diff --git a/static/ui.js b/static/ui.js
index df380b466..c6a629ca5 100644
--- a/static/ui.js
+++ b/static/ui.js
@@ -8997,17 +8997,33 @@ function renderMessages(options){
   // streaming.
   if(_preservedLiveTurn){
     const _rebuilt=document.getElementById('liveAssistantTurn');
-    const _preservedSeg=_preservedLiveTurn.querySelector('[data-live-assistant="1"]');
+    // Pick the PARSER-OWNED live segment, not just the first one. On reconnect /
+    // post-tool activity boundaries a live turn can carry MULTIPLE
+    // [data-live-assistant="1"] segments, and the smd parser writes into the
+    // LAST (tail) one (see ensureAssistantRow in messages.js — it re-attaches to
+    // the last live segment). Prefer the preserved segment whose
+    // data-live-segment-seq matches the rebuilt tail (same logical segment), then
+    // fall back to the last preserved live segment. Using querySelector() (first)
+    // here would move the wrong segment and leave the parser-owned tail detached
+    // in a multi-segment turn.
+    const _rebuiltSegs=_rebuilt?_rebuilt.querySelectorAll('[data-live-assistant="1"]'):null;
+    const _rebuiltSeg=(_rebuiltSegs&&_rebuiltSegs.length)?_rebuiltSegs[_rebuiltSegs.length-1]:null;
+    const _preservedSegs=_preservedLiveTurn.querySelectorAll('[data-live-assistant="1"]');
+    let _preservedSeg=_preservedSegs.length?_preservedSegs[_preservedSegs.length-1]:null;
+    const _rebuiltSeq=_rebuiltSeg?_rebuiltSeg.getAttribute('data-live-segment-seq'):null;
+    if(_rebuiltSeq){
+      for(const _seg of _preservedSegs){
+        if(_seg.getAttribute('data-live-segment-seq')===_rebuiltSeq){_preservedSeg=_seg;break;}
+      }
+    }
     const _preservedLen=_liveAssistantSegmentTextLength(_preservedSeg||_preservedLiveTurn);
     if(_preservedLen>0){
-      const _rebuiltSegs=_rebuilt?_rebuilt.querySelectorAll('[data-live-assistant="1"]'):null;
-      const _rebuiltSeg=(_rebuiltSegs&&_rebuiltSegs.length)?_rebuiltSegs[_rebuiltSegs.length-1]:null;
       const _rebuiltLen=_rebuilt?_liveAssistantSegmentTextLength(_rebuiltSeg||_rebuilt):-1;
       if(_rebuiltLen<=_preservedLen){
         if(S.session) _preservedLiveTurn.dataset.sessionId=S.session.session_id;
         if(_rebuilt&&_rebuiltSeg&&_preservedSeg){
           // Segment-level swap: keep the rebuilt turn (and its other segments /
-          // tool groups) but restore the parser-referenced live segment.
+          // tool groups) but restore the parser-referenced (tail) live segment.
           _rebuiltSeg.replaceWith(_preservedSeg);
         }else if(_rebuilt){
           // Rebuilt turn has no live segment to target — replace the whole turn.
diff --git a/tests/test_issue3877_midstream_flicker.py b/tests/test_issue3877_midstream_flicker.py
index 0b60a42f6..e5d67b4db 100644
--- a/tests/test_issue3877_midstream_flicker.py
+++ b/tests/test_issue3877_midstream_flicker.py
@@ -135,6 +135,29 @@ def test_reattach_swaps_at_segment_level_to_preserve_rebuilt_structure():
     assert "_preservedSeg" in reattach and "_rebuiltSeg" in reattach
 
 
+def test_reattach_targets_the_parser_owned_tail_segment_not_the_first():
+    """Multi-live-segment turns (reconnect / post-tool activity boundaries) can have
+    several [data-live-assistant="1"] segments; the smd parser writes into the LAST
+    (tail) one. The re-attach must select the preserved TAIL segment — preferring the
+    one whose data-live-segment-seq matches the rebuilt tail — not the first via a bare
+    querySelector(). Picking the first would move the wrong segment and leave the
+    parser-owned tail detached (Codex CORE finding on the #3877-reopen fix)."""
+    body = _function_body(UI_JS, "renderMessages")
+    reattach = body[body.find("Re-attach the preserved live turn (#3877)") :]
+    # Preserved segment is chosen from querySelectorAll (tail), not querySelector (first).
+    assert "_preservedSegs=_preservedLiveTurn.querySelectorAll('[data-live-assistant=\"1\"]')" in reattach
+    assert "_preservedSegs[_preservedSegs.length-1]" in reattach, (
+        "must default to the LAST preserved live segment (the parser-owned tail)"
+    )
+    # And prefer the segment whose live-segment-seq matches the rebuilt tail.
+    assert "data-live-segment-seq" in reattach and "_rebuiltSeq" in reattach, (
+        "must prefer the preserved segment matching the rebuilt tail's "
+        "data-live-segment-seq before falling back to the last segment"
+    )
+    # The first-only querySelector form must NOT be how _preservedSeg is derived.
+    assert "_preservedSeg=_preservedLiveTurn.querySelector(" not in reattach
+
+
 def test_reattach_runs_after_rebuild_loop():
     """The re-attach must run AFTER the rebuild (so a freshly-rebuilt live turn exists to
     compare against / replace) but is still inside renderMessages."""