diff --git a/static/messages.js b/static/messages.js
index 21b68b4a..49670a37 100644
--- a/static/messages.js
+++ b/static/messages.js
@@ -436,39 +436,68 @@ async function send(){
   const userMsg={role:'user',content:displayText,attachments:uploaded.length?uploadedNames:undefined,_ts:Date.now()/1000};
   S.toolCalls=[];  // clear tool calls from previous turn
   clearLiveToolCards();  // clear any leftover live cards from last turn
-  S.messages.push(userMsg);renderMessages();appendThinking('',{pending:true});setBusy(true);
-  // First optimistic pass: make the local user turn visible before /api/chat/start
-  // can save pending state on the server.
-  if(typeof upsertActiveSessionForLocalTurn==='function'){
-    upsertActiveSessionForLocalTurn({title:displayText.slice(0,64),messageCount:S.messages.length,timestampMs:Date.now()});
-  }
-  const optimisticMessages=[...S.messages];
-  INFLIGHT[activeSid]={messages:optimisticMessages,uploaded:uploadedNames,toolCalls:[]};
-  if(typeof saveInflightState==='function'){
-    saveInflightState(activeSid,{streamId:null,messages:INFLIGHT[activeSid].messages,uploaded:uploadedNames,toolCalls:[]});
-  }
-  if(typeof renderSessionListFromCache==='function') renderSessionListFromCache();
-  startApprovalPolling(activeSid);
-  startClarifyPolling(activeSid);
-  _fetchYoloState(activeSid);  // sync YOLO pill with backend state
-  S.activeStreamId = null;  // will be set after stream starts
-  if(typeof updateSendBtn==='function') updateSendBtn();
-
-  // Set provisional title from user message immediately so session appears
-  // in the sidebar right away with a meaningful name. /api/chat/start persists
-  // the server-side provisional title and may refine this optimistic text.
-  if(S.session&&(S.session.title==='Untitled'||!S.session.title)){
-    const provisionalTitle=displayText.slice(0,64);
-    applySessionTitleUpdate(activeSid, provisionalTitle, {force:true, rememberProvisional:true});
-    if(typeof upsertActiveSessionForLocalTurn==='function'){
-      // Second optimistic pass: carry the provisional title into the cached row
-      // without re-fetching /api/sessions before pending state exists server-side.
-      upsertActiveSessionForLocalTurn({title:provisionalTitle,messageCount:S.messages.length,timestampMs:Date.now()});
+  let optimisticMessages;
+  try{
+    S.messages.push(userMsg);renderMessages();appendThinking('',{pending:true});setBusy(true);
+    // First optimistic pass: make the local user turn visible before /api/chat/start
+    // can save pending state on the server.
+    _runOptionalPreStartUiStep('upsertActiveSessionForLocalTurn.initial', ()=>{
+      if(typeof upsertActiveSessionForLocalTurn==='function'){
+        upsertActiveSessionForLocalTurn({title:displayText.slice(0,64),messageCount:S.messages.length,timestampMs:Date.now()});
+      }
+    });
+    optimisticMessages=[...S.messages];
+    INFLIGHT[activeSid]={messages:optimisticMessages,uploaded:uploadedNames,toolCalls:[]};
+    if(typeof saveInflightState==='function'){
+      saveInflightState(activeSid,{streamId:null,messages:INFLIGHT[activeSid].messages,uploaded:uploadedNames,toolCalls:[]});
     }
-  } else if(typeof upsertActiveSessionForLocalTurn==='function'){
-    upsertActiveSessionForLocalTurn({title:S.session&&S.session.title||displayText.slice(0,64),messageCount:S.messages.length,timestampMs:Date.now()});
-  } else {
-    renderSessionListFromCache();  // ensure it's visible even if already titled
+    _runOptionalPreStartUiStep('renderSessionListFromCache.initial', ()=>{
+      if(typeof renderSessionListFromCache==='function') renderSessionListFromCache();
+    });
+    _runOptionalPreStartUiStep('startApprovalPolling.prestart', ()=>startApprovalPolling(activeSid));
+    _runOptionalPreStartUiStep('startClarifyPolling.prestart', ()=>startClarifyPolling(activeSid));
+    _runOptionalPreStartUiStep('fetchYoloState.prestart', ()=>_fetchYoloState(activeSid));  // sync YOLO pill with backend state
+    S.activeStreamId = null;  // will be set after stream starts
+    _runOptionalPreStartUiStep('updateSendBtn.prestart', ()=>{
+      if(typeof updateSendBtn==='function') updateSendBtn();
+    });
+
+    // Set provisional title from user message immediately so session appears
+    // in the sidebar right away with a meaningful name. /api/chat/start persists
+    // the server-side provisional title and may refine this optimistic text.
+    if(S.session&&(S.session.title==='Untitled'||!S.session.title)){
+      const provisionalTitle=displayText.slice(0,64);
+      _runOptionalPreStartUiStep('applySessionTitleUpdate.provisional', ()=>{
+        applySessionTitleUpdate(activeSid, provisionalTitle, {force:true, rememberProvisional:true});
+      });
+      _runOptionalPreStartUiStep('upsertActiveSessionForLocalTurn.provisional', ()=>{
+        if(typeof upsertActiveSessionForLocalTurn==='function'){
+          // Second optimistic pass: carry the provisional title into the cached row
+          // without re-fetching /api/sessions before pending state exists server-side.
+          upsertActiveSessionForLocalTurn({title:provisionalTitle,messageCount:S.messages.length,timestampMs:Date.now()});
+        }
+      });
+    } else if(typeof upsertActiveSessionForLocalTurn==='function'){
+      _runOptionalPreStartUiStep('upsertActiveSessionForLocalTurn.titled', ()=>{
+        upsertActiveSessionForLocalTurn({title:S.session&&S.session.title||displayText.slice(0,64),messageCount:S.messages.length,timestampMs:Date.now()});
+      });
+    } else {
+      _runOptionalPreStartUiStep('renderSessionListFromCache.prestart', ()=>{
+        renderSessionListFromCache();  // ensure it's visible even if already titled
+      });
+    }
+  }catch(preStartError){
+    // The user turn must reach /api/chat/start even if local optimistic UI
+    // bookkeeping (render cache, storage quota, sidebar reconciliation, etc.)
+    // throws. Otherwise the pane can show a user bubble + spinner while the
+    // backend never receives the turn.
+    const message=preStartError&&preStartError.message?preStartError.message:String(preStartError||'unknown error');
+    try{console.warn('[webui] pre-start optimistic UI failed; continuing to /api/chat/start', message);}catch(_){ }
+    if(!S.messages.includes(userMsg)) S.messages.push(userMsg);
+    optimisticMessages=[...S.messages];
+    INFLIGHT[activeSid]={messages:optimisticMessages,uploaded:uploadedNames,toolCalls:[]};
+    try{setBusy(true);}catch(_){S.busy=true;}
+    S.activeStreamId=null;
   }
 
   // Start the agent via POST, get a stream_id back
diff --git a/tests/test_inflight_send_start_race.py b/tests/test_inflight_send_start_race.py
index fcedbe40..731d6d2d 100644
--- a/tests/test_inflight_send_start_race.py
+++ b/tests/test_inflight_send_start_race.py
@@ -24,7 +24,7 @@ def _function_body(src: str, name: str) -> str:
 def test_send_preserves_optimistic_messages_across_chat_start_await():
     """send() must not dereference INFLIGHT[activeSid] after await without a fallback."""
     body = _function_body(MESSAGES_JS, "send")
-    setup_idx = body.index("const optimisticMessages=[...S.messages];")
+    setup_idx = body.index("optimisticMessages=[...S.messages];")
     inflight_idx = body.index("INFLIGHT[activeSid]={messages:optimisticMessages")
     await_idx = body.index("const startData=await api('/api/chat/start'")
     save_idx = body.index("saveInflightState(activeSid,{streamId", await_idx)
@@ -66,6 +66,42 @@ def test_send_clears_stale_busy_state_before_queue_branch():
     )
 
 
+def test_pre_start_optimistic_ui_helpers_cannot_block_chat_start():
+    """Optional optimistic UI helpers must not strand a local bubble before /api/chat/start."""
+    body = _function_body(MESSAGES_JS, "send")
+    helper_body = _function_body(MESSAGES_JS, "_runOptionalPreStartUiStep")
+
+    optimistic_idx = body.index("S.messages.push(userMsg);renderMessages();appendThinking('',{pending:true});setBusy(true);")
+    chat_start_idx = body.index("api('/api/chat/start'")
+    pre_start = body[optimistic_idx:chat_start_idx]
+
+    assert "try" in helper_body and "catch" in helper_body, (
+        "optional pre-start UI helper wrapper must catch errors before /api/chat/start"
+    )
+    assert "_runOptionalPreStartUiStep" in pre_start, (
+        "send() should wrap optimistic sidebar/title/polling helpers before /api/chat/start"
+    )
+    assert "upsertActiveSessionForLocalTurn" in pre_start and "applySessionTitleUpdate" in pre_start
+
+
+def test_pre_start_optimistic_block_cannot_prevent_chat_start():
+    """Any pre-start UI/storage exception must still fall through to /api/chat/start."""
+    body = _function_body(MESSAGES_JS, "send")
+    optimistic_idx = body.index("S.messages.push(userMsg);renderMessages();appendThinking('',{pending:true});setBusy(true);")
+    chat_start_idx = body.index("api('/api/chat/start'")
+    pre_start = body[optimistic_idx:chat_start_idx]
+
+    assert "}catch(preStartError){" in pre_start, (
+        "The whole optimistic pre-start block needs a catch, not only individual optional helpers"
+    )
+    assert "continuing to /api/chat/start" in pre_start, (
+        "The recovery path should document that chat/start must still execute"
+    )
+    assert pre_start.rindex("}catch(preStartError){") < chat_start_idx, (
+        "pre-start catch must be before the /api/chat/start call"
+    )
+
+
 def test_server_absent_optimistic_first_turn_rows_are_not_kept_forever():
     """A local first-turn sidebar row must expire when /api/chat/start never persisted it."""
     body = _function_body(SESSIONS_JS, "_mergeOptimisticFirstTurnSessions")