diff --git a/tests/test_memory_extractor_vector_cross_tenant.py b/tests/test_memory_extractor_vector_cross_tenant.py
index 6b1d243..49702c1 100644
--- a/tests/test_memory_extractor_vector_cross_tenant.py
+++ b/tests/test_memory_extractor_vector_cross_tenant.py
@@ -32,16 +32,20 @@ def _load_extractor():
     return mod
 
 
-def _install_llm_stub(facts_json):
+def _install_llm_stub(monkeypatch, facts_json):
     mod = types.ModuleType("src.llm_core")
 
     async def llm_call_async(*a, **k):
         return facts_json
 
     mod.llm_call_async = llm_call_async
+    # Use monkeypatch.setitem so sys.modules is restored at teardown. A raw
+    # assignment here permanently replaced the real src.llm_core with this
+    # stripped stub, leaking "My home is in Lisbon" (and hiding _detect_provider)
+    # into every later-collected test that imports the real module.
     src_pkg = sys.modules.get("src") or types.ModuleType("src")
-    sys.modules["src"] = src_pkg
-    sys.modules["src.llm_core"] = mod
+    monkeypatch.setitem(sys.modules, "src", src_pkg)
+    monkeypatch.setitem(sys.modules, "src.llm_core", mod)
 
 
 class FakeSession:
@@ -95,7 +99,7 @@ def test_vector_match_from_other_tenant_does_not_drop_users_fact(monkeypatch):
     ])
     # The vector store reports user B's new fact as a near-duplicate of a1.
     vec = FakeVector(match_id="a1")
-    _install_llm_stub('["My home is in Lisbon"]')
+    _install_llm_stub(monkeypatch, '["My home is in Lisbon"]')
 
     memory_extractor = _load_extractor()