diff --git a/backend/internal/garmin/pyscript/tests/test_wrapper.py b/backend/internal/garmin/pyscript/tests/test_wrapper.py
index cb1ab51..8c1c32f 100644
--- a/backend/internal/garmin/pyscript/tests/test_wrapper.py
+++ b/backend/internal/garmin/pyscript/tests/test_wrapper.py
@@ -1,5 +1,6 @@
 import os
 import queue
+import threading
 from unittest.mock import MagicMock, patch
 
 import pytest
@@ -146,13 +147,62 @@ def test_startup_login_missing_credentials_returns_immediately():
 
 
 def test_startup_login_times_out_without_blocking():
+    # _startup_login now waits on its own private, function-local queue
+    # (never the shared wrapper._login_result_queue -- see the
+    # no-shared-queue-leakage test below), so forcing its timeout path
+    # means intercepting the queue.Queue() constructor it calls internally
+    # rather than patching a module-level queue object.
     env = {"GARMIN_EMAIL": "test@example.com", "GARMIN_PASSWORD": "secret"}
     with patch.dict("os.environ", env):
         with patch("wrapper.Garmin") as mock_garmin_cls:
             mock_garmin_cls.return_value = MagicMock()
-            with patch.object(wrapper._login_result_queue, "get", side_effect=queue.Empty):
+            with patch("wrapper.queue.Queue") as mock_queue_cls:
+                mock_queue_cls.return_value.get.side_effect = queue.Empty
                 # Should return promptly (bounded by the 10s timeout passed to
                 # queue.get, which is mocked here to raise immediately) rather
                 # than blocking main()'s stdin dispatch loop from starting.
                 wrapper._startup_login()
     assert wrapper._auth_state == "unauthenticated"
+
+
+def test_startup_login_does_not_leak_into_shared_authenticate_queue():
+    """Regression test: _startup_login used to push its background thread's
+    result onto the same module-level _login_result_queue that
+    _handle_authenticate/_handle_complete_mfa share for the MFA handoff. A
+    startup login that was still in flight when a user's first explicit
+    'authenticate' call came in could have that call accidentally dequeue
+    the startup thread's stale result instead of its own fresh one.
+    _startup_login now uses its own private queue, so a still-in-flight
+    startup attempt can never interfere with a later authenticate() call."""
+    release = threading.Event()
+
+    env = {"GARMIN_EMAIL": "test@example.com", "GARMIN_PASSWORD": "secret"}
+    with patch.dict("os.environ", env):
+        with patch("wrapper.Garmin") as mock_garmin_cls:
+            # A startup login whose underlying garminconnect call is still
+            # blocked (simulating "slow to resolve") when _startup_login's
+            # own bounded wait (mocked to time out immediately, so this test
+            # doesn't need to sleep the real 10s) returns.
+            slow_client = MagicMock()
+            slow_client.login.side_effect = lambda **kwargs: release.wait(5)
+            mock_garmin_cls.return_value = slow_client
+            with patch("wrapper.queue.Queue") as mock_queue_cls:
+                mock_queue_cls.return_value.get.side_effect = queue.Empty
+                wrapper._startup_login()
+
+    assert wrapper._auth_state == "unauthenticated"
+    # Nothing from the still-in-flight startup attempt ever touches the
+    # shared queue that authenticate()/complete_mfa() rely on.
+    assert wrapper._login_result_queue.empty()
+
+    release.set()  # let the stale startup thread finish harmlessly in the background
+
+    # A fresh, unrelated authenticate() call must get its own result, not
+    # anything left over from the startup attempt above.
+    with patch.dict("os.environ", env):
+        with patch("wrapper.Garmin") as mock_garmin_cls2:
+            mock_garmin_cls2.return_value = MagicMock()
+            resp = wrapper.dispatch({"id": 20, "cmd": "authenticate"})
+
+    assert resp == {"id": 20, "result": {"status": "success", "message": "Authenticated successfully."}}
+    assert wrapper._auth_state == "authenticated"
diff --git a/backend/internal/garmin/pyscript/wrapper.py b/backend/internal/garmin/pyscript/wrapper.py
index 6d786c1..533dab4 100644
--- a/backend/internal/garmin/pyscript/wrapper.py
+++ b/backend/internal/garmin/pyscript/wrapper.py
@@ -42,7 +42,17 @@ def _startup_login():
     must never block indefinitely here -- if it times out, the thread
     keeps running and will update _auth_state whenever it eventually
     finishes (success or failure), same as today, just without wedging
-    the whole subprocess unresponsive in the meantime."""
+    the whole subprocess unresponsive in the meantime.
+
+    Deliberately uses its own private, function-local result queue rather
+    than the module-level _login_result_queue that _handle_authenticate and
+    _handle_complete_mfa share -- those two are legitimately two halves of
+    one explicit, MFA-capable login flow and must share a queue for the MFA
+    handoff to work, but this is a plain background tokenstore resume with
+    no MFA involved. Sharing the queue here would let this thread's stale
+    result (arriving after the 10s timeout below) be dequeued by an
+    unrelated, later authenticate/complete_mfa call instead of that call's
+    own fresh result."""
     global _client, _auth_state
     email = os.environ.get("GARMIN_EMAIL")
     password = os.environ.get("GARMIN_PASSWORD")
@@ -50,18 +60,19 @@ def _startup_login():
         return
 
     _client = Garmin(email, password)
+    result_queue = queue.Queue()  # private to this call, never shared with authenticate/complete_mfa
 
     def _do_login():
         try:
             _client.login(tokenstore=TOKENSTORE)
-            _login_result_queue.put(("success", None))
+            result_queue.put(("success", None))
         except Exception as exc:
-            _login_result_queue.put(("error", str(exc)))
+            result_queue.put(("error", str(exc)))
 
     threading.Thread(target=_do_login, daemon=True).start()
 
     try:
-        status, err = _login_result_queue.get(timeout=10)
+        status, err = result_queue.get(timeout=10)
         if status == "success":
             _auth_state = "authenticated"
         else: