diff --git a/src/aituner/harness.py b/src/aituner/harness.py
index 2d7ac11..3c7a404 100644
--- a/src/aituner/harness.py
+++ b/src/aituner/harness.py
@@ -2104,12 +2104,6 @@ def _validation_exhausted_guard(
     if baseline_rate <= 0 or incumbent_rate <= 0:
         return default
     gain = incumbent_rate / baseline_rate
-    if gain < _STRONG_INCUMBENT_MIN_GAIN:
-        return {
-            **default,
-            "reason": "incumbent_gain_not_large_enough_for_validation_stop",
-            "incumbent_gain_vs_baseline": gain,
-        }
 
     best_index = next(
         (
@@ -2130,6 +2124,21 @@ def _validation_exhausted_guard(
         for item in recent_diagnostics[best_index + 1 :]
         if item.get("status") in {"completed", "failed"}
     ]
+    incumbent = next(
+        (
+            item
+            for item in recent_diagnostics
+            if item.get("trial_id") == state.best_trial_id
+        ),
+        {},
+    )
+    gmu_ceiling_incumbent = _is_gpu_memory_utilization_ceiling_incumbent(incumbent)
+    if gain < _STRONG_INCUMBENT_MIN_GAIN and not gmu_ceiling_incumbent:
+        return {
+            **default,
+            "reason": "incumbent_gain_not_large_enough_for_validation_stop",
+            "incumbent_gain_vs_baseline": gain,
+        }
     if len(after_best) < _MIN_POST_INCUMBENT_VALIDATION_TRIALS:
         return {
             **default,
@@ -2154,6 +2163,7 @@ def _validation_exhausted_guard(
     families: set[str] = set()
     for item in after_best:
         families.update(_validation_families(item))
+    families.update(_validation_families(incumbent))
     has_topology = "topology" in families
     has_runtime = bool(families & {"runtime", "max-num-seqs", "max-num-batched-tokens"})
     enough_evidence = (
@@ -2202,6 +2212,17 @@ def _validation_families(item: dict[str, Any]) -> set[str]:
     return families
 
 
+def _is_gpu_memory_utilization_ceiling_incumbent(item: dict[str, Any]) -> bool:
+    config_patch = item.get("config_patch")
+    if not isinstance(config_patch, dict):
+        return False
+    flag_patch = config_patch.get("flag_patch")
+    if not isinstance(flag_patch, dict):
+        return False
+    gmu = _parse_float_like(flag_patch.get("gpu-memory-utilization"), default=0.0)
+    return gmu >= _GMU_SAFE_CEILING - EPSILON
+
+
 def _strong_incumbent_guard(
     state: StudyState,
     recent_diagnostics: list[dict[str, Any]],
diff --git a/tests/test_core_flow.py b/tests/test_core_flow.py
index 4eaa5d9..bbc4d5a 100644
--- a/tests/test_core_flow.py
+++ b/tests/test_core_flow.py
@@ -1068,6 +1068,110 @@ class CoreFlowTests(unittest.TestCase):
             self.assertTrue(context["harness_stop"]["should_stop"])
             self.assertEqual(context["harness_stop"]["reason"], "post_incumbent_validation_exhausted")
 
+    def test_harness_stop_after_gmu_incumbent_and_non_improving_topology_validation(self) -> None:
+        with tempfile.TemporaryDirectory() as tmp:
+            tmp_path = Path(tmp)
+            study_path = _write_study_assets(
+                tmp_path,
+                engine_overrides={
+                    "tunable_flags": [
+                        "tensor-parallel-size",
+                        "data-parallel-size",
+                        "gpu-memory-utilization",
+                    ],
+                    "topology_constraints": {
+                        "allowed_tensor_parallel_sizes": [1, 2, 4, 8],
+                        "allowed_data_parallel_sizes": [1, 2],
+                        "allowed_tp_dp_products": [1, 2, 4, 8],
+                    },
+                },
+            )
+            study = load_study_spec(study_path)
+            state = StudyState(
+                study_id=study.study_id,
+                best_trial_id="trial-0007",
+                best_request_rate=6.8667,
+                best_request_rate_per_gpu=3.4333,
+                trials=[
+                    TrialSummary(
+                        trial_id="trial-0001",
+                        status="completed",
+                        best_request_rate=2.2,
+                        best_request_rate_per_gpu=2.2,
+                        config_patch={"env_patch": {}, "flag_patch": {}},
+                    ),
+                    TrialSummary(
+                        trial_id="trial-0002",
+                        status="completed",
+                        best_request_rate=6.5167,
+                        best_request_rate_per_gpu=3.2583,
+                        config_patch={
+                            "env_patch": {},
+                            "flag_patch": {"tensor-parallel-size": 2},
+                        },
+                    ),
+                    TrialSummary(
+                        trial_id="trial-0003",
+                        status="completed",
+                        best_request_rate=8.3667,
+                        best_request_rate_per_gpu=2.0917,
+                        config_patch={
+                            "env_patch": {},
+                            "flag_patch": {"tensor-parallel-size": 4},
+                        },
+                    ),
+                    TrialSummary(
+                        trial_id="trial-0007",
+                        status="completed",
+                        best_request_rate=6.8667,
+                        best_request_rate_per_gpu=3.4333,
+                        config_patch={
+                            "env_patch": {},
+                            "flag_patch": {
+                                "tensor-parallel-size": 2,
+                                "gpu-memory-utilization": 0.97,
+                            },
+                        },
+                    ),
+                    TrialSummary(
+                        trial_id="trial-0008",
+                        status="completed",
+                        best_request_rate=4.1833,
+                        best_request_rate_per_gpu=1.0458,
+                        config_patch={
+                            "env_patch": {},
+                            "flag_patch": {
+                                "tensor-parallel-size": 4,
+                                "data-parallel-size": 2,
+                            },
+                        },
+                    ),
+                    TrialSummary(
+                        trial_id="trial-0009",
+                        status="completed",
+                        best_request_rate=8.3667,
+                        best_request_rate_per_gpu=1.0458,
+                        config_patch={
+                            "env_patch": {},
+                            "flag_patch": {"tensor-parallel-size": 8},
+                        },
+                    ),
+                ],
+            )
+            context = build_harness_context(
+                study=study,
+                window_summary={"prompt_tokens_p95": 1500},
+                state=state,
+            )
+            self.assertTrue(context["harness_stop"]["should_stop"])
+            self.assertEqual(
+                context["harness_stop"]["reason"],
+                "post_incumbent_validation_exhausted",
+            )
+            proposal = build_harness_stop_proposal(context)
+            self.assertIsNotNone(proposal)
+            self.assertTrue(proposal.should_stop)
+
     def test_harness_validation_uses_full_state_baseline_when_history_window_moves(self) -> None:
         with tempfile.TemporaryDirectory() as tmp:
             tmp_path = Path(tmp)