diff --git a/static/style.css b/static/style.css
index dd212efe..1d6c0d26 100644
--- a/static/style.css
+++ b/static/style.css
@@ -4118,6 +4118,13 @@ body.resizing .sidebar{transition:none!important;}
   white-space:nowrap;
   font-size:10px;
 }
+/* Narrow viewports: the ~50px of fixed time-label chrome squeezes the collapsed
+   row's flexible preview (the most informative part) toward zero. Hide the label
+   under ~420px; the full timing is still available in Compact/expanded review and
+   via the title tooltip on wider screens. (#5739 Fable UX fix.) */
+@media (max-width:420px){
+  .transparent-event-time{ display:none; }
+}
 .transparent-event-row .thinking-card-label{
   display:inline;
   flex:0 0 auto;
diff --git a/static/ui.js b/static/ui.js
index c443f035..c1ab9f1d 100644
--- a/static/ui.js
+++ b/static/ui.js
@@ -5568,6 +5568,18 @@ function _activityClockLabel(ts){
   if(!Number.isFinite(stamp)||stamp<=0)return'';
   try{return new Date(stamp*1000).toLocaleTimeString([], {hour:'numeric',minute:'2-digit'});}catch(_){return'';}
 }
+// Full date+time label for the worklog event-time tooltip (title attr). Guards the
+// same valid-Date range as _timestampSeconds so a bad epoch never yields "Invalid
+// Date" in the tooltip. (#5739)
+function _activityFullClockLabel(ts){
+  const stamp=Number(ts);
+  if(!Number.isFinite(stamp)||stamp<=0||stamp>8.64e12)return'';
+  try{
+    const d=new Date(stamp*1000);
+    if(isNaN(d.getTime()))return'';
+    return d.toLocaleString([], {year:'numeric',month:'short',day:'numeric',hour:'numeric',minute:'2-digit'});
+  }catch(_){return'';}
+}
 function _timestampSeconds(value){
   if(value===undefined||value===null||value==='') return null;
   if(value instanceof Date){
@@ -5663,6 +5675,11 @@ function _syncTransparentEventTimestamp(row, header, opts){
     timeEl.className='transparent-event-time';
   }
   timeEl.textContent=label;
+  // Full date+time tooltip: the bare clock label is date-ambiguous when a settled
+  // session is reviewed days later (or a run crosses midnight), and timing is the
+  // whole point of this label. (#5739 Fable UX fix.)
+  const fullLabel=_activityFullClockLabel(ts);
+  if(fullLabel) timeEl.setAttribute('title',fullLabel); else timeEl.removeAttribute('title');
   timeEl.setAttribute('data-event-at',String(ts));
   timeEl.setAttribute('data-event-at-source',source);
   row.setAttribute('data-event-at',String(ts));
diff --git a/tests/test_issue5700_worklog_event_timestamps.py b/tests/test_issue5700_worklog_event_timestamps.py
index 4ef424c8..5059d129 100644
--- a/tests/test_issue5700_worklog_event_timestamps.py
+++ b/tests/test_issue5700_worklog_event_timestamps.py
@@ -247,6 +247,7 @@ global._toolWorklogListEl = () => null;
 global._activityKeyForLiveTurn = () => 'activity-key';
 global._assistantTurnBlocks = () => null;
 eval(extractFunc('_activityClockLabel'));
+eval(extractFunc('_activityFullClockLabel'));
 eval(extractFunc('_timestampSeconds'));
 eval(extractFunc('_firstValidTimestampSeconds'));
 eval(extractFunc('_transparentEventTimestampSeconds'));
@@ -458,6 +459,31 @@ process.stdout.write(JSON.stringify({
     assert data["normalLabel"]
 
 
+def test_full_clock_label_tooltip_includes_date_and_rejects_bad_epochs():
+    # The worklog time label carries a full date+time tooltip (title attr) so a
+    # settled session reviewed days later (or a run crossing midnight) is not
+    # date-ambiguous. Bad epochs must yield '' (never 'Invalid Date'). (#5739 Fable.)
+    data = _run_node(
+        """
+process.stdout.write(JSON.stringify({
+  normal: _activityFullClockLabel(1700000901),
+  huge: _activityFullClockLabel(1e20),
+  zero: _activityFullClockLabel(0),
+  negative: _activityFullClockLabel(-1),
+}));
+"""
+    )
+
+    # a normal epoch yields a non-empty full label with the year present, no "Invalid Date"
+    assert data["normal"]
+    assert "Invalid Date" not in data["normal"]
+    assert "2023" in data["normal"] or "2024" in data["normal"] or "2025" in data["normal"] or "2026" in data["normal"]
+    # out-of-range / non-positive epochs yield empty string (no tooltip)
+    assert data["huge"] == ""
+    assert data["zero"] == ""
+    assert data["negative"] == ""
+
+
 def test_settled_anchor_scene_rows_use_row_created_at_timestamps():
     data = _run_node(
         """