diff --git a/src/aituner/spec.py b/src/aituner/spec.py
index 3b7fbca..e39c0a1 100644
--- a/src/aituner/spec.py
+++ b/src/aituner/spec.py
@@ -151,10 +151,12 @@ class TraceSpec:
     timestamp_field: str
     max_concurrency: int
     max_requests_per_probe: int | None = None
+    synthetic_prompt_cap_tokens: int | None = None
 
     @classmethod
     def from_dict(cls, data: Mapping[str, Any]) -> "TraceSpec":
         max_requests = data.get("max_requests_per_probe")
+        synthetic_prompt_cap = data.get("synthetic_prompt_cap_tokens")
         return cls(
             windows_path=_require_str(data.get("windows_path"), context="trace.windows_path"),
             window_id=_require_str(data.get("window_id"), context="trace.window_id"),
@@ -167,6 +169,9 @@ class TraceSpec:
                 data.get("max_concurrency", 64), context="trace.max_concurrency"
             ),
             max_requests_per_probe=int(max_requests) if max_requests is not None else None,
+            synthetic_prompt_cap_tokens=(
+                int(synthetic_prompt_cap) if synthetic_prompt_cap is not None else None
+            ),
         )
 
 
diff --git a/src/aituner/trace.py b/src/aituner/trace.py
index 8eec760..1fbd707 100644
--- a/src/aituner/trace.py
+++ b/src/aituner/trace.py
@@ -81,6 +81,14 @@ def _coerce_messages(row: Mapping[str, Any]) -> list[dict[str, Any]]:
     raise TraceError("trace row is missing chat messages/prompt text")
 
 
+def _synthetic_prompt_from_tokens(token_count: int) -> str:
+    if token_count <= 0:
+        return "hello"
+    # Keep it ASCII and structurally simple so the same trace can be replayed
+    # on any OpenAI-compatible engine without extra tokenizer assets.
+    return " ".join(["token"] * token_count)
+
+
 def _coerce_completion_tokens(row: Mapping[str, Any]) -> int | None:
     for key in ("max_completion_tokens", "max_tokens", "output_length", "completion_tokens"):
         value = row.get(key)
@@ -123,9 +131,24 @@ def load_trace_requests(study: StudySpec, *, study_spec_path: Path) -> tuple[Win
             sampling_u = row.get(study.trace.u_field, 1.0)
             if isinstance(sampling_u, bool) or not isinstance(sampling_u, (int, float)):
                 raise TraceError(f"trace row {idx} is missing numeric {study.trace.u_field}")
+            prompt_tokens_hint = _coerce_prompt_tokens(row)
+            try:
+                messages = _coerce_messages(row)
+            except TraceError:
+                capped_prompt_tokens = prompt_tokens_hint or 0
+                if study.trace.synthetic_prompt_cap_tokens is not None:
+                    capped_prompt_tokens = min(
+                        capped_prompt_tokens, study.trace.synthetic_prompt_cap_tokens
+                    )
+                messages = [
+                    {
+                        "role": "user",
+                        "content": _synthetic_prompt_from_tokens(capped_prompt_tokens),
+                    }
+                ]
             body: dict[str, Any] = {
                 "model": study.model.served_model_name,
-                "messages": _coerce_messages(row),
+                "messages": messages,
                 "stream": True,
                 "stream_options": {"include_usage": True},
             }
@@ -141,7 +164,7 @@ def load_trace_requests(study: StudySpec, *, study_spec_path: Path) -> tuple[Win
                     arrival_s=float(timestamp),
                     sampling_u=float(sampling_u),
                     body=body,
-                    prompt_tokens_hint=_coerce_prompt_tokens(row),
+                    prompt_tokens_hint=prompt_tokens_hint,
                     completion_tokens_hint=completion_tokens,
                 )
             )
diff --git a/tests/test_core_flow.py b/tests/test_core_flow.py
index 9734ee5..7b7c4bd 100644
--- a/tests/test_core_flow.py
+++ b/tests/test_core_flow.py
@@ -153,6 +153,85 @@ class CoreFlowTests(unittest.TestCase):
             self.assertIn("queueing_knee_by_bucket", prompt)
             self.assertTrue(study_root.exists())
 
+    def test_length_only_trace_rows_are_synthesized(self) -> None:
+        with tempfile.TemporaryDirectory() as tmp:
+            tmp_path = Path(tmp)
+            trace_dir = tmp_path / "trace_windows" / "traces"
+            trace_dir.mkdir(parents=True)
+            trace_path = trace_dir / "chat_len_only.jsonl"
+            with trace_path.open("w", encoding="utf-8") as handle:
+                handle.write(
+                    json.dumps(
+                        {
+                            "timestamp": 0.0,
+                            "sampling_u": 0.1,
+                            "input_length": 32,
+                            "output_length": 16
+                        }
+                    )
+                    + "\n"
+                )
+            windows_path = tmp_path / "trace_windows" / "windows.json"
+            windows_path.write_text(
+                json.dumps(
+                    {
+                        "windows": [
+                            {
+                                "window_id": "w1",
+                                "trace_type": "chat",
+                                "trace_file": "traces/chat_len_only.jsonl",
+                                "window_start": 0.0,
+                                "window_end": 10.0
+                            }
+                        ]
+                    }
+                ),
+                encoding="utf-8",
+            )
+            study_path = tmp_path / "study.json"
+            study_path.write_text(
+                json.dumps(
+                    {
+                        "study_id": "study-len-only",
+                        "hardware": {"gpu_count": 1},
+                        "model": {
+                            "model_id": "m1",
+                            "served_model_name": "dummy-model"
+                        },
+                        "engine": {
+                            "engine_name": "vllm",
+                            "exec_path": "/usr/local/bin/vllm",
+                            "host": "127.0.0.1",
+                            "port": 8000,
+                            "ready_timeout_s": 10,
+                            "request_timeout_s": 10,
+                            "healthcheck_path": "/v1/models",
+                            "launch_args": [],
+                            "base_envs": {},
+                            "base_flags": {},
+                            "tunable_envs": [],
+                            "tunable_flags": []
+                        },
+                        "trace": {
+                            "windows_path": str(windows_path),
+                            "window_id": "w1",
+                            "max_concurrency": 1,
+                            "synthetic_prompt_cap_tokens": 8
+                        },
+                        "slo": {"target_pass_rate": 0.95},
+                        "search": {"low": 0.0, "high": 1.0, "tolerance": 0.1, "max_probes": 2, "sample_seed": 1},
+                        "llm": {"system_prompt": "", "max_history_trials": 1}
+                    }
+                ),
+                encoding="utf-8",
+            )
+            study = load_study_spec(study_path)
+            _, requests = load_trace_requests(study, study_spec_path=study_path)
+            self.assertEqual(len(requests), 1)
+            message = requests[0].body["messages"][0]["content"]
+            self.assertEqual(message.count("token"), 8)
+            self.assertEqual(requests[0].body["max_tokens"], 16)
+
     def test_slo_evaluation_step_and_fixed_rules(self) -> None:
         with tempfile.TemporaryDirectory() as tmp:
             study = load_study_spec(_write_study_assets(Path(tmp)))