diff --git a/scripts/prepare_trace_windows.py b/scripts/prepare_trace_windows.py
index beca987..09d2d48 100644
--- a/scripts/prepare_trace_windows.py
+++ b/scripts/prepare_trace_windows.py
@@ -203,24 +203,29 @@ def extract_windows(windows: list[dict[str, Any]], *, sample_seed: int) -> dict[
                 if not trace_raw or not prompt_raw:
                     continue
                 trace_row = json.loads(trace_raw)
-                prompt_row = json.loads(prompt_raw)
-                merged = _merge_trace_and_prompt(trace_row, prompt_row)
-                timestamp = float(merged.get("timestamp") or 0.0)
+                timestamp = float(trace_row.get("timestamp") or 0.0)
+                matched_window: dict[str, Any] | None = None
                 for window in bucket:
                     start = float(window["window_start"])
                     end = float(window["window_end"])
                     if start <= timestamp < end:
-                        out = dict(merged)
-                        out["source_timestamp"] = timestamp
-                        out["timestamp"] = timestamp - start
-                        out["sampling_u"] = stable_uniform(
-                            seed=sample_seed,
-                            window_id=str(window["window_id"]),
-                            index=len(extracted[str(window["window_id"])]),
-                            row=merged,
-                        )
-                        extracted[str(window["window_id"])].append(out)
+                        matched_window = window
                         break
+                if matched_window is None:
+                    continue
+                prompt_row = json.loads(prompt_raw)
+                merged = _merge_trace_and_prompt(trace_row, prompt_row)
+                out = dict(merged)
+                start = float(matched_window["window_start"])
+                out["source_timestamp"] = timestamp
+                out["timestamp"] = timestamp - start
+                out["sampling_u"] = stable_uniform(
+                    seed=sample_seed,
+                    window_id=str(matched_window["window_id"]),
+                    index=len(extracted[str(matched_window["window_id"])]),
+                    row=merged,
+                )
+                extracted[str(matched_window["window_id"])].append(out)
     return extracted