diff --git a/CHANGELOG.md b/CHANGELOG.md
index b8eaaf34..1e121d30 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,9 @@
 
 ## [Unreleased]
 
+### Changed
+- Live chat Activity now shows observable run events, model/run metadata, clearer working elapsed text, and explicit waiting/stale states instead of an empty `Thinking…` placeholder when no reasoning text is available.
+
 ## [v0.51.124] — 2026-05-24 — Release CV (stage-batch6 — 3-PR Windows-only stack — agent paths / docs / port hardening)
 
 ### Added
diff --git a/static/style.css b/static/style.css
index d010f4e1..a11495ec 100644
--- a/static/style.css
+++ b/static/style.css
@@ -2322,6 +2322,15 @@ body.resizing .sidebar{transition:none!important;}
 .tool-call-group:not(.tool-call-group-collapsed) .tool-call-group-chevron{transform:rotate(90deg);}
 .tool-call-group-body{display:block;padding-left:var(--space-3);}
 .tool-call-group.tool-call-group-collapsed .tool-call-group-body{display:none;}
+.agent-activity-status{display:grid;grid-template-columns:18px minmax(0,1fr) auto;align-items:start;gap:var(--space-2);padding:5px 0;color:var(--muted);font-size:var(--font-size-xs);line-height:1.45;border-bottom:1px solid color-mix(in srgb,var(--border-subtle) 60%,transparent);}
+.agent-activity-status:last-child{border-bottom:0;}
+.agent-activity-status-icon{display:inline-flex;align-items:center;justify-content:center;min-height:18px;opacity:.72;color:var(--muted);}
+.agent-activity-status-copy{display:flex;flex-direction:column;min-width:0;gap:1px;}
+.agent-activity-status-label{color:var(--text);font-weight:600;overflow:hidden;text-overflow:ellipsis;white-space:nowrap;}
+.agent-activity-status-detail{color:var(--muted);opacity:.72;overflow:hidden;text-overflow:ellipsis;white-space:nowrap;}
+.agent-activity-status-time{font-variant-numeric:tabular-nums;opacity:.55;white-space:nowrap;}
+.agent-activity-status-waiting .agent-activity-status-label{color:var(--muted);}
+.agent-activity-status-error .agent-activity-status-label{color:var(--error);}
 .tool-call-group-label{font-weight:600;color:var(--muted);position:relative;display:inline-block;overflow:hidden;}
 .tool-call-group[data-live-tool-call-group="1"] .tool-call-group-label{
   color:var(--muted);
diff --git a/static/ui.js b/static/ui.js
index c6a5cd2a..4e7abe16 100644
--- a/static/ui.js
+++ b/static/ui.js
@@ -2208,6 +2208,7 @@ function _startCompressionElapsedTimer(){if(!_compressionElapsedTimer)_compressi
 function _clearCompressionElapsedTimer(){if(_compressionElapsedTimer){clearInterval(_compressionElapsedTimer);_compressionElapsedTimer=null;}}
 let _activityElapsedTimer=null;
 let _activityElapsedTimerGroup=null;
+function _activityNowSeconds(){return Date.now()/1000;}
 function _activityElapsedStartedAt(group){
   if(!group)return null;
   const raw=(group.dataset&&group.dataset.turnStartedAt!==undefined&&group.dataset.turnStartedAt!=='')
@@ -2219,7 +2220,52 @@ function _activityElapsedStartedAt(group){
 function _activityElapsedLabel(group){
   const started=_activityElapsedStartedAt(group);
   if(!started)return'';
-  return _formatActiveElapsedTimer((Date.now()/1000)-started);
+  return _formatActiveElapsedTimer(_activityNowSeconds()-started);
+}
+function _activityMarkObserved(group, ts){
+  if(!group||group.getAttribute('data-live-tool-call-group')!=='1')return;
+  const stamp=Number(ts||_activityNowSeconds());
+  if(Number.isFinite(stamp)&&stamp>0) group.setAttribute('data-last-activity-at',String(stamp));
+}
+function _activityLastObservedAge(group){
+  const stamp=Number(group&&group.getAttribute('data-last-activity-at'));
+  if(!Number.isFinite(stamp)||stamp<=0)return null;
+  return Math.max(0,_activityNowSeconds()-stamp);
+}
+function _activityClockLabel(ts){
+  const stamp=Number(ts||_activityNowSeconds());
+  if(!Number.isFinite(stamp)||stamp<=0)return'';
+  try{return new Date(stamp*1000).toLocaleTimeString([], {hour:'numeric',minute:'2-digit'});}catch(_){return'';}
+}
+function _activityStatusNode({kind='info',label='',detail='',status='done',ts=null,id=''}){
+  const row=document.createElement('div');
+  row.className=`agent-activity-status agent-activity-status-${kind} agent-activity-status-${status}`;
+  if(id) row.setAttribute('data-activity-event-id',id);
+  if(ts) row.setAttribute('data-activity-at',String(ts));
+  const iconMap={run:li('play',13),model:li('bot',13),waiting:'<span class="tool-card-running-dot"></span>',thinking:li('lightbulb',13),tool:li('wrench',13),done:li('check',13),warning:li('alert-triangle',13)};
+  row.innerHTML=`<span class="agent-activity-status-icon">${iconMap[kind]||iconMap.info||li('circle',13)}</span><span class="agent-activity-status-copy"><span class="agent-activity-status-label">${esc(label)}</span>${detail?`<span class="agent-activity-status-detail">${esc(detail)}</span>`:''}</span><span class="agent-activity-status-time">${esc(_activityClockLabel(ts))}</span>`;
+  return row;
+}
+function _appendActivityEvent(group, event){
+  if(!group)return null;
+  const body=group.querySelector('.tool-call-group-body');
+  if(!body)return null;
+  const eventId=event&&event.id;
+  let row=eventId?body.querySelector(`.agent-activity-status[data-activity-event-id="${CSS.escape(eventId)}"]`):null;
+  const next=_activityStatusNode(event||{});
+  if(row){row.replaceWith(next);row=next;}
+  else{body.appendChild(next);row=next;}
+  _activityMarkObserved(group,event&&event.ts);
+  return row;
+}
+function _ensureLiveActivityBaseline(group){
+  if(!group||group.getAttribute('data-live-tool-call-group')!=='1')return;
+  const started=_activityElapsedStartedAt(group)||_activityNowSeconds();
+  if(!group.getAttribute('data-turn-started-at')) group.setAttribute('data-turn-started-at',String(started));
+  if(!group.getAttribute('data-last-activity-at')) group.setAttribute('data-last-activity-at',String(started));
+  _appendActivityEvent(group,{id:'run-started',kind:'run',label:'Run started',detail:'Observable activity will appear here as the agent works.',status:'done',ts:started});
+  const modelLabel=(S.session&&S.session.model)?getModelLabel(S.session.model):'';
+  if(modelLabel)_appendActivityEvent(group,{id:'run-model',kind:'model',label:`Model: ${modelLabel}`,detail:S.activeProfile&&S.activeProfile!=='default'?`Profile: ${S.activeProfile}`:'',status:'done',ts:started});
 }
 function _setActivityElapsedStartedAt(group){
   if(!group||group.getAttribute('data-live-tool-call-group')!=='1')return;
@@ -2240,8 +2286,10 @@ function _updateActiveActivityElapsedTimer(){
     group.removeAttribute('data-active-turn-elapsed');
   }
   if(durationEl){
-    durationEl.textContent=label?`Working ${label}`:'';
-    durationEl.style.display=label?'':'none';
+    const activeText=label?`Working for ${label}`:'';
+    const progressText=_activityLiveProgressLabel(group);
+    durationEl.textContent=[progressText, activeText].filter(Boolean).join(' · ');
+    durationEl.style.display=durationEl.textContent?'':'none';
   }
 }
 function _startActivityElapsedTimer(group){
@@ -5219,7 +5267,10 @@ function ensureActivityGroup(inner, opts){
   }else if(activityKey&&!group.getAttribute('data-activity-disclosure-key')){
     group.setAttribute('data-activity-disclosure-key',activityKey);
   }
-  if(live) _setActivityElapsedStartedAt(group);
+  if(live){
+    _setActivityElapsedStartedAt(group);
+    _ensureLiveActivityBaseline(group);
+  }
   _syncToolCallGroupSummary(group);
   if(live) _startActivityElapsedTimer(group);
   return group;
@@ -6657,7 +6708,9 @@ function _syncToolCallGroupSummary(group){
   const label=group.querySelector('.tool-call-group-label');
   const durationEl=group.querySelector('.tool-call-group-duration');
   if(label){
-    if(toolCount) label.textContent=`Activity: ${toolCount} tool${toolCount===1?'':'s'}`;
+    if(group.getAttribute('data-live-tool-call-group')==='1'){
+      label.textContent=toolCount?`Activity: ${toolCount} tool${toolCount===1?'':'s'}`:'Activity · Running';
+    }else if(toolCount) label.textContent=`Activity: ${toolCount} tool${toolCount===1?'':'s'}`;
     else label.textContent='Activity';
     label.setAttribute('data-sweep-label', label.textContent);
   }
@@ -6691,9 +6744,14 @@ function _activityProgressLabelForToolName(name){
 
 function _activityLiveProgressLabel(group){
   if(!group||group.getAttribute('data-live-tool-call-group')!=='1') return '';
+  const idleAge=_activityLastObservedAge(group);
+  if(idleAge!==null&&idleAge>=90) return `No recent activity for ${_formatActiveElapsedTimer(idleAge)}`;
   const running=group.querySelector('.tool-card.tool-card-running .tool-card-name');
   const latest=running || Array.from(group.querySelectorAll('.tool-card-name')).pop();
-  return _activityProgressLabelForToolName(latest?latest.textContent:'');
+  const waiting=group.querySelector('.agent-activity-status-waiting .agent-activity-status-label');
+  if(latest) return _activityProgressLabelForToolName(latest.textContent);
+  if(waiting&&waiting.textContent) return waiting.textContent;
+  return 'Starting agent';
 }
 
 // ── Live tool card helpers (called during SSE streaming) ──
@@ -6759,6 +6817,19 @@ function appendLiveToolCard(tc){
   const anchor=children.filter(el=>el.matches('[data-live-assistant="1"],.tool-call-group,.tool-card-row,.agent-activity-thinking')).pop();
   const group=ensureActivityGroup(inner,{live:true,collapsed:true,anchor,activityKey:_activityKeyForLiveTurn()});
   const body=group.querySelector('.tool-call-group-body');
+  const toolName=_toolDisplayName(tc);
+  const toolEventId=tid?`tool-${tid}`:`tool-${String(tc.name||'tool').replace(/[^a-z0-9_-]/gi,'_')}`;
+  const toolDone=tc.done!==false;
+  _appendActivityEvent(group,{
+    id:toolEventId,
+    kind:'tool',
+    label:toolDone?`Tool finished: ${toolName}`:`Running tool: ${toolName}`,
+    detail:tc.preview||tc.snippet||'',
+    status:toolDone?(tc.is_error?'error':'done'):'waiting',
+    ts:_activityNowSeconds(),
+  });
+  const waiting=body.querySelector('.agent-activity-status[data-activity-event-id="thinking-placeholder"] .agent-activity-status-label');
+  if(waiting&&!toolDone) waiting.textContent='Waiting on tool result';
   // Update existing card in place (tool_complete after tool_start)
   if(tid){
     const existing=body.querySelector(`.tool-card-row[data-live-tid="${CSS.escape(tid)}"]`);
@@ -7583,6 +7654,7 @@ function appendThinking(text='', options){
     return;
   }
   const thinkingText=String(text||'').trim()||'Thinking…';
+  const cleanThinking=_sanitizeThinkingDisplayText(thinkingText);
   const allChildren=Array.from(blocks.children);
   const anchor=allChildren.filter(el=>
     el.id!=='toolRunningRow' &&
@@ -7591,6 +7663,20 @@ function appendThinking(text='', options){
   const group=ensureActivityGroup(blocks,{live:true,collapsed:true,anchor,activityKey:_activityKeyForLiveTurn()});
   const body=group&&group.querySelector('.tool-call-group-body');
   if(!body) return;
+  if(!cleanThinking||cleanThinking==='Thinking…'){
+    const label=body.querySelector('.tool-card.tool-card-running')?'Waiting on tool result':'Waiting on model';
+    const detail=body.querySelector('.tool-card-row')
+      ? 'The agent is running; tool results and response text will appear here.'
+      : 'No tool activity has been reported yet.';
+    _appendActivityEvent(group,{id:'thinking-placeholder',kind:'waiting',label,detail,status:'waiting',ts:_activityNowSeconds()});
+    const active=body.querySelector('.agent-activity-thinking[data-thinking-active="1"]');
+    if(active) active.removeAttribute('data-thinking-active');
+    _syncToolCallGroupSummary(group);
+    scrollIfPinned();
+    return;
+  }
+  const placeholder=body.querySelector('.agent-activity-status[data-activity-event-id="thinking-placeholder"]');
+  if(placeholder) placeholder.remove();
   let row=body.querySelector('.agent-activity-thinking[data-thinking-active="1"]');
   if(!row){
     const thinkingCards=Array.from(body.querySelectorAll('.agent-activity-thinking'));
@@ -7598,12 +7684,13 @@ function appendThinking(text='', options){
     if(row) row.setAttribute('data-thinking-active','1');
   }
   if(!row){
-    row=_thinkingActivityNode(thinkingText, false);
+    row=_thinkingActivityNode(cleanThinking, false);
     row.setAttribute('data-thinking-active','1');
     body.appendChild(row);
   }else{
-    _renderThinkingInto(row,thinkingText);
+    _renderThinkingInto(row,cleanThinking);
   }
+  _activityMarkObserved(group);
   _syncToolCallGroupSummary(group);
   scrollIfPinned();
   if(_scrollPinned){
diff --git a/tests/test_live_activity_timeline.py b/tests/test_live_activity_timeline.py
new file mode 100644
index 00000000..f2226de1
--- /dev/null
+++ b/tests/test_live_activity_timeline.py
@@ -0,0 +1,45 @@
+"""Regression coverage for live Activity timeline UX.
+
+The live Activity disclosure should surface observable run telemetry instead of a
+blank Thinking placeholder while preserving the quiet tool/thinking metadata
+family.
+"""
+
+import pathlib
+
+
+REPO = pathlib.Path(__file__).parent.parent
+UI_JS = (REPO / "static" / "ui.js").read_text(encoding="utf-8")
+STYLE_CSS = (REPO / "static" / "style.css").read_text(encoding="utf-8")
+
+
+def test_live_activity_group_has_observable_baseline_events():
+    assert "function _ensureLiveActivityBaseline(group)" in UI_JS
+    assert "Run started" in UI_JS
+    assert "Observable activity will appear here as the agent works." in UI_JS
+    assert "Model: ${modelLabel}" in UI_JS
+    assert "_ensureLiveActivityBaseline(group);" in UI_JS
+
+
+def test_empty_thinking_placeholder_becomes_status_row_not_raw_thinking_card():
+    assert "data-activity-event-id=\"thinking-placeholder\"" in UI_JS
+    assert "Waiting on model" in UI_JS
+    assert "No tool activity has been reported yet." in UI_JS
+    assert "Waiting on tool result" in UI_JS
+    assert "_thinkingActivityNode(cleanThinking, false)" in UI_JS
+
+
+def test_tool_events_update_activity_timeline_and_summary():
+    assert "Tool finished: ${toolName}" in UI_JS
+    assert "Running tool: ${toolName}" in UI_JS
+    assert "No recent activity for ${_formatActiveElapsedTimer(idleAge)}" in UI_JS
+    assert "Activity · Running" in UI_JS
+    assert "Working for ${label}" in UI_JS
+
+
+def test_activity_status_rows_have_quiet_metadata_styling():
+    assert ".agent-activity-status{" in STYLE_CSS
+    assert "grid-template-columns:18px minmax(0,1fr) auto" in STYLE_CSS
+    assert ".agent-activity-status-detail" in STYLE_CSS
+    assert ".agent-activity-status-time" in STYLE_CSS
+    assert ".agent-activity-status-error .agent-activity-status-label{color:var(--error);}" in STYLE_CSS