diff --git a/sso-mfa/k8s/lldap/GROUP-MAINTENANCE.md b/sso-mfa/k8s/lldap/GROUP-MAINTENANCE.md
index 147a3fc..5bb9be2 100644
--- a/sso-mfa/k8s/lldap/GROUP-MAINTENANCE.md
+++ b/sso-mfa/k8s/lldap/GROUP-MAINTENANCE.md
@@ -70,6 +70,21 @@ suspension (`netkingdom-`) and LLDAP administrative groups are refused by the
 existing service-group validator before login. Use their respective owners for
 those operations; the old unrestricted membership helper is not a fallback.
 
+## Sign-in diagnosis: one uid's groups
+
+To see which directory groups a login will carry (NK-WP-0051,
+[`docs/sign-in-diagnosis.md`](../../../docs/sign-in-diagnosis.md)):
+
+```bash
+python3 lldap_admin.py groups --user signin-probe-negative \
+  --root /absolute/private/custody
+```
+
+It is read-only and has no `--apply`. It prints `uid` and `groups` (sorted
+display names, never DNs, mail or credentials). An unknown uid is refused, not
+reported as an empty list. Matching the names to an OpenBao role stays a
+metadata read in ops-mason.
+
 ## Preservation, interruption and recovery
 
 The digest binds origin, operation, target and the observed directory users,
diff --git a/sso-mfa/k8s/lldap/lldap_admin.py b/sso-mfa/k8s/lldap/lldap_admin.py
index 35db314..0f846db 100644
--- a/sso-mfa/k8s/lldap/lldap_admin.py
+++ b/sso-mfa/k8s/lldap/lldap_admin.py
@@ -147,8 +147,13 @@ def snapshot(provider, token):
 
 
 def validate_operation(operation, action=None, group=None, user=None):
-    if operation not in {'bootstrap', 'membership', 'verify'}:
+    if operation not in {'bootstrap', 'membership', 'verify', 'groups'}:
         raise Refused()
+    if operation == 'groups':
+        # Read-only sign-in diagnosis preview (docs/sign-in-diagnosis.md).
+        if action is not None or group is not None or not isinstance(user, str) or not re.fullmatch(r'[a-z][a-z0-9._-]{0,127}', user):
+            raise Refused()
+        return
     if operation != 'membership':
         if any(v is not None for v in (action, group, user)):
             raise Refused()
@@ -167,7 +172,7 @@ def validate_operation(operation, action=None, group=None, user=None):
 
 def maintain(provider, base, operation, *, action=None, group=None, user=None, apply=False, expected_digest=None):
     validate_operation(operation, action, group, user)
-    if (apply and (operation == 'verify' or action == 'list')) or (expected_digest is not None and not apply):
+    if (apply and (operation in ('verify', 'groups') or action == 'list')) or (expected_digest is not None and not apply):
         raise Refused()
     if apply and (not isinstance(expected_digest, str) or not re.fullmatch(r'[0-9a-f]{64}', expected_digest)):
         raise Refused()
@@ -182,6 +187,11 @@ def maintain(provider, base, operation, *, action=None, group=None, user=None, a
                 'native_acceptance': 'unverified',
                 'remaining': ['governed_lifecycle', 'actual_login_and_mfa', 'issued_claims_and_consumer_access',
                               'session_revocation', 'independent_recovery']}
+    if operation == 'groups':
+        if user not in before['users']:
+            raise Refused()
+        names = sorted(name for name, item in before['groups'].items() if user in item['members'])
+        return {'result': 'groups_observed', 'uid': user, 'groups': names, 'native_acceptance': 'unverified'}
     if operation == 'membership' and (group not in before['groups'] or (user is not None and user not in before['users'])):
         raise Refused()
     binding = {'base': endpoint(base), 'operation': operation, 'action': action, 'group': group, 'user': user, 'before': before}
@@ -234,11 +244,13 @@ def main(argv=None):
     try:
         parser = Parser(description=__doc__)
         sub = parser.add_subparsers(dest='operation', required=True, parser_class=Parser)
-        for operation in ('bootstrap', 'membership', 'verify'):
+        for operation in ('bootstrap', 'membership', 'verify', 'groups'):
             command = sub.add_parser(operation)
             command.add_argument('--root', required=True, help='Explicit private custody directory outside Git')
             command.add_argument('--lldap-url', default='https://lldap.coulomb.social')
-            if operation != 'verify':
+            if operation == 'groups':
+                command.add_argument('--user', required=True)
+            elif operation != 'verify':
                 command.add_argument('--apply', action='store_true')
                 command.add_argument('--expected-digest')
             if operation == 'membership':
diff --git a/tests/test_lldap_group_maintenance.py b/tests/test_lldap_group_maintenance.py
index 0f34f7f..034ce95 100644
--- a/tests/test_lldap_group_maintenance.py
+++ b/tests/test_lldap_group_maintenance.py
@@ -186,6 +186,34 @@ def test_list_is_nonmutating_and_verification_does_not_claim_acceptance():
     assert provider.writes == []
 
 
+def test_groups_preview_lists_display_names_for_one_uid_without_writes():
+    provider = Directory()
+    result = admin.maintain(provider, 'https://lldap.coulomb.social', 'groups', user='tegwick')
+    assert result == {'result': 'groups_observed', 'uid': 'tegwick',
+                      'groups': ['activity-core-operators', 'lldap_admin'], 'native_acceptance': 'unverified'}
+    assert provider.writes == []
+
+
+@pytest.mark.parametrize('kwargs', [{'user': 'nobody'}, {'user': 'Bad uid'}, {'user': None},
+                                    {'user': 'tegwick', 'group': 'lldap_admin'},
+                                    {'user': 'tegwick', 'apply': True, 'expected_digest': '0' * 64}])
+def test_groups_preview_refuses_unknown_or_malformed_requests(kwargs):
+    provider = Directory()
+    with pytest.raises(admin.Refused):
+        admin.maintain(provider, 'https://lldap.coulomb.social', 'groups', **kwargs)
+    assert provider.writes == []
+
+
+def test_groups_cli_has_no_apply_and_prints_only_diagnosis_fields():
+    out = io.StringIO()
+    with patch.object(admin, 'private_password', return_value='fixture'), patch.object(admin, 'LLDAPProvisioner', return_value=Directory()), contextlib.redirect_stdout(out):
+        assert admin.main(['groups', '--root', '/fixture', '--user', 'agent-codex']) == 0
+    assert json.loads(out.getvalue()) == {'groups': ['tenant:one:users'], 'native_acceptance': 'unverified',
+                                          'result': 'groups_observed', 'uid': 'agent-codex'}
+    with contextlib.redirect_stderr(io.StringIO()):
+        assert admin.main(['groups', '--root', '/fixture', '--user', 'agent-codex', '--apply']) == 1
+
+
 @pytest.mark.parametrize('payload', [{}, {'users': [], 'groups': None}, {'users': [{'id': 'same'}, {'id': 'same'}], 'groups': []},
     {'users': [], 'groups': [{'id': True, 'displayName': 'a', 'users': []}]},
     {'users': [], 'groups': [{'id': 1, 'displayName': 'a', 'users': [{'id': 'unknown'}]}]},
diff --git a/workplans/NK-WP-0051-sign-in-diagnosis.md b/workplans/NK-WP-0051-sign-in-diagnosis.md
index 3634722..3c5f8f2 100644
--- a/workplans/NK-WP-0051-sign-in-diagnosis.md
+++ b/workplans/NK-WP-0051-sign-in-diagnosis.md
@@ -137,7 +137,7 @@ This repo only points at that requirement from `docs/sign-in-diagnosis.md`.
 
 ```task
 id: NK-WP-0051-T04
-status: todo
+status: done
 depends_on: [NK-WP-0051-T01]
 priority: high
 state_hub_task_id: "65f85a11-8512-5700-b258-feb2ead84372"