Stage 374: PR #2431 — fix(chat): render pending assistant placeholder before /api/chat/start returns by @Michaelyklam (fixes #2429)

Co-authored-by: Michael Lam <michael@example.local>

static/messages.js

@@ -273,7 +273,7 @@ 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();setBusy(true);
+  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'){

static/ui.js

@@ -7033,11 +7033,12 @@ function finalizeThinkingCard(){
     _syncToolCallGroupSummary(group);
   }
 }
-function appendThinking(text=''){
+function appendThinking(text='', options){
   // Guard: ignore if session was switched during an async SSE stream.
   // The old stream's reasoning events can still fire after switch;
   // without this check they would pollute the new session's DOM.
-  if(!S.session||!S.activeStreamId) return;
+  const allowPendingPlaceholder=!!(options&&options.pending===true);
+  if(!S.session||(!S.activeStreamId&&!allowPendingPlaceholder)) return;
   $('emptyState').style.display='none';
   let turn=$('liveAssistantTurn');
   if(!turn){

tests/test_sidebar_first_turn_visibility.py

@@ -16,7 +16,7 @@ class TestSidebarFirstTurnVisibility:
             "send() must optimistically upsert the active session into the sidebar "
             "as soon as the local user message is pushed."
         )
-        push_idx = src.index("S.messages.push(userMsg);renderMessages();appendThinking();setBusy(true);")
+        push_idx = src.index("S.messages.push(userMsg);renderMessages();appendThinking('',{pending:true});setBusy(true);")
         helper_idx = src.index("upsertActiveSessionForLocalTurn", push_idx)
         start_idx = src.index("api('/api/chat/start'", push_idx)
         assert helper_idx < start_idx, (
@@ -29,6 +29,26 @@ class TestSidebarFirstTurnVisibility:
             "that race can overwrite the optimistic first-turn row with an empty list."
         )
 
+    def test_messages_send_renders_pending_assistant_placeholder_before_chat_start(self):
+        messages = read("static/messages.js")
+        ui = read("static/ui.js")
+        send_start = messages.index("async function send()")
+        push_idx = messages.index("appendThinking('',{pending:true})", send_start)
+        start_idx = messages.index("api('/api/chat/start'", send_start)
+        assert push_idx < start_idx, (
+            "send() should render an assistant-side pending placeholder before "
+            "/api/chat/start or the first SSE event returns."
+        )
+        append_start = ui.index("function appendThinking(text='', options)")
+        append_end = ui.index("function updateThinking", append_start)
+        append_body = ui[append_start:append_end]
+        assert "allowPendingPlaceholder" in append_body
+        assert "options.pending===true" in append_body.replace(" ", ""), (
+            "appendThinking() must allow the explicit pre-stream placeholder path "
+            "without weakening the stale-stream guard for ordinary SSE updates."
+        )
+        assert "(!S.activeStreamId&&!allowPendingPlaceholder)" in append_body.replace(" ", "")
+
     def test_sessions_js_has_local_turn_upsert_helper(self):
         src = read("static/sessions.js")
         assert "function upsertActiveSessionForLocalTurn" in src