diff --git a/static/ui.js b/static/ui.js
index caadaf88..88a53c33 100644
--- a/static/ui.js
+++ b/static/ui.js
@@ -5537,14 +5537,9 @@ function renderCompressionUi(){
   el.style.display='none';
 }
 // Session render cache: avoids full markdown+DOM rebuild when switching back
-// to a session that was already rendered with the same message count.
+// to a session whose rendered transcript inputs are unchanged.
 // Keyed by session_id. Only used on cross-session navigation, never for
 // in-session updates (new messages, edits, stream events).
-//
-// Known limitation: cache key is session_id + message count. Edits and retries
-// that mutate message content without changing the count will serve stale HTML
-// on back-navigation until the user triggers an in-session update. Acceptable
-// for the common read-only back-navigation case; not suitable as a general cache.
 const _sessionHtmlCache=new Map();
 let _sessionHtmlCacheSid=null; // session_id currently rendered in the DOM
 function clearMessageRenderCache(){
@@ -5552,6 +5547,55 @@ function clearMessageRenderCache(){
   _sessionHtmlCacheSid=null;
 }
 
+function _messageRenderCacheSignature(){
+  let hash=2166136261;
+  function add(value){
+    const s=String(value==null?'':value);
+    for(let i=0;i<s.length;i++){
+      hash^=s.charCodeAt(i);
+      hash=Math.imul(hash,16777619)>>>0;
+    }
+    hash^=31;
+    hash=Math.imul(hash,16777619)>>>0;
+  }
+  const messages=Array.isArray(S.messages)?S.messages:[];
+  add(messages.length);
+  for(const m of messages){
+    if(!m||typeof m!=='object'){ add('missing'); continue; }
+    add(m.role);add(m.timestamp);add(m._ts);add(m._error);add(m._statusCard);
+    add(msgContent(m));
+    if(Array.isArray(m.content)){
+      add('content-array');
+      m.content.forEach(part=>{
+        if(!part||typeof part!=='object'){ add(part); return; }
+        add(part.type);add(part.id);add(part.name);add(part.text);add(part.content);
+      });
+    }
+    if(Array.isArray(m.tool_calls)){
+      add('message-tool-calls');add(m.tool_calls.length);
+      m.tool_calls.forEach(tc=>{add(tc&&tc.id);add(tc&&tc.name);add(tc&&tc.type);add(JSON.stringify(tc&&tc.function||{}));});
+    }
+    if(Array.isArray(m._partial_tool_calls)){
+      add('partial-tool-calls');add(m._partial_tool_calls.length);
+      m._partial_tool_calls.forEach(tc=>{add(tc&&tc.id);add(tc&&tc.name);add(tc&&tc.snippet);});
+    }
+    if(_messageHasReasoningPayload(m)) add(m.reasoning||m.thinking||m._reasoning||'reasoning');
+    if(Array.isArray(m.attachments)) m.attachments.forEach(a=>add(a&&typeof a==='object'?JSON.stringify(a):a));
+  }
+  const toolCalls=Array.isArray(S.toolCalls)?S.toolCalls:[];
+  add('settled-tool-calls');add(toolCalls.length);
+  toolCalls.forEach(tc=>{
+    if(!tc||typeof tc!=='object'){ add(tc); return; }
+    add(tc.tid);add(tc.id);add(tc.name);add(tc.done);add(tc.is_diff);add(tc.assistant_msg_idx);add(tc.snippet);add(JSON.stringify(tc.args||{}));
+  });
+  if(S.session){
+    add(S.session.message_count);add(S.session.updated_at);add(S.session.compression_anchor_visible_idx);
+    add(JSON.stringify(S.session.compression_anchor_message_key||null));
+    add(S.session.compression_anchor_summary||'');
+  }
+  return `${messages.length}:${toolCalls.length}:${hash.toString(16)}`;
+}
+
 function _clipCliToolSnippet(text, maxLen=20000){
   const s=String(text||'');
   if(s.length<=maxLen) return s;
@@ -5698,6 +5742,7 @@ function renderMessages(options){
   const msgCount=S.messages.length;
   if(sid!==_messageRenderWindowSid) _resetMessageRenderWindow(sid);
   const renderWindowSize=_currentMessageRenderWindowSize();
+  const renderSignature=_messageRenderCacheSignature();
   const hasTransientTranscriptUi=!!(
     (window._compressionUi&&(!window._compressionUi.sessionId||window._compressionUi.sessionId===sid)) ||
     (window._handoffUi&&(!window._handoffUi.sessionId||window._handoffUi.sessionId===sid))
@@ -5713,7 +5758,7 @@ function renderMessages(options){
   // before those cards can be inserted.
   if(sid&&sid!==_sessionHtmlCacheSid&&!INFLIGHT[sid]&&!hasTransientTranscriptUi){
     const cached=_sessionHtmlCache.get(sid);
-    if(cached&&cached.msgCount===msgCount&&cached.renderWindowSize===renderWindowSize){
+    if(cached&&cached.msgCount===msgCount&&cached.renderWindowSize===renderWindowSize&&cached.signature===renderSignature){
       inner.innerHTML=cached.html;
       _sessionHtmlCacheSid=sid;
       _wireMessageWindowLoadEarlierButton();
@@ -6324,7 +6369,7 @@ function renderMessages(options){
     const _html=inner.innerHTML;
     // Only cache sessions with <300KB rendered HTML; evict oldest beyond 8 sessions.
     if(_html.length<300_000){
-      _sessionHtmlCache.set(sid,{html:_html,msgCount,renderWindowSize});
+      _sessionHtmlCache.set(sid,{html:_html,msgCount,renderWindowSize,signature:renderSignature});
       if(_sessionHtmlCache.size>8){_sessionHtmlCache.delete(_sessionHtmlCache.keys().next().value);}
     }
   }
diff --git a/tests/test_issue2613_render_cache_signature.py b/tests/test_issue2613_render_cache_signature.py
new file mode 100644
index 00000000..bb812291
--- /dev/null
+++ b/tests/test_issue2613_render_cache_signature.py
@@ -0,0 +1,26 @@
+from pathlib import Path
+
+
+UI_JS = Path("static/ui.js").read_text(encoding="utf-8")
+
+
+def test_session_html_cache_uses_render_signature_not_only_count():
+    assert "function _messageRenderCacheSignature()" in UI_JS
+    assert "const renderSignature=_messageRenderCacheSignature();" in UI_JS
+    assert "cached.signature===renderSignature" in UI_JS
+    assert "signature:renderSignature" in UI_JS
+
+
+def test_render_signature_tracks_message_content_and_settled_tool_cards():
+    signature_fn = UI_JS[UI_JS.index("function _messageRenderCacheSignature()"):UI_JS.index("function _clipCliToolSnippet")]
+    assert "msgContent(m)" in signature_fn
+    assert "m.tool_calls" in signature_fn
+    assert "m._partial_tool_calls" in signature_fn
+    assert "S.toolCalls" in signature_fn
+    assert "tc.snippet" in signature_fn
+    assert "compression_anchor_summary" in signature_fn
+
+
+def test_documentation_no_longer_allows_same_count_stale_html():
+    assert "Known limitation: cache key is session_id + message count" not in UI_JS
+    assert "mutate message content without changing the count will serve stale HTML" not in UI_JS
diff --git a/tests/test_issue734_message_windowing.py b/tests/test_issue734_message_windowing.py
index 992c5046..0352f0b6 100644
--- a/tests/test_issue734_message_windowing.py
+++ b/tests/test_issue734_message_windowing.py
@@ -33,7 +33,8 @@ def test_windowed_render_keeps_streaming_and_tool_activity_anchored_to_rendered_
 
 def test_window_state_participates_in_cache_and_cached_button_is_rewired():
     assert "cached.renderWindowSize===renderWindowSize" in UI_JS
-    assert "_sessionHtmlCache.set(sid,{html:_html,msgCount,renderWindowSize})" in UI_JS
+    assert "cached.signature===renderSignature" in UI_JS
+    assert "_sessionHtmlCache.set(sid,{html:_html,msgCount,renderWindowSize,signature:renderSignature})" in UI_JS
     assert "function _wireMessageWindowLoadEarlierButton()" in UI_JS
     assert "_wireMessageWindowLoadEarlierButton();" in UI_JS
     assert UI_JS.count("_wireMessageWindowLoadEarlierButton();") >= 2