Merge pull request #2928

# Conflicts:
#	CHANGELOG.md

CHANGELOG.md

@@ -8,6 +8,7 @@
 - When the session index is missing, WebUI now starts a background rebuild while preserving the first sidebar full-scan result, so the index is primed for later requests without temporarily hiding existing sessions.
 - Live token-usage hints now cap the cumulative in-flight tool-result prompt estimate per assistant turn, preventing many large tool callbacks from temporarily inflating the context ring before exact provider accounting arrives.
 - Streaming checkpoint saves now run under the session's profile environment, so periodic background checkpoints no longer risk falling back to the process-global profile when a WebUI tab is using a non-default profile.
+- Chat streaming now keeps a single live EventSource registered per session/stream, preventing reconnect or context-compaction paths from stacking subscribers and rendering one assistant token stream multiple times.
 
 ## [v0.51.135] — 2026-05-25 — Release DG (stage-batch17 — 9-PR small-fix batch)
 

static/messages.js

@@ -609,10 +609,11 @@ async function send(){
 
 const LIVE_STREAMS={};
 
-function closeLiveStream(sessionId, streamId){
+function closeLiveStream(sessionId, streamId, source){
   const live=LIVE_STREAMS[sessionId];
   if(!live) return;
   if(streamId&&live.streamId!==streamId) return;
+  if(source&&live.source!==source) return;
   try{live.source.close();}catch(_){ }
   delete LIVE_STREAMS[sessionId];
 }
@@ -747,8 +748,8 @@ function attachLiveStream(activeSid, streamId, uploaded=[], options={}){
     if(_persistTimer) return;
     _persistTimer=setTimeout(()=>{_persistTimer=null;persistInflightState();},2000);
   }
-  function _closeSource(){
-    closeLiveStream(activeSid, streamId);
+  function _closeSource(source){
+    closeLiveStream(activeSid, streamId, source);
   }
   function _stripLiveVisibleAssistantEchoFromThinking(text, snippets){
     let out=String(text||'');
@@ -1455,6 +1456,12 @@ function attachLiveStream(activeSid, streamId, uploaded=[], options={}){
   }
 
   function _wireSSE(source){
+    const existingLive=LIVE_STREAMS[activeSid];
+    if(existingLive&&existingLive.source&&existingLive.source!==source){
+      try{existingLive.source.close();}catch(_){ }
+    }
+    LIVE_STREAMS[activeSid]={streamId,source};
+
     // Note on #631 Bug B: the original PR description stated the server
     // "replays buffered token events" on reconnect, and proposed resetting
     // the accumulators here so the re-sent tokens wouldn't double the prefix.
@@ -2081,13 +2088,14 @@ function attachLiveStream(activeSid, streamId, uploaded=[], options={}){
 
     source.addEventListener('error',async e=>{
       if(_terminalStateReached || _streamFinalized){
-        _closeSource();
+        _closeSource(source);
         return;
       }
       if(typeof recordClientSSEError==='function') recordClientSSEError('chat-response',{ready_state:source?source.readyState:null,session_id:activeSid,stream_id:streamId,reason:'chat EventSource.onerror'});
       source.close();
       if(_deferStreamErrorIfOffline()) return;
       if(_deferStreamErrorIfPageHidden()) return;
+      _closeSource(source);
       // Attempt one reconnect if the stream is still active server-side
       if(!_reconnectAttempted && streamId){
         _reconnectAttempted=true;

tests/test_1694_terminal_cleanup_ownership.py

@@ -121,3 +121,23 @@ def test_done_handler_is_idempotent_for_replay_or_duplicate_done_events():
     assert guard_idx != -1 and guard_idx < sound_idx, (
         "completion sound must be behind the duplicate-done finalization guard"
     )
+
+
+def test_attach_live_stream_registers_one_source_per_session_stream():
+    """Reconnect/compaction paths must not stack same-stream EventSources.
+
+    The stream channel broadcasts each token to every subscriber. If the browser
+    opens four live EventSources for the same run, one assistant paragraph is
+    appended four times even though the run journal contains it once.
+    """
+    close_body = _function_body("closeLiveStream")
+    attach_body = _function_body("attachLiveStream")
+    wire_body = _function_body("_wireSSE")
+    error_body = _event_body("error")
+
+    assert "const LIVE_STREAMS={};" in MESSAGES_JS
+    assert "LIVE_STREAMS[activeSid]={streamId,source};" in wire_body
+    assert "existingLive.source.close();" in wire_body
+    assert "if(source&&live.source!==source) return;" in close_body
+    assert "existingLive&&existingLive.streamId===streamId" in attach_body
+    assert "_closeSource(source);" in error_body