diff --git a/cylc/flow/scheduler_cli.py b/cylc/flow/scheduler_cli.py
index 09b405aaa21..2ab793f18e7 100644
--- a/cylc/flow/scheduler_cli.py
+++ b/cylc/flow/scheduler_cli.py
@@ -20,7 +20,7 @@
 from functools import lru_cache
 from shlex import quote
 import sys
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, List
 
 from cylc.flow import LOG, RSYNC_LOG
 from cylc.flow.exceptions import ServiceFileError
@@ -360,14 +360,22 @@ def scheduler_cli(options: 'Values', workflow_id: str) -> None:
     sys.exit(ret)
 
 
+def _protect_remote_cmd_args(cmd: List[str]) -> List[str]:
+    """Protect command args from second shell interpretation.
+
+    Escape quoting for --set="FOO='foo'" args.
+    (Separate function for unit testing.)
+    """
+    return [quote(c) for c in cmd]
+
+
 def _distribute(host):
     """Re-invoke this command on a different host if requested."""
     # Check whether a run host is explicitly specified, else select one.
     if not host:
         host = select_workflow_host()[0]
     if is_remote_host(host):
-        # quote here for (e.g.) --set='TEST="test"'
-        cmd = [quote(c) for c in sys.argv[1:]]
+        cmd = _protect_remote_cmd_args(sys.argv[1:])
         # Prevent recursive host selection
         cmd.append("--host=localhost")
         _remote_cylc_cmd(cmd, host=host)
diff --git a/tests/unit/test_scheduler_cli.py b/tests/unit/test_scheduler_cli.py
new file mode 100644
index 00000000000..f4b9cab2f63
--- /dev/null
+++ b/tests/unit/test_scheduler_cli.py
@@ -0,0 +1,24 @@
+# THIS FILE IS PART OF THE CYLC WORKFLOW ENGINE.
+# Copyright (C) NIWA & British Crown (Met Office) & Contributors.
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+"""Tests for Cylc scheduler CLI."""
+
+from cylc.flow.scheduler_cli import _protect_remote_cmd_args
+
+
+def test__protect_remote_cmd_args():
+    cmd = ['cylc', 'play', '-n', '--set=FOO="foo"', 'wf']
+    exp = ['cylc', 'play', '-n', '\'--set=FOO="foo"\'', 'wf']
+    assert _protect_remote_cmd_args(cmd) == exp