Add generic decode-only harness guidance

tests/test_core_flow.py

@@ -486,6 +486,52 @@ class CoreFlowTests(unittest.TestCase):
             self.assertIn("There is no TTFT SLO for this study.", prompt)
             self.assertIn("decode-only", prompt)
 
+    def test_decode_only_harness_defaults_to_decode_tpot(self) -> None:
+        with tempfile.TemporaryDirectory() as tmp:
+            tmp_path = Path(tmp)
+            study_path = _write_study_assets(
+                tmp_path,
+                trace_overrides={"request_mode": "decode_only"},
+                slo_overrides={
+                    "ttft_rule": None,
+                    "tpot_rule": {"kind": "fixed_ms", "threshold_ms": 20},
+                },
+                engine_overrides={
+                    "tunable_flags": [
+                        "tensor-parallel-size",
+                        "data-parallel-size",
+                        "max-num-seqs",
+                        "max-num-batched-tokens",
+                    ],
+                    "topology_constraints": {
+                        "allowed_tensor_parallel_sizes": [1, 2, 4, 8],
+                        "allowed_data_parallel_sizes": [1, 2, 4, 8],
+                        "allowed_tp_dp_products": [8],
+                        "require_tp_dp_product_equals_gpu_count": True,
+                    },
+                },
+            )
+            study = load_study_spec(study_path)
+            window, requests = load_trace_requests(study, study_spec_path=study_path)
+            context = build_harness_context(
+                study=study,
+                window_summary=summarize_window(requests, window),
+                state=StudyState(study_id=study.study_id),
+            )
+            active = {
+                harness["knob_family"]
+                for harness in context["knob_harnesses"]
+                if harness["active_now"]
+            }
+            self.assertIn("tensor-parallel-size", active)
+            self.assertIn("data-parallel-size", active)
+            self.assertIn("max-num-seqs", active)
+            self.assertIn("max-num-batched-tokens", active)
+            self.assertIn(
+                "For decode_only studies, ignore TTFT",
+                "\n".join(context["proposal_rules"]),
+            )
+
     def test_load_study_spec_rejects_mismatched_served_model_name(self) -> None:
         with tempfile.TemporaryDirectory() as tmp:
             tmp_path = Path(tmp)