diff --git a/docs/command-line.rst b/docs/command-line.rst
index 287b78b..9f3df4f 100644
--- a/docs/command-line.rst
+++ b/docs/command-line.rst
@@ -40,7 +40,9 @@ Diagnostic and one-off actions
    legacy or key-specific relay output sequentially for one second and test
    every configured WAVE file. Disconnect the external bell circuit and use a
    safe audio level first. ``--check`` remains non-destructive when both flags
-   are supplied.
+   are supplied. ``--test`` can run alongside ``school-bell.service`` when
+   they do not claim the same GPIO pins. If a configured pin is already in
+   use, stop the service before retrying the test.
 
 ``--debug``
    Enable verbose local logging. It can reveal hostnames and operational
diff --git a/src/school_bell/main.py b/src/school_bell/main.py
index 830e5af..e4407f0 100644
--- a/src/school_bell/main.py
+++ b/src/school_bell/main.py
@@ -115,6 +115,43 @@ def _safe_startup_message(error, config=None):
     return (message or 'No error details available')[:500]
 
 
+class _GPIOBusyDuringTest(RuntimeError):
+    """Signal that an operational test cannot claim the configured GPIO."""
+
+
+def _is_gpio_busy(error):
+    """Return whether an exception chain reports an already claimed GPIO."""
+    seen = set()
+    while error is not None and id(error) not in seen:
+        seen.add(id(error))
+        if 'gpio busy' in str(error).lower():
+            return True
+        error = error.__cause__ or error.__context__
+    return False
+
+
+def _report_gpio_busy(logger):
+    """Explain the common service/test GPIO conflict without a traceback."""
+    message = (
+        'Cannot run --test because a configured GPIO pin is already in use. '
+        'If school-bell.service is running, stop it with '
+        '`sudo systemctl stop school-bell` and try again.'
+    )
+    logger.error(
+        message,
+        extra={
+            'event': 'startup_failed',
+            'status': 'failure',
+            'fields': {
+                'exception_type': 'GPIOBusy',
+                'startup_phase': 'service_initialization',
+                'error_message': message,
+            },
+        },
+    )
+    return message
+
+
 def _configure_startup_monitoring(logger, config):
     """Best-effort remote logging setup from a parsed configuration."""
     if not isinstance(config, dict):
@@ -235,6 +272,14 @@ def _initialize_service(args, logger, prog, info):
         phase = 'service_initialization'
         return SchoolBell(**runtime_config)
     except Exception as error:
+        if (
+            phase == 'service_initialization'
+            and args.test
+            and not args.check
+            and _is_gpio_busy(error)
+        ):
+            message = _report_gpio_busy(logger)
+            raise _GPIOBusyDuringTest(message) from None
         _report_startup_failure(logger, error, phase, config)
         raise
 
@@ -365,7 +410,10 @@ def main():
     logger = init_logger(prog, args.debug)
     if args.check:
         return _check_configuration(args, logger, prog, info)
-    obj = _initialize_service(args, logger, prog, info)
+    try:
+        obj = _initialize_service(args, logger, prog, info)
+    except _GPIOBusyDuringTest:
+        return 1
 
     # play a test file or run the schedule
     try:
diff --git a/tests/test_main.py b/tests/test_main.py
index dbf9591..c56b8e4 100644
--- a/tests/test_main.py
+++ b/tests/test_main.py
@@ -138,6 +138,47 @@ def __init__(self, **config):
     }
 
 
+def test_test_mode_reports_busy_gpio_without_traceback(monkeypatch, caplog):
+    class BusySchoolBell:
+        def __init__(self, **config):
+            raise RuntimeError("'GPIO busy'")
+
+    monkeypatch.setattr(main_module, 'SchoolBell', BusySchoolBell)
+    monkeypatch.setattr(
+        main_module, '_configure_startup_monitoring', lambda *_: None
+    )
+    monkeypatch.setattr(sys, 'argv', [
+        'school-bell', json.dumps({'schedule': {}, 'wav': {}}), '--test',
+    ])
+
+    assert main_module.main() == 1
+
+    assert 'Cannot run --test' in caplog.text
+    assert 'sudo systemctl stop school-bell' in caplog.text
+    record = next(
+        record for record in caplog.records
+        if getattr(record, 'event', None) == 'startup_failed'
+    )
+    assert record.exc_info is None
+
+
+def test_non_test_gpio_busy_error_is_reraised(monkeypatch):
+    class BusySchoolBell:
+        def __init__(self, **config):
+            raise RuntimeError('GPIO busy')
+
+    monkeypatch.setattr(main_module, 'SchoolBell', BusySchoolBell)
+    monkeypatch.setattr(
+        main_module, '_configure_startup_monitoring', lambda *_: None
+    )
+    monkeypatch.setattr(
+        sys, 'argv', ['school-bell', json.dumps({'schedule': {}, 'wav': {}})]
+    )
+
+    with pytest.raises(RuntimeError, match='GPIO busy'):
+        main_module.main()
+
+
 def test_startup_event_redacts_sensitive_configuration(monkeypatch):
     records, handler = _capture_startup_events(monkeypatch)
     secret = 'do-not-log-this-token'