fix(webui): preserve causal message order (NAN-29) (#5503)

nanobot/webui/transcript.py

@@ -1813,11 +1813,16 @@ def replay_transcript_to_ui_messages(
                 break
             content = str(candidate.get("content") or "")
             has_answer = len(content) > 0
+            if has_answer:
+                break
+            # A completed reasoning field is closed even while its assistant
+            # placeholder remains streaming for the rest of the turn.
             if (
                 candidate.get("reasoningStreaming")
-                or candidate.get("reasoning") is not None
-                or has_answer
-                or candidate.get("isStreaming")
+                or (
+                    candidate.get("isStreaming")
+                    and candidate.get("reasoning") is None
+                )
             ):
                 prev[i] = {
                     **candidate,
@@ -1827,15 +1832,6 @@ def replay_transcript_to_ui_messages(
                     **turn_fields,
                 }
                 return
-            if not has_answer and candidate.get("isStreaming"):
-                prev[i] = {
-                    **candidate,
-                    "reasoning": chunk,
-                    "reasoningStreaming": True,
-                    "activitySegmentId": candidate.get("activitySegmentId") or _ensure_activity_segment(),
-                    **turn_fields,
-                }
-                return
             break
         segment = _ensure_activity_segment()
         prev.append(
@@ -1915,19 +1911,6 @@ def replay_transcript_to_ui_messages(
             and not m.get("media")
         )
 
-    def is_tool_trace_at(index: int) -> bool:
-        m = messages[index] if 0 <= index < len(messages) else None
-        return bool(m and m.get("kind") == "trace")
-
-    def prune_reasoning_only() -> None:
-        nonlocal messages
-        kept: list[dict[str, Any]] = []
-        for i, m in enumerate(messages):
-            if is_reasoning_only_placeholder(m) and not is_tool_trace_at(i + 1):
-                continue
-            kept.append(m)
-        messages = kept
-
     def stamp_completion(
         *,
         latency_ms: int | None = None,
@@ -2442,7 +2425,6 @@ def replay_transcript_to_ui_messages(
             for i, m in enumerate(messages):
                 if m.get("isStreaming"):
                     messages[i] = {**m, "isStreaming": False}
-            prune_reasoning_only()
             lat = rec.get("latency_ms")
             usage = rec.get("usage")
             sanitized_usage = (