diff --git a/components/src/dynamo/replay/api.py b/components/src/dynamo/replay/api.py
index 0ac237c70b99..baf1a285538a 100644
--- a/components/src/dynamo/replay/api.py
+++ b/components/src/dynamo/replay/api.py
@@ -97,7 +97,7 @@ class _TraceReplayOptions(_CommonReplayOptions, total=False):
     trace_num_prefix_groups: int
     report_jsonl_path: str | os.PathLike[str] | None
     max_sim_time_ms: float | None
-    # Internal to retune sweeps; not part of the public API.
+    # Internal; not part of the public API.
     _planner_warmup_observations: "Sequence[TrafficObservation] | None"
     _deterministic: bool
 
@@ -420,8 +420,9 @@ def run_trace_replay(
     # Internal: a retune sweep's inner planner is warmed with the history it
     # already bucketed instead of its load_predictor_warmup_trace.
     _planner_warmup_observations=None,
-    # Internal: a retune sweep replays the baseline and every candidate with
-    # the canonical router seed and ordinal request ids (common random numbers).
+    # Internal: replay with the canonical router seed and ordinal request ids
+    # (common random numbers). Retune sweeps and every trace replay through
+    # the Sweeper's Dynamo runner set it.
     _deterministic=False,
 ) -> ReplayReport | dict[str, Any]:
     """Run trace replay.
diff --git a/components/src/dynamo/replay/simulation.py b/components/src/dynamo/replay/simulation.py
index 5b1a5bd49936..251e033199b4 100644
--- a/components/src/dynamo/replay/simulation.py
+++ b/components/src/dynamo/replay/simulation.py
@@ -409,6 +409,9 @@ def _run_trace(
         # embedded value instead of imposing the synthetic-workload default.
         if trace_format not in {"weka", "dynamo"} and trace_block_size is None:
             trace_block_size = self.trace_block_size
+        # Trace replays use ordinal request ids and the canonical router seed,
+        # so a candidate's score depends only on its configuration and the
+        # trace.
         if deployment.deployment_mode == "agg":
             return run_trace_replay(
                 trace_files=trace_files,
@@ -423,6 +426,7 @@ def _run_trace(
                 execution_model=execution_model,
                 extra_engine_args=self._engine_args(deployment, "aggregated"),
                 num_workers=deployment.num_workers,
+                _deterministic=True,
                 **common,
             )
         return run_trace_replay(
@@ -440,6 +444,7 @@ def _run_trace(
             decode_engine_args=self._engine_args(deployment, "decode"),
             num_prefill_workers=deployment.num_prefill_workers,
             num_decode_workers=deployment.num_decode_workers,
+            _deterministic=True,
             **common,
         )
 
diff --git a/components/src/dynamo/replay/tests/test_simulation.py b/components/src/dynamo/replay/tests/test_simulation.py
index 4c7c7b3b7cea..23725992c6d5 100644
--- a/components/src/dynamo/replay/tests/test_simulation.py
+++ b/components/src/dynamo/replay/tests/test_simulation.py
@@ -97,7 +97,35 @@ def _agg_deployment() -> BackendDeploymentSpec:
     )
 
 
-def test_trace_runner_preserves_current_replay_arguments(monkeypatch) -> None:
+def _disagg_deployment() -> BackendDeploymentSpec:
+    return BackendDeploymentSpec(
+        deployment_mode="disagg",
+        backend="vllm",
+        backend_version="0.11.0",
+        prefill_engine_args={
+            "worker_type": "prefill",
+            "timing_model": {"type": "polynomial"},
+        },
+        decode_engine_args={
+            "worker_type": "decode",
+            "timing_model": {"type": "polynomial"},
+        },
+        num_prefill_workers=1,
+        num_decode_workers=2,
+    )
+
+
+@pytest.mark.parametrize(
+    ("deployment", "workers"),
+    [
+        (_agg_deployment(), {"num_workers": 3}),
+        (_disagg_deployment(), {"num_prefill_workers": 1, "num_decode_workers": 2}),
+    ],
+    ids=["agg", "disagg"],
+)
+def test_trace_runner_preserves_current_replay_arguments(
+    monkeypatch, deployment, workers
+) -> None:
     seen = {}
 
     def fake_run_trace_replay(**kwargs):
@@ -113,7 +141,7 @@ def fake_run_trace_replay(**kwargs):
     monkeypatch.setattr(simulation, "KvRouterConfig", _FakeRouterConfig)
     monkeypatch.setattr(simulation, "run_trace_replay", fake_run_trace_replay)
     spec = ReplaySpec(
-        backend_deployment=_agg_deployment(),
+        backend_deployment=deployment,
         workload={
             "trace_path": "tiny.jsonl",
             "trace_format": "dynamo",
@@ -159,7 +187,8 @@ def fake_run_trace_replay(**kwargs):
 
     assert seen["trace_files"] == "tiny.jsonl"
     assert seen["trace_format"] == "dynamo"
-    assert seen["num_workers"] == 3
+    assert {name: seen[name] for name in workers} == workers
+    assert seen["_deterministic"] is True
     assert seen["router_mode"] == "kv_router"
     assert seen["planner_config"] == {"mode": "agg"}
     assert seen["arrival_speedup_ratio"] == 2.0