diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5d1a6fddd7..47168fd7ab 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,8 +3,11 @@
 
 ## [Unreleased]
 
+## [v0.51.342] — 2026-06-09 — Release LF (transcript + sidebar reliability: blank-transcript, missing-index stall, stale watermark)
+
 ### Fixed
 
+- **Chat transcript no longer renders as just a stack of date separators with no messages (#3875).** The live-to-final / Worklog redesign (#3401) folds intermediate assistant segments into a collapsed Worklog card and hides the source segment. When a turn's *only* content was folded into a collapsed Worklog (e.g. an autonomous or interrupted run whose final assistant message is empty, or a reload where the session's tool-call metadata did not hydrate), every segment was hidden and the turn painted as nothing — leaving the transcript a bare column of `SUNDAY` / `SATURDAY` / date dividers. `renderMessages()` now enforces a fail-safe invariant: a settled assistant turn never renders with zero visible content. When a turn has no visible segment, its folded Worklog group is expanded (or, as a last resort, its hidden segments are un-hidden) so the content is never silently swallowed. Turns that already show a visible answer are untouched, so the intended collapsed-Worklog UX is preserved. (#3875)
 - **Sidebar no longer stalls for seconds on a state.db that is missing the agent's message index.** The CLI-session scan orders candidate sessions by a correlated `MAX(timestamp)` subquery over `messages`, which collapses to a full per-session table scan when `idx_messages_session ON messages(session_id, timestamp)` is absent — stalling `/api/sessions` for several seconds on every refresh (the 5s cache TTL never settles, so repeated "Slow WebUI request still running" warnings appear). A normally-migrated hermes-agent state.db has this index, but a db that lost its migrations (older hermes-agent, or a hand-rebuilt/reimported db) does not. WebUI now primes the index with `CREATE INDEX IF NOT EXISTS` before the scan — a no-op when it already exists, and a ~20ms self-heal otherwise (measured 13.3s → 0.009s on a no-index 8k-session db). Best-effort: degrades silently on a read-only db, a locked db, or a minimal schema without a `timestamp` column. (#3887)
 - **A stale truncation watermark no longer hides messages sent after a retry/undo/edit.** `retry`, `undo`, and the Edit-truncate handler set a `truncation_watermark` to suppress the *replaced* tail from the append-only state.db merge, but nothing cleared it when the user then sent a genuinely new turn — so it froze at the old edit boundary. A frozen watermark then dropped the post-edit turns whenever the sidecar was later reconstructed empty (recovery/reconcile), permanently losing them from the WebUI transcript even though state.db still had the full data. WebUI now retires the watermark once the new turn is durably committed to `session.messages` — at the agent-result merge, the eager user-message checkpoint, or the cold-load recovery commit (the edit boundary is historical once a real turn lands). Cleared to `None`, distinct from the `0.0` truncate-to-empty sentinel, so the #2914 "don't replay a deleted transcript" behavior is unchanged. (#3831)
 
diff --git a/static/ui.js b/static/ui.js
index 81399b3827..8033eb5622 100644
--- a/static/ui.js
+++ b/static/ui.js
@@ -8726,6 +8726,69 @@ function renderMessages(options){
       }
     }
   }
+  // Fail-safe invariant (#3875): a settled assistant turn must never render with
+  // ZERO visible content. The Worklog redesign (#3401) folds intermediate
+  // assistant segments into a collapsed Worklog card and hides the source segment
+  // (`assistant-segment-worklog-source` → display:none). That is correct WHEN the
+  // turn also has a visible final answer. But when a turn's ONLY content is folded
+  // into a collapsed Worklog (e.g. an autonomous/interrupted run whose final
+  // assistant message is empty, or a reload where S.toolCalls didn't hydrate so the
+  // worklog card built with no expandable tool steps), every segment is hidden and
+  // the turn paints as nothing — leaving the transcript a bare stack of date
+  // separators (#3875 brick). Reveal such turns so their content is never silently
+  // swallowed: expand the turn's Worklog group(s) when the turn has no other
+  // visible content. This NEVER touches a turn that has any visible segment, so the
+  // intended collapsed-Worklog UX is preserved whenever a visible answer exists.
+  // The live turn is excluded by its `liveAssistantTurn` id (it drives its own
+  // state during a stream), so this sweep is safe to run even while busy — a
+  // historical blank turn must not re-paint blank during a follow-up stream
+  // (Opus advisor, stage-342).
+  {
+    const _turnHasVisibleContent=(turn)=>{
+      const segs=turn.querySelectorAll('.assistant-segment');
+      for(const seg of segs){
+        // A segment shows real content only when it is NOT worklog-folded AND its
+        // body/files/status actually painted (the anchor-only placeholder class
+        // carries no visible body).
+        if(seg.classList.contains('assistant-segment-worklog-source')) continue;
+        if(seg.classList.contains('assistant-segment-anchor')) continue;
+        if((seg.textContent||'').trim()) return true;
+      }
+      return false;
+    };
+    for(const turn of inner.querySelectorAll('.assistant-turn')){
+      if(turn.id==='liveAssistantTurn') continue; // live turn drives its own state
+      if(_turnHasVisibleContent(turn)) continue;
+      // No visible content — surface the folded Worklog so the turn isn't blank.
+      const groups=turn.querySelectorAll('.tool-worklog-group,.tool-call-group');
+      let revealed=false;
+      for(const group of groups){
+        if(!(group.textContent||'').trim()) continue; // empty group can't help
+        if(group.classList.contains('tool-call-group-collapsed')){
+          group.classList.remove('tool-call-group-collapsed');
+          group.classList.add('open');
+          const summary=group.querySelector('.tool-call-group-summary,.activity-summary');
+          if(summary) summary.setAttribute('aria-expanded','true');
+        }
+        // `revealed` means "this turn has a non-empty Worklog group that the user
+        // can see" — NOT "we just expanded something". An already-open non-empty
+        // group is itself visible (it slips past _turnHasVisibleContent only
+        // because that check inspects .assistant-segment nodes, not group bodies),
+        // so the turn isn't truly blank and the last-resort un-hide below is
+        // unnecessary. Keep this assignment OUTSIDE the if(collapsed) branch.
+        revealed=true;
+      }
+      // Last resort: no usable worklog group either, but hidden worklog-source
+      // segments carry the real text — un-hide them so nothing is lost.
+      if(!revealed){
+        for(const seg of turn.querySelectorAll('.assistant-segment-worklog-source')){
+          if(!(seg.textContent||'').trim()) continue;
+          seg.classList.remove('assistant-segment-worklog-source');
+          seg.removeAttribute('aria-hidden');
+        }
+      }
+    }
+  }
   // Only force-scroll when not actively streaming — mid-stream re-renders
   // (tool completion, session switch) must not override the user's scroll position.
   // scrollIfPinned() respects _scrollPinned, so it's a no-op if user scrolled up.
diff --git a/tests/test_issue3875_blank_transcript_failsafe.py b/tests/test_issue3875_blank_transcript_failsafe.py
new file mode 100644
index 0000000000..550150d374
--- /dev/null
+++ b/tests/test_issue3875_blank_transcript_failsafe.py
@@ -0,0 +1,89 @@
+"""Regression coverage for #3875 — chat transcript renders as only date separators.
+
+#3875 (BUG: messages stopped displaying correctly): the chat transcript rendered as
+nothing but a stack of date-change separators (SUNDAY / SATURDAY / dates) with no
+message bodies between them. Two users confirmed it; only a restart appeared to help
+(it didn't — it was a render-state bug, not server state).
+
+Root cause: the live-to-final / Worklog redesign (#3401, v0.51.294) folds intermediate
+assistant segments into a collapsed Worklog card and hides the source segment via the
+``assistant-segment-worklog-source`` class (``display:none``). That is correct WHEN the
+turn also has a visible final answer. But when a turn's ONLY content is folded into a
+collapsed Worklog — e.g. an autonomous/interrupted run whose final assistant message is
+empty, or a reload where ``S.toolCalls`` did not hydrate so the Worklog card has no
+expandable tool steps — every segment is hidden and the turn paints as nothing, leaving
+the transcript a bare stack of date separators.
+
+The fix is a defensive fail-safe at the END of ``renderMessages``: a settled assistant
+turn must never render with ZERO visible content. When a turn has no visible segment,
+its folded Worklog group is expanded (or, as a last resort, its hidden worklog-source
+segments are un-hidden) so the content is never silently swallowed. The fail-safe never
+touches a turn that already has any visible segment, so the intended collapsed-Worklog
+UX is preserved whenever a visible answer exists.
+
+These are static source-structure assertions over the shipped ``renderMessages`` so the
+invariant cannot silently regress.
+"""
+from __future__ import annotations
+
+from pathlib import Path
+
+REPO = Path(__file__).resolve().parent.parent
+UI_JS = (REPO / "static" / "ui.js").read_text(encoding="utf-8")
+
+
+def _function_body(src: str, name: str) -> str:
+    marker = f"function {name}("
+    start = src.find(marker)
+    assert start != -1, f"{name} not found"
+    brace = src.find("{", start)
+    assert brace != -1, f"{name} body not found"
+    depth = 0
+    for idx in range(brace, len(src)):
+        ch = src[idx]
+        if ch == "{":
+            depth += 1
+        elif ch == "}":
+            depth -= 1
+            if depth == 0:
+                return src[brace + 1 : idx]
+    raise AssertionError(f"{name} body not closed")
+
+
+def test_render_messages_has_blank_turn_failsafe():
+    """#3875: renderMessages must carry the no-blank-turn fail-safe invariant."""
+    body = _function_body(UI_JS, "renderMessages")
+    # The fail-safe is anchored by its issue tag so it is greppable + intentional.
+    assert "Fail-safe invariant (#3875)" in body, (
+        "the #3875 no-blank-turn fail-safe is missing from renderMessages"
+    )
+
+
+def test_failsafe_reveals_folded_worklog_for_blank_turns():
+    """The fail-safe must expand the folded Worklog group when a turn has no visible content."""
+    body = _function_body(UI_JS, "renderMessages")
+    # It must scan turns and skip any turn that already has visible content.
+    assert "_turnHasVisibleContent" in body
+    # A turn is only acted on when it lacks visible content (the skip-guard).
+    assert "if(_turnHasVisibleContent(turn)) continue;" in body
+    # The reveal action removes the collapsed class on the Worklog group.
+    assert "tool-call-group-collapsed" in body
+    assert "removeAttribute('aria-hidden')" in body
+
+
+def test_failsafe_preserves_collapsed_worklog_when_visible_answer_exists():
+    """The fail-safe must NOT touch turns that already render visible content.
+
+    The skip-guard (`if(_turnHasVisibleContent(turn)) continue;`) is what preserves the
+    intended collapsed-Worklog UX: a turn with any visible answer is left untouched, so
+    this fix only ever ADDS visibility to otherwise-blank turns and can never re-expand a
+    Worklog the user expects collapsed.
+    """
+    body = _function_body(UI_JS, "renderMessages")
+    # The visible-content check skips worklog-source (folded) + anchor-only placeholder
+    # segments, and treats any other non-empty segment as "visible".
+    failsafe = body[body.find("Fail-safe invariant (#3875)") :]
+    assert "assistant-segment-worklog-source" in failsafe
+    assert "assistant-segment-anchor" in failsafe
+    # The live turn drives its own state and must be excluded from the sweep.
+    assert "liveAssistantTurn" in failsafe