diff --git a/bash/patchman-client b/bash/patchman-client
index d6686d3..7b07bba 100755
--- a/bash/patchman-client
+++ b/bash/patchman-client
@@ -1100,8 +1100,7 @@ if ! check_command_exists awk || \
     exit 1
 fi
 
-os_lock_dir=/var/lock
-lock_dir=$(dirname "$(readlink -f ${os_lock_dir})")/patchman
+lock_dir=/run/patchman
 mkdir -p "${lock_dir}"
 if [ ! -d "${lock_dir}" ] ; then
     echo "Lock directory does not exist, exiting: ${lock_dir}"
diff --git a/contrib/systemd/patchman-client.service b/contrib/systemd/patchman-client.service
index 5da7ea2..3f57147 100644
--- a/contrib/systemd/patchman-client.service
+++ b/contrib/systemd/patchman-client.service
@@ -18,7 +18,7 @@ NoNewPrivileges=yes
 ProtectSystem=strict
 ProtectHome=yes
 PrivateTmp=yes
-ReadOnlyPaths=/
+RuntimeDirectory=patchman
 
 [Install]
 WantedBy=multi-user.target
diff --git a/go/cmd/submit.go b/go/cmd/submit.go
index 95d1a10..9067e3f 100644
--- a/go/cmd/submit.go
+++ b/go/cmd/submit.go
@@ -86,7 +86,7 @@ The server URL and API key can be provided via:
 		}
 
 		// Acquire exclusive lock to prevent concurrent runs
-		lockDir := "/run/lock/patchman"
+		lockDir := "/run/patchman"
 		os.MkdirAll(lockDir, 0o755)
 		lockPath := filepath.Join(lockDir, "patchman.lock")
 		lockFile, err := os.OpenFile(lockPath, os.O_CREATE|os.O_WRONLY, 0o644)
diff --git a/python/patchman_client/client.py b/python/patchman_client/client.py
index 99682fd..cf3911a 100755
--- a/python/patchman_client/client.py
+++ b/python/patchman_client/client.py
@@ -42,7 +42,7 @@
 
 __version__ = '2.0.0'
 DEFAULT_CONFIG = '/etc/patchman/patchman-client.conf'
-LOCK_DIR = '/run/lock/patchman'
+LOCK_DIR = '/run/patchman'
 LOCK_FILE = os.path.join(LOCK_DIR, 'patchman.lock')
 
 
diff --git a/tests/test_systemd_lock_consistency.py b/tests/test_systemd_lock_consistency.py
new file mode 100644
index 0000000..d6010bd
--- /dev/null
+++ b/tests/test_systemd_lock_consistency.py
@@ -0,0 +1,62 @@
+"""Guard against #866: hardened unit must carve out a writable lock dir and
+all clients must agree on it."""
+import os
+import re
+
+REPO = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+SERVICE = os.path.join(REPO, 'contrib', 'systemd', 'patchman-client.service')
+BASH = os.path.join(REPO, 'bash', 'patchman-client')
+PY = os.path.join(REPO, 'python', 'patchman_client', 'client.py')
+GO = os.path.join(REPO, 'go', 'cmd', 'submit.go')
+
+
+def _read(path):
+    with open(path, encoding='utf-8') as handle:
+        return handle.read()
+
+
+def _service_values(key):
+    values = []
+    for line in _read(SERVICE).splitlines():
+        line = line.strip()
+        if line.startswith('#') or '=' not in line:
+            continue
+        name, _, value = line.partition('=')
+        if name.strip() == key:
+            values.append(value.strip())
+    return values
+
+
+def _runtime_lock_dir():
+    runtime = _service_values('RuntimeDirectory')
+    assert runtime, 'patchman-client.service must set RuntimeDirectory'
+    return '/run/' + runtime[0]
+
+
+def test_hardened_unit_carves_out_a_writable_runtime_dir():
+    # if the unit locks the filesystem down, it must provide a writable /run dir
+    protect = _service_values('ProtectSystem')
+    if protect and protect[0] == 'strict':
+        assert _service_values('RuntimeDirectory'), \
+            'ProtectSystem=strict needs RuntimeDirectory= or /run is read-only (#866)'
+    # a blanket ReadOnlyPaths=/ re-mounts the runtime dir read-only; forbid it
+    assert '/' not in _service_values('ReadOnlyPaths'), \
+        'ReadOnlyPaths=/ makes the lock dir read-only even with RuntimeDirectory (#866)'
+
+
+def test_bash_client_lock_dir_matches_runtime_dir():
+    match = re.search(r'^lock_dir=(\S+)', _read(BASH), re.MULTILINE)
+    assert match, 'could not find lock_dir in bash client'
+    assert match.group(1) == _runtime_lock_dir()
+
+
+def test_python_client_lock_dir_matches_runtime_dir():
+    match = re.search(r"^LOCK_DIR\s*=\s*'([^']+)'", _read(PY), re.MULTILINE)
+    assert match, 'could not find LOCK_DIR in python client'
+    assert match.group(1) == _runtime_lock_dir()
+
+
+def test_go_client_lock_dir_matches_runtime_dir():
+    match = re.search(r'lockDir\s*:=\s*"([^"]+)"', _read(GO))
+    assert match, 'could not find lockDir in go client'
+    assert match.group(1) == _runtime_lock_dir()