diff --git a/screen_locker/install_systemd.sh b/screen_locker/install_systemd.sh
index 3fd3f83..ff9a7db 100755
--- a/screen_locker/install_systemd.sh
+++ b/screen_locker/install_systemd.sh
@@ -4,10 +4,8 @@
 SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 SCREEN_LOCK_PATH="$SCRIPT_DIR/screen_lock.py"
 SERVICE_FILE="$SCRIPT_DIR/workout-locker.service"
-TIMER_FILE="$SCRIPT_DIR/workout-locker.timer"
 USER_SERVICE_DIR="$HOME/.config/systemd/user"
 SERVICE_NAME="workout-locker.service"
-TIMER_NAME="workout-locker.timer"
 
 # Check if service is already installed
 if [ -f "$USER_SERVICE_DIR/$SERVICE_NAME" ]; then
@@ -26,9 +24,14 @@ fi
 # Create user systemd directory if it doesn't exist
 mkdir -p "$USER_SERVICE_DIR"
 
-# Copy service and timer files to user systemd directory
+# Remove old timer if it was previously installed
+if systemctl --user is-active "workout-locker.timer" &>/dev/null; then
+	systemctl --user disable --now "workout-locker.timer" 2>/dev/null || true
+fi
+rm -f "$USER_SERVICE_DIR/workout-locker.timer"
+
+# Copy service file to user systemd directory
 cp "$SERVICE_FILE" "$USER_SERVICE_DIR/$SERVICE_NAME"
-cp "$TIMER_FILE" "$USER_SERVICE_DIR/$TIMER_NAME"
 
 # Update paths in the service file to use absolute paths
 REPO_ROOT="$(cd "$SCRIPT_DIR/../.." && pwd)"
@@ -39,18 +42,16 @@ sed -i "s|ExecStart=/usr/bin/python3.*|ExecStart=/usr/bin/python3 -m python_pkg.
 # Reload systemd daemon
 systemctl --user daemon-reload
 
-# Enable the service to start on login and the timer for periodic checks
+# Enable the service to start on login (one-shot, no periodic timer)
 systemctl --user enable "$SERVICE_NAME"
-systemctl --user enable --now "$TIMER_NAME"
 
 echo "✓ Workout locker service installed"
 echo "✓ Service will start automatically on next login"
 echo ""
 echo "To start now: systemctl --user start workout-locker"
 echo "To check status: systemctl --user status workout-locker"
-echo "To check timer: systemctl --user list-timers workout-locker.timer"
 echo "To stop: systemctl --user stop workout-locker"
-echo "To disable autostart: systemctl --user disable workout-locker workout-locker.timer"
+echo "To disable autostart: systemctl --user disable workout-locker"
 
 # Check autostart installation status
 echo ""
diff --git a/screen_locker/screen_lock.py b/screen_locker/screen_lock.py
index 43f5a95..462cc2c 100755
--- a/screen_locker/screen_lock.py
+++ b/screen_locker/screen_lock.py
@@ -50,6 +50,21 @@ __all__ = [
 _logger = logging.getLogger(__name__)
 
 
+def _assert_not_under_pytest() -> None:
+    """Raise if the screen locker is being created inside a pytest run.
+
+    Defence-in-depth: prevents a real fullscreen Tk window from locking
+    the user's screen when tests forget to mock ``tk.Tk``.
+    The check is cheap (one dict lookup) and only fires during testing.
+    """
+    if "pytest" in sys.modules and getattr(tk, "__name__", "") == "tkinter":
+        msg = (
+            "SAFETY: ScreenLocker.__init__ called under pytest with "
+            "real tkinter — tk.Tk is not mocked"
+        )
+        raise RuntimeError(msg)
+
+
 class ScreenLocker(
     ShutdownMixin,
     PhoneVerificationMixin,
@@ -64,6 +79,7 @@ class ScreenLocker(
         verify_only: bool = False,
     ) -> None:
         """Initialize screen locker with optional demo mode."""
+        _assert_not_under_pytest()
         script_dir = Path(__file__).resolve().parent
         self.log_file = script_dir / "workout_log.json"
         self.verify_only = verify_only
diff --git a/screen_locker/tests/conftest.py b/screen_locker/tests/conftest.py
index c93d70c..7c54947 100644
--- a/screen_locker/tests/conftest.py
+++ b/screen_locker/tests/conftest.py
@@ -1,4 +1,12 @@
-"""Shared fixtures and helpers for screen_locker tests."""
+"""Shared fixtures and helpers for screen_locker tests.
+
+Safety:
+  ``_block_real_tk_and_exit`` (autouse) replaces the **entire** ``tk``
+  module reference inside ``screen_lock`` with a MagicMock and stubs
+  ``sys.exit``.  This makes it physically impossible for any test to
+  create a real Tk root window, go fullscreen, or grab input — even if
+  the test forgets to request the explicit ``mock_tk`` fixture.
+"""
 
 from __future__ import annotations
 
@@ -12,7 +20,40 @@ import pytest
 from python_pkg.screen_locker.screen_lock import ScreenLocker
 
 if TYPE_CHECKING:
-    from collections.abc import Generator
+    from collections.abc import Generator, Iterator
+
+
+def _make_mock_tk() -> MagicMock:
+    """Build a MagicMock that stands in for the ``tkinter`` module."""
+    mock = MagicMock()
+    mock_root = MagicMock()
+    mock_root.winfo_screenwidth.return_value = 1920
+    mock_root.winfo_screenheight.return_value = 1080
+    mock.Tk.return_value = mock_root
+
+    mock_frame = MagicMock()
+    mock_frame.winfo_children.return_value = []
+    mock.Frame.return_value = mock_frame
+
+    # Keep real TclError so ``except tk.TclError`` still works.
+    mock.TclError = tk.TclError
+    return mock
+
+
+@pytest.fixture(autouse=True)
+def _block_real_tk_and_exit() -> Iterator[None]:
+    """Replace the whole ``tk`` module and ``sys.exit`` for every test.
+
+    Patching the entire module (not just ``tk.Tk``) ensures that
+    **nothing** in tkinter can touch the real display server.
+    """
+    mock = _make_mock_tk()
+
+    with (
+        patch("python_pkg.screen_locker.screen_lock.tk", mock),
+        patch("python_pkg.screen_locker.screen_lock.sys.exit"),
+    ):
+        yield
 
 
 @pytest.fixture
diff --git a/screen_locker/tests/test_init_and_log.py b/screen_locker/tests/test_init_and_log.py
index a781e69..c74e110 100644
--- a/screen_locker/tests/test_init_and_log.py
+++ b/screen_locker/tests/test_init_and_log.py
@@ -10,12 +10,29 @@ from unittest.mock import MagicMock, patch
 
 import pytest
 
+from python_pkg.screen_locker.screen_lock import _assert_not_under_pytest
 from python_pkg.screen_locker.tests.conftest import create_locker
 
 if TYPE_CHECKING:
     from pathlib import Path
 
 
+class TestAssertNotUnderPytest:
+    """Tests for the _assert_not_under_pytest runtime guard."""
+
+    def test_raises_when_tk_is_real(self) -> None:
+        """Guard fires if tk.Tk is the real tkinter class under pytest."""
+        with (
+            patch("python_pkg.screen_locker.screen_lock.tk", tk),
+            pytest.raises(RuntimeError, match="SAFETY"),
+        ):
+            _assert_not_under_pytest()
+
+    def test_silent_when_tk_is_mocked(self) -> None:
+        """Guard stays silent when tk is already mocked (normal test run)."""
+        _assert_not_under_pytest()
+
+
 class TestScreenLockerInit:
     """Tests for ScreenLocker initialization."""
 
diff --git a/screen_locker/workout-locker.timer b/screen_locker/workout-locker.timer
deleted file mode 100644
index 4a01809..0000000
--- a/screen_locker/workout-locker.timer
+++ /dev/null
@@ -1,10 +0,0 @@
-[Unit]
-Description=Periodically check if workout was done today
-
-[Timer]
-OnBootSec=5s
-OnUnitActiveSec=15min
-Persistent=true
-
-[Install]
-WantedBy=timers.target