diff --git a/tests/conftest.py b/tests/conftest.py
index 5fcf02113..97e249b41 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -159,3 +159,43 @@ def _serve_test_static():
             os.environ["ODYSSEUS_TEST_STATIC_ORIGIN"] = previous_origin
         server.shutdown()
         server.server_close()
+
+
+@pytest.fixture(autouse=True)
+def _no_leaked_module_stubs():
+    """Fail the test that leaves a bare ``src.*``/``core.*`` stub behind.
+
+    Several test modules install empty stand-in modules so an import-heavy
+    production module can be loaded under the mocks above. When one of those
+    writes is not undone, the stub stays in ``sys.modules`` for the rest of the
+    session and every later test that imports the real module silently gets an
+    empty one instead. The suite still passes as a whole, because the victims
+    usually run before the leak; it only breaks under a different collection
+    order, which is why this class of bug reaches CI green.
+
+    This fixture is declared in the root conftest, so it is set up before any
+    test-module fixture and torn down after all of them — a stub that a test's
+    own teardown removes is not reported. The leaked entries are dropped here
+    as well as reported, so the failure stays attributed to the test that
+    introduced it instead of cascading into the rest of the run.
+
+    Bare stubs present before the test starts are ignored: this guards against
+    new leaks, it does not police import state the session began with.
+    """
+    from tests.helpers.import_state import bare_module_stubs, clear_module
+
+    before = bare_module_stubs()
+    yield
+    leaked = sorted(bare_module_stubs() - before)
+    if not leaked:
+        return
+    for name in leaked:
+        clear_module(name)
+    pytest.fail(
+        "test left bare module stub(s) in sys.modules: "
+        + ", ".join(leaked)
+        + ". Register the stub through monkeypatch.setitem(sys.modules, ...) "
+        "or tests.helpers.import_state.preserve_import_state so it is undone "
+        "at teardown.",
+        pytrace=False,
+    )
diff --git a/tests/helpers/import_state.py b/tests/helpers/import_state.py
index 0eea62d9d..f58c86c37 100644
--- a/tests/helpers/import_state.py
+++ b/tests/helpers/import_state.py
@@ -31,6 +31,7 @@ safe for callers that pass both a parent package and a child module.
 """
 
 import sys
+import types
 from contextlib import contextmanager
 
 _ABSENT = object()
@@ -167,3 +168,33 @@ def preserve_import_state(*module_names):
         # Phase 2: restore all parent-package attributes.
         for name, (_, saved_attr) in saved.items():
             _restore_parent_attr(name, saved_attr)
+
+
+# Names under these prefixes are the ones a leaked stub actually breaks: a
+# later test doing ``import src.x`` or ``import core.x`` silently gets the
+# empty stub instead of the real module.
+_GUARDED_PREFIXES = ("src.", "core.")
+
+
+def bare_module_stubs():
+    """Return the ``src.*``/``core.*`` names currently bound to a bare stub.
+
+    A bare stub is a plain :class:`types.ModuleType` with no on-disk
+    ``__file__`` — the object ``types.ModuleType(name)`` produces. That is the
+    same "is this a fake?" test the ``clear_fake_*`` helpers above use, so a
+    module imported from disk is never reported.
+
+    ``MagicMock`` stand-ins are deliberately out of scope: they answer every
+    attribute, so they fail loudly at use rather than silently, and several
+    test modules install them on purpose.
+    """
+    found = set()
+    for name, mod in list(sys.modules.items()):
+        if not name.startswith(_GUARDED_PREFIXES):
+            continue
+        if type(mod) is not types.ModuleType:
+            continue
+        if getattr(mod, "__file__", None):
+            continue
+        found.add(name)
+    return found
diff --git a/tests/test_scheduler_restart_doublefire.py b/tests/test_scheduler_restart_doublefire.py
index 9f0c87372..ca90c55bc 100644
--- a/tests/test_scheduler_restart_doublefire.py
+++ b/tests/test_scheduler_restart_doublefire.py
@@ -21,12 +21,21 @@ def _test_utcnow():
     return datetime.now(timezone.utc).replace(tzinfo=None)
 
 
-def _stub_heavy():
+def _stub_heavy(monkeypatch):
+    """Stub the heavy modules ``task_scheduler`` imports, for this test only.
+
+    Registered through ``monkeypatch.setitem`` so every entry is removed at
+    teardown. A bare ``sys.modules[name] = ...`` leaves an empty module behind
+    for the rest of the session, and any later test that imports the real one
+    silently gets the stub instead - a failure that only shows up under a
+    different collection order.
+    """
     for name in [
         "src.builtin_actions", "src.ai_interaction", "src.endpoint_resolver",
         "src.agent_loop", "src.session_manager",
     ]:
-        sys.modules.setdefault(name, types.ModuleType(name))
+        if name not in sys.modules:
+            monkeypatch.setitem(sys.modules, name, types.ModuleType(name))
 
 
 def _setup_isolated_db():
@@ -74,7 +83,7 @@ def test_scheduler_utcnow_preserves_naive_utc_contract():
 
 def _drive_scheduler(monkeypatch, pre_start_setup=None):
     """Build a TaskScheduler bypassing __init__ and run start() + two polls."""
-    _stub_heavy()
+    _stub_heavy(monkeypatch)
     cd, ScheduledTask, TaskRun = _setup_isolated_db()
 
     from src.task_scheduler import TaskScheduler