Stop after strong incumbent harness gains

tests/test_core_flow.py

@@ -365,6 +365,58 @@ class CoreFlowTests(unittest.TestCase):
                 ]
             )
 
+    def test_harness_strong_incumbent_guard_after_large_gain(self) -> None:
+        with tempfile.TemporaryDirectory() as tmp:
+            tmp_path = Path(tmp)
+            study_path = _write_study_assets(tmp_path)
+            study = load_study_spec(study_path)
+            state = StudyState(
+                study_id=study.study_id,
+                best_trial_id="trial-0002",
+                best_request_rate_per_gpu=0.21,
+                trials=[
+                    TrialSummary(
+                        trial_id="trial-0001",
+                        status="completed",
+                        parallel_size=1,
+                        best_request_rate=0.035,
+                        best_request_rate_per_gpu=0.035,
+                        config_patch={"env_patch": {}, "flag_patch": {}},
+                    ),
+                    TrialSummary(
+                        trial_id="trial-0002",
+                        status="completed",
+                        parallel_size=2,
+                        best_request_rate=0.42,
+                        best_request_rate_per_gpu=0.21,
+                        config_patch={
+                            "env_patch": {},
+                            "flag_patch": {
+                                "tensor-parallel-size": 2,
+                                "data-parallel-size": 1,
+                            },
+                        },
+                    ),
+                ],
+            )
+            context = build_harness_context(
+                study=study,
+                window_summary={
+                    "prompt_tokens_p95": 7628,
+                    "prompt_tokens_p99": 8102,
+                    "prompt_tail_ratio_p95_p50": 3.83,
+                },
+                state=state,
+            )
+            guard = context["convergence_guard"]["strong_incumbent"]
+            self.assertTrue(guard["guard_active"])
+            self.assertGreaterEqual(guard["incumbent_gain_vs_baseline"], 3.0)
+            self.assertTrue(
+                context["convergence_guard"][
+                    "should_stop_if_no_harness_can_justify_a_new_adjacent_probe"
+                ]
+            )
+
     def test_trace_input_length_filter_keeps_only_matching_rows(self) -> None:
         with tempfile.TemporaryDirectory() as tmp:
             tmp_path = Path(tmp)