diff --git a/cylc/flow/network/schema.py b/cylc/flow/network/schema.py
index cfa6986189a..19298918cd3 100644
--- a/cylc/flow/network/schema.py
+++ b/cylc/flow/network/schema.py
@@ -1444,11 +1444,15 @@ class TimePoint(String):
 class WorkflowStopMode(Enum):
     """The mode used to stop a running workflow."""
 
+    # NOTE: using a different enum because:
+    # * Graphene requires special enums.
+    # * We only want to offer a subset of stop modes.
+
     # Note: contains only the REQUEST_* values from StopMode
-    Clean = StopMode.REQUEST_CLEAN
-    Kill = StopMode.REQUEST_KILL
-    Now = StopMode.REQUEST_NOW
-    NowNow = StopMode.REQUEST_NOW_NOW
+    Clean = StopMode.REQUEST_CLEAN.value
+    Kill = StopMode.REQUEST_KILL.value
+    Now = StopMode.REQUEST_NOW.value
+    NowNow = StopMode.REQUEST_NOW_NOW.value
 
     @property
     def description(self):
@@ -1722,7 +1726,7 @@ class Meta:
     class Arguments:
         workflows = List(WorkflowID, required=True)
         mode = WorkflowStopMode(
-            default_value=StopMode.REQUEST_CLEAN
+            default_value=WorkflowStopMode.Clean.value  # type: ignore
         )
         cycle_point = CyclePoint(
             description='Stop after the workflow reaches this cycle.'
diff --git a/cylc/flow/scheduler.py b/cylc/flow/scheduler.py
index 19e9ac9cf3b..ba08237943b 100644
--- a/cylc/flow/scheduler.py
+++ b/cylc/flow/scheduler.py
@@ -856,6 +856,11 @@ def command_stop(
                 pass
         else:
             # immediate shutdown
+            try:
+                mode = StopMode(mode)
+            except ValueError:
+                LOG.error(f'Invalid stop mode {mode}')
+                return
             self._set_stop(mode)
             if mode is StopMode.REQUEST_KILL:
                 self.time_next_kill = time()
diff --git a/cylc/flow/scripts/cylc.py b/cylc/flow/scripts/cylc.py
index 8d8b2007e02..036cf4c05a9 100644
--- a/cylc/flow/scripts/cylc.py
+++ b/cylc/flow/scripts/cylc.py
@@ -137,8 +137,6 @@ def get_version(long=False):
         ' use cylc graph <flow1> --diff <flow2>',
     'gscan':
         'cylc gscan has been removed, use the web UI',
-    'gui':
-        'cylc gui has been removed, use the web UI',
     'insert':
         'inserting tasks is now done automatically',
     'jobscript':
@@ -329,7 +327,7 @@ def cli_version(long_fmt=False):
     """Wrapper for get_version."""
     print(get_version(long_fmt))
     if long_fmt:
-        list_plugins()
+        print(list_plugins())
     sys.exit(0)
 
 
@@ -357,23 +355,26 @@ def list_plugins():
         for entry_point in entry_points
     }
 
+    lines = []
     if dists:
-        print('\nPlugins:')
+        lines.append('\nPlugins:')
         maxlen1 = max(len(dist.project_name) for dist in dists) + 2
         maxlen2 = max(len(dist.version) for dist in dists) + 2
         for dist in dists:
-            print(
+            lines.append(
                 f'  {dist.project_name.ljust(maxlen1)}'
                 f' {dist.version.ljust(maxlen2)}'
                 f' {dist.module_path}'
             )
 
-        print('\nEntry Points:')
+        lines.append('\nEntry Points:')
         for entry_point_name, entry_points in entry_point_groups.items():
             if entry_points:
-                print(f'  {entry_point_name}:')
+                lines.append(f'  {entry_point_name}:')
                 for entry_point in entry_points:
-                    print(f'    {entry_point}')
+                    lines.append(f'    {entry_point}')
+
+    return '\n'.join(lines)
 
 
 @contextmanager