diff --git a/tests/test_web_api.py b/tests/test_web_api.py
index 6489535..1fc47f4 100644
--- a/tests/test_web_api.py
+++ b/tests/test_web_api.py
@@ -445,6 +445,30 @@ def test_session_list_includes_folder_id(server_url):
         assert s["folder_id"] is None
 
 
+def test_reap_stale_chat_sessions_passes_user_id(server_url):
+    """Regression for the daemon-thread crash where the reaper called
+    remove_chat_session(sid) without user_id and the call raised
+    TypeError on every cycle. Pre-fix this caused stale sessions to
+    accumulate forever (the reaper thread died on first invocation)."""
+    with _client(server_url) as c:
+        _register(c, "alice")
+        sid = c.post("/api/prompt",
+                     json={"prompt": "", "session_id": ""}).json()["session_id"]
+        # Force this session into the "stale + idle" state by rewinding
+        # its last_active far enough that is_stale() returns True.
+        from web import api as _apimod
+        sess = _apimod._chat_sessions.get(sid)
+        assert sess is not None, "session should be in the in-memory cache"
+        import time as _time
+        sess.last_active = _time.monotonic() - 1e9  # very old
+        # Reaper must run cleanly — pre-fix this raised
+        # TypeError: remove_chat_session() missing 1 required positional
+        # argument: 'user_id'
+        _apimod.reap_stale_chat_sessions()
+        # And it must actually evict the session
+        assert sid not in _apimod._chat_sessions
+
+
 def test_cross_user_isolation(server_url):
     with _client(server_url) as ca:
         _register(ca, "alice")
diff --git a/web/api.py b/web/api.py
index 4fd042f..9025f37 100644
--- a/web/api.py
+++ b/web/api.py
@@ -1133,11 +1133,17 @@ def get_available_models() -> list[dict]:
 
 
 def reap_stale_chat_sessions():
-    """Called periodically by server.py's reaper thread."""
-    stale: list[str] = []
+    """Called periodically by server.py's reaper thread.
+
+    `remove_chat_session` requires the owning user_id for ownership-check
+    parity with the per-user DELETE endpoint, so we capture it from the
+    cached ChatSession object — collecting `(sid, user_id)` pairs under the
+    lock and applying outside it (remove_chat_session re-acquires).
+    """
+    stale: list[tuple[str, int]] = []
     with _chat_lock:
         for sid, session in _chat_sessions.items():
             if session.is_stale() and session.is_idle():
-                stale.append(sid)
-    for sid in stale:
-        remove_chat_session(sid)
+                stale.append((sid, session.user_id))
+    for sid, user_id in stale:
+        remove_chat_session(sid, user_id)