diff --git a/src/aituner/engine.py b/src/aituner/engine.py
index aa18895..b192ce7 100644
--- a/src/aituner/engine.py
+++ b/src/aituner/engine.py
@@ -31,10 +31,7 @@ def _serialize_flag_parts(name: str, value: Any) -> list[str]:
     if isinstance(value, bool):
         return [flag] if value else [f"--no-{_normalize_flag_name(name)}"]
     if isinstance(value, list):
-        parts: list[str] = []
-        for item in value:
-            parts.extend([flag, str(item)])
-        return parts
+        return [flag, *[str(item) for item in value]]
     return [flag, str(value)]
 
 
diff --git a/tests/test_core_flow.py b/tests/test_core_flow.py
index d1c51c2..6218f2c 100644
--- a/tests/test_core_flow.py
+++ b/tests/test_core_flow.py
@@ -9,12 +9,14 @@ from pathlib import Path
 from unittest import mock
 
 from aituner.cli import main as cli_main
+from aituner.engine import build_launch_recipe
 from aituner.http_client import _auth_headers, _openai_url, _should_bypass_proxy
 from aituner.job import append_job, build_trial_job
 from aituner.llm import _extract_response_text, build_prompt, parse_proposal_text
 from aituner.search import ThresholdProbe, binary_search_max_feasible
 from aituner.slo import RequestOutcome, evaluate_request, summarize_evaluations
 from aituner.spec import (
+    ConfigPatch,
     LLMEndpointSpec,
     Proposal,
     SpecError,
@@ -544,6 +546,22 @@ class CoreFlowTests(unittest.TestCase):
             self.assertEqual(summary["slo_pass_rate"], 0.5)
             self.assertFalse(summary["feasible"])
 
+    def test_build_launch_recipe_serializes_list_flags_once(self) -> None:
+        with tempfile.TemporaryDirectory() as tmp:
+            study = load_study_spec(_write_study_assets(Path(tmp)))
+            recipe = build_launch_recipe(
+                study.engine,
+                ConfigPatch(
+                    flag_patch={
+                        "cuda-graph-sizes": [1, 2, 4],
+                    }
+                ),
+            )
+            self.assertIn("--cuda-graph-sizes", recipe.argv)
+            flag_index = recipe.argv.index("--cuda-graph-sizes")
+            self.assertEqual(recipe.argv[flag_index + 1 : flag_index + 4], ["1", "2", "4"])
+            self.assertEqual(recipe.argv.count("--cuda-graph-sizes"), 1)
+
     def test_prepare_trace_windows_materializes_repo_local_assets(self) -> None:
         with tempfile.TemporaryDirectory() as tmp:
             tmp_path = Path(tmp)