diff --git a/manage_labels.py b/manage_labels.py
index 056b08b..0a2f52f 100755
--- a/manage_labels.py
+++ b/manage_labels.py
@@ -4,9 +4,14 @@
 Auth follows the project convention: credentials are pulled from the macOS
 keychain via `git credential fill` (HTTPS), then sent as Basic auth.
 
-Usage:
-    ./manage_labels.py          # create labels, then apply the mapping below
-    ./manage_labels.py --dry    # print actions without writing
+Modes (default = create labels then apply the one-off MAPPING, preserving the
+original behavior):
+
+    ./manage_labels.py                     # create labels + apply MAPPING
+    ./manage_labels.py --create-labels     # idempotent label creation only
+    ./manage_labels.py --apply-mapping     # one-off MAPPING labeling only
+    ./manage_labels.py --add-label 42 chore   # add one label to one issue
+    ./manage_labels.py --dry ...           # print actions without writing
 """
 import os
 import sys
@@ -34,7 +39,7 @@ LABELS = [
      "description": "Issue is being worked on"},
 ]
 
-# issue number -> label names to apply
+# issue number -> label names to apply (one-off backfill)
 MAPPING = {
     23: ["chore"],
     22: ["chore"],
@@ -56,6 +61,11 @@ MAPPING = {
 
 BASE_URL = repo_api_url(HOST, ORG, REPO)
 
+USAGE = (
+    "usage: manage_labels.py [--dry] "
+    "[--create-labels | --apply-mapping | --add-label <issue> <label>]"
+)
+
 
 def api(method, path, auth, payload=None):
     """Thin wrapper around auth.api_request that prepends BASE_URL and
@@ -68,19 +78,15 @@ def api(method, path, auth, payload=None):
         return None
 
 
-def main():
-    dry = "--dry" in sys.argv
-    auth = get_auth_header(HOST)
-    if auth is None:
-        print("Could not get credentials from git credential fill",
-              file=sys.stderr)
-        sys.exit(1)
-
-    # 1. Existing labels -> name:id
+def _labels_by_name(auth):
+    """Return {label name: id} for the repo's existing labels."""
     existing = api("GET", "/labels?limit=100", auth) or []
-    by_name = {l["name"]: l["id"] for l in existing}
+    return {lb["name"]: lb["id"] for lb in existing}
 
-    # 2. Create missing labels
+
+def create_labels(auth, dry=False):
+    """Idempotently create the LABELS set; return the resulting name->id map."""
+    by_name = _labels_by_name(auth)
     for spec in LABELS:
         if spec["name"] in by_name:
             print(f"label exists: {spec['name']}")
@@ -92,8 +98,13 @@ def main():
         if created:
             by_name[created["name"]] = created["id"]
             print(f"created label: {created['name']} (id {created['id']})")
+    return by_name
 
-    # 3. Apply mapping
+
+def apply_mapping(auth, by_name=None, dry=False):
+    """Apply the one-off MAPPING (PUT replaces each issue's label set)."""
+    if by_name is None:
+        by_name = _labels_by_name(auth)
     for issue, names in sorted(MAPPING.items(), reverse=True):
         ids = [by_name[n] for n in names if n in by_name]
         missing = [n for n in names if n not in by_name]
@@ -105,9 +116,59 @@ def main():
         # PUT replaces the issue's labels with exactly this set (idempotent).
         res = api("PUT", f"/issues/{issue}/labels", auth, {"labels": ids})
         if res is not None:
-            applied = [l["name"] for l in res]
+            applied = [lb["name"] for lb in res]
             print(f"#{issue} labeled: {applied}")
 
 
+def add_label(auth, issue, label, dry=False):
+    """Ad-hoc: ADD a single existing label to one issue (append, not replace)."""
+    by_name = _labels_by_name(auth)
+    if label not in by_name:
+        print(f"  unknown label '{label}'; create it first (--create-labels)",
+              file=sys.stderr)
+        return False
+    if dry:
+        print(f"[dry] #{issue} += {label}")
+        return True
+    # POST appends to the issue's existing labels (does not replace).
+    res = api("POST", f"/issues/{issue}/labels", auth, {"labels": [by_name[label]]})
+    if res is not None:
+        print(f"#{issue} += {label}")
+        return True
+    return False
+
+
+def main(argv=None):
+    argv = list(sys.argv[1:] if argv is None else argv)
+    dry = "--dry" in argv or "--dry-run" in argv
+
+    auth = get_auth_header(HOST)
+    if auth is None:
+        print("Could not get credentials from git credential fill",
+              file=sys.stderr)
+        sys.exit(1)
+
+    if "--create-labels" in argv:
+        create_labels(auth, dry=dry)
+    elif "--apply-mapping" in argv:
+        apply_mapping(auth, dry=dry)
+    elif "--add-label" in argv:
+        i = argv.index("--add-label")
+        if i + 2 >= len(argv):
+            print(USAGE, file=sys.stderr)
+            sys.exit(2)
+        try:
+            issue = int(argv[i + 1])
+        except ValueError:
+            print(f"--add-label: issue must be a number, got '{argv[i + 1]}'",
+                  file=sys.stderr)
+            sys.exit(2)
+        add_label(auth, issue, argv[i + 2], dry=dry)
+    else:
+        # Default (backward compatible): create labels, then apply the mapping.
+        by_name = create_labels(auth, dry=dry)
+        apply_mapping(auth, by_name, dry=dry)
+
+
 if __name__ == "__main__":
     main()
diff --git a/tests/test_manage_labels.py b/tests/test_manage_labels.py
index c261c5e..71fee7c 100644
--- a/tests/test_manage_labels.py
+++ b/tests/test_manage_labels.py
@@ -137,5 +137,89 @@ class TestConstants(unittest.TestCase):
                              f"Label '{label['name']}' has invalid color")
 
 
+# ---------------------------------------------------------------------------
+# Modes: --create-labels / --apply-mapping / --add-label (#6)
+# ---------------------------------------------------------------------------
+class TestModes(unittest.TestCase):
+
+    def _methods(self, mock_api):
+        return [(c[0][0], c[0][1]) for c in mock_api.call_args_list]
+
+    @patch("manage_labels.get_auth_header", return_value=FAKE_AUTH)
+    @patch("manage_labels.api")
+    def test_create_labels_only_no_mapping(self, mock_api, _auth):
+        def se(method, path, auth, payload=None):
+            if method == "GET":
+                return []  # no existing labels
+            if method == "POST" and path == "/labels":
+                return {"id": 1, "name": payload["name"]}
+            return None
+        mock_api.side_effect = se
+        manage_labels.main(["--create-labels"])
+        methods = self._methods(mock_api)
+        self.assertTrue(any(m == ("POST", "/labels") for m in methods))
+        self.assertFalse(any(m[0] == "PUT" for m in methods))  # no mapping applied
+
+    @patch("manage_labels.get_auth_header", return_value=FAKE_AUTH)
+    @patch("manage_labels.api")
+    def test_apply_mapping_only_no_label_creation(self, mock_api, _auth):
+        existing = [_make_label(l["name"], i + 1)
+                    for i, l in enumerate(manage_labels.LABELS)]
+
+        def se(method, path, auth, payload=None):
+            if method == "GET":
+                return existing
+            if method == "PUT":
+                return [{"name": "applied"}]
+            return None
+        mock_api.side_effect = se
+        manage_labels.main(["--apply-mapping"])
+        methods = self._methods(mock_api)
+        self.assertFalse(any(m == ("POST", "/labels") for m in methods))
+        put_calls = [m for m in methods if m[0] == "PUT"]
+        self.assertEqual(len(put_calls), len(manage_labels.MAPPING))
+
+    @patch("manage_labels.get_auth_header", return_value=FAKE_AUTH)
+    @patch("manage_labels.api")
+    def test_add_label_appends_to_issue(self, mock_api, _auth):
+        existing = [_make_label("chore", 5)]
+
+        def se(method, path, auth, payload=None):
+            if method == "GET":
+                return existing
+            if method == "POST":
+                return [{"name": "chore"}]
+            return None
+        mock_api.side_effect = se
+        manage_labels.main(["--add-label", "42", "chore"])
+        posts = [c for c in mock_api.call_args_list
+                 if c[0][0] == "POST" and c[0][1] == "/issues/42/labels"]
+        self.assertEqual(len(posts), 1)
+        self.assertEqual(posts[0][0][3], {"labels": [5]})  # append, id 5
+        # POST appends; no PUT (which would replace the whole set).
+        self.assertFalse(any(c[0][0] == "PUT" for c in mock_api.call_args_list))
+
+    @patch("manage_labels.get_auth_header", return_value=FAKE_AUTH)
+    @patch("manage_labels.api")
+    def test_add_label_unknown_makes_no_write(self, mock_api, _auth):
+        mock_api.side_effect = lambda *a, **k: [] if a[0] == "GET" else None
+        manage_labels.main(["--add-label", "42", "ghost"])
+        # Only the GET label lookup; no POST/PUT for an undefined label.
+        self.assertTrue(all(c[0][0] == "GET" for c in mock_api.call_args_list))
+
+    @patch("manage_labels.get_auth_header", return_value=FAKE_AUTH)
+    @patch("manage_labels.api")
+    def test_add_label_dry_makes_no_write(self, mock_api, _auth):
+        mock_api.side_effect = lambda *a, **k: [_make_label("chore", 5)] if a[0] == "GET" else None
+        manage_labels.main(["--dry", "--add-label", "42", "chore"])
+        self.assertTrue(all(c[0][0] == "GET" for c in mock_api.call_args_list))
+
+    @patch("manage_labels.get_auth_header", return_value=FAKE_AUTH)
+    @patch("manage_labels.api")
+    def test_add_label_non_numeric_issue_exits(self, mock_api, _auth):
+        with self.assertRaises(SystemExit):
+            manage_labels.main(["--add-label", "notanum", "chore"])
+
+
 if __name__ == "__main__":
     unittest.main()