.venv', 'model-cache'}
+    allowed = set(state['files']) | {'.venv', 'model-cache', '__pycache__'}
     for path in target.iterdir():
         if path.is_symlink() or path.name not in allowed:
             raise ValueError(f'Foreign or linked semantic retrieval path: {path.name}')
+        if path.name == '__pycache__' and (not path.is_dir() or any(
+                child.is_symlink() or not child.is_file() or not re.fullmatch(
+                    r'(?:server|benchmark)\.[A-Za-z0-9_-]+(?:\.opt-\d+)?\.pyc', child.name)
+                for child in path.iterdir())):
+            raise ValueError('Foreign or linked semantic retrieval path: __pycache__')
 
 
 def sync(root, home, clients, enabled, dry_run, update, summary=False):
diff --git a/scripts/tests/test-semantic-retrieval.py b/scripts/tests/test-semantic-retrieval.py
index cd3313c..6a7e77e 100644
--- a/scripts/tests/test-semantic-retrieval.py
+++ b/scripts/tests/test-semantic-retrieval.py
@@ -4,6 +4,7 @@ import io
 import json
 from contextlib import redirect_stdout
 from pathlib import Path
+import py_compile
 import tempfile
 import unittest
 from unittest.mock import patch
@@ -85,6 +86,25 @@ class SemanticRetrievalTests(unittest.TestCase):
             self.assertFalse(target.exists())
             self.assertFalse(state_path.exists())
 
+    def test_update_accepts_python_cache_but_rejects_foreign_cache_content(self):
+        with tempfile.TemporaryDirectory() as directory:
+            home = Path(directory) / 'home'
+            target = home / '.my-ai-configuration/semantic-retrieval'
+            target.mkdir(parents=True)
+            files = {}
+            for name in MODULE.FILES:
+                content = (ROOT / 'shared/retrieval' / name).read_bytes()
+                (target / name).write_bytes(content)
+                files[name] = hashlib.sha256(content).hexdigest()
+            MODULE.save_state(target.parent / 'semantic-retrieval.json', {'version': 1, 'clients': ['codex'], 'files': files}, False)
+            py_compile.compile(str(target / 'server.py'), doraise=True)
+            cache = target / '__pycache__'
+            with patch.object(MODULE, 'ensure_runtime', return_value=Path('python')), patch.object(MODULE, 'ensure_models'):
+                MODULE.sync(ROOT, home, ['codex'], True, False, True)
+            (cache / 'foreign.txt').write_text('keep')
+            with self.assertRaisesRegex(ValueError, '__pycache__'):
+                MODULE.sync(ROOT, home, ['codex'], True, False, True)
+
     def test_negative_benchmark_keeps_active_runtime(self):
         with tempfile.TemporaryDirectory() as directory:
             home = Path(directory) / 'home'