fix: recover empty validation compilations

tests/test_validation_harness.py

@@ -178,6 +178,42 @@ class ValidationHarnessTests(unittest.TestCase):
         self.assertEqual(len(metrics), 4)
         self.assertTrue(any("IMPORTANT FINALIZATION RETRY" in prompt for _, prompt in calls))
 
+    def test_empty_compiler_output_is_retried_and_never_becomes_a_502(self):
+        body = api.ChatRequest(primary_model="primary", validator_models=["validator"], harness=True, message="Write the requested result")
+        calls = []
+
+        def fake_stream(payload, request_id, cancel_event=None, parent_id=None):
+            model = payload["model"]
+            prompt = payload["messages"][-1]["content"]
+            calls.append((model, prompt))
+            if model == "validator":
+                content = "Make the result more specific."
+            elif "IMPORTANT FINALIZATION RETRY" in prompt:
+                content = "refined final answer after empty compiler output"
+            elif "VALIDATION REPORTS:" in prompt:
+                content = ""
+            else:
+                content = "primary draft"
+            return {"message": {"role": "assistant", "content": content}, "done": True}
+
+        def fake_metric(conversation_id, request_id, model, state, status=None, error=""):
+            return {"request_id": request_id, "model": model, "status": status}
+
+        with patch.object(api, "_require_installed_model", side_effect=lambda name: name), patch.object(
+            api, "_stream_chat_request", side_effect=fake_stream
+        ), patch.object(api, "_persist_metric", side_effect=fake_metric), patch.object(
+            api, "_persist_chat_stage"
+        ), patch.object(api, "_persist_chat_event"), patch.object(api, "_chat_state"):
+            final, metrics, reports = api._run_validation_harness(
+                body, "root-request", "conversation", "primary", ["validator"], threading.Event(), []
+            )
+
+        self.assertEqual(final, "refined final answer after empty compiler output")
+        self.assertEqual(len(reports), 1)
+        self.assertEqual(len(metrics), 4)
+        self.assertTrue(any("Primary returned empty output" in prompt for _, prompt in calls) or any("IMPORTANT FINALIZATION RETRY" in prompt for _, prompt in calls))
+
+
     def test_status_omits_full_catalog_by_default(self):
         with patch.object(api, "_local_tags", return_value=[]), patch.object(api, "_local_ps", return_value=[]), patch.object(
             api, "_ensure_catalog", return_value={"models": [], "families": {}, "source": "test"}