diff --git a/scripts/lab_loop.py b/scripts/lab_loop.py
index 0f4cfc9..cc0fbb2 100644
--- a/scripts/lab_loop.py
+++ b/scripts/lab_loop.py
@@ -68,8 +68,8 @@ class Gitea:
         if missing:
             raise LoopError(f"Missing repository labels: {', '.join(missing)}")
         self.request(
-            "PATCH",
-            f"/repos/{self.repo}/issues/{number}",
+            "PUT",
+            f"/repos/{self.repo}/issues/{number}/labels",
             {"labels": [label_map[name] for name in sorted(names)]},
         )
 
diff --git a/tests/test_lab_loop.py b/tests/test_lab_loop.py
index 0216acc..7e91248 100644
--- a/tests/test_lab_loop.py
+++ b/tests/test_lab_loop.py
@@ -1,9 +1,35 @@
 import unittest
 
-from scripts.lab_loop import LoopError, issue_label_names, transition, validate_owner
+from scripts.lab_loop import Gitea, LoopError, issue_label_names, transition, validate_owner
+
+
+class RecordingGitea(Gitea):
+    def __init__(self):
+        super().__init__("https://example.invalid/api/v1", "owner/repo", "secret")
+        self.calls = []
+
+    def labels(self):
+        return {"agent:timmy": 1, "state:claimed": 2, "priority:P0": 3}
+
+    def request(self, method, path, payload=None):
+        self.calls.append((method, path, payload))
 
 
 class LoopProtocolTests(unittest.TestCase):
+    def test_set_labels_uses_gitea_replace_labels_endpoint(self):
+        api = RecordingGitea()
+        api.set_labels(7, {"agent:timmy", "state:claimed", "priority:P0"})
+        self.assertEqual(
+            api.calls,
+            [
+                (
+                    "PUT",
+                    "/repos/owner/repo/issues/7/labels",
+                    {"labels": [1, 3, 2]},
+                )
+            ],
+        )
+
     def test_transition_replaces_agent_and_state_but_preserves_kind_and_priority(self):
         labels = {"agent:timmy", "state:claimed", "priority:P1", "kind:build"}
         self.assertEqual(