diff --git a/CHANGELOG.md b/CHANGELOG.md
index b0c1f9b7..d2fbbb87 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- **Post-start streaming errors in the browser no longer hide active streams after a successful `/api/chat/start` (#4481).** If browser-side bookkeeping after stream start throws, it is now logged as recoverable and `attachLiveStream(activeSid, streamId, uploadedNames)` still runs when `stream_id` is available, so a transient UI exception cannot clear the send state or prevent live tokens from attaching.
+
 ## [v0.51.513] — 2026-06-19 — Release RX (credential-pool quota status for all pooled providers)
 
 ### Added
diff --git a/static/messages.js b/static/messages.js
index de6a81b2..27803391 100644
--- a/static/messages.js
+++ b/static/messages.js
@@ -898,6 +898,16 @@ function _runOptionalPreStartUiStep(label, fn){
   }
 }
 
+function _runOptionalPostStartUiStep(label, fn){
+  try{
+    return typeof fn==='function'?fn():undefined;
+  }catch(e){
+    const message=e&&e.message?e.message:String(e||'unknown error');
+    try{console.warn('[webui] optional post-start UI step failed', label, message);}catch(_){ }
+    return undefined;
+  }
+}
+
 function _sessionTitleLooksDefaultOrProvisional(titleText, provisionalText){
   const title=String(titleText||'').replace(/\s+/g,' ').trim();
   if(!title||title==='Untitled'||title==='New Chat')return true;
@@ -1248,8 +1258,12 @@ async function send(){
 
   // Start the agent via POST, get a stream_id back
   let streamId;
+  let postStartData;
+  let modelStateForPostStart;
+  let explicitPickForPostStart;
   try{
     const _modelState=_chatPayloadModelState();
+    modelStateForPostStart=_modelState;
     const _pendingPick=(typeof _readPendingSessionModel==='function')
       ? _readPendingSessionModel(activeSid)
       : null;
@@ -1262,6 +1276,7 @@ async function send(){
     // once read so a later send of an unchanged dropdown isn't treated as an explicit
     // pick. (#3739/#3737, Codex catch)
     if(_explicitPick && typeof _clearPendingSessionModel==='function') _clearPendingSessionModel(activeSid);
+    explicitPickForPostStart=_explicitPick;
     const startData=await api('/api/chat/start',{method:'POST',body:JSON.stringify({
       session_id:activeSid,message:msgText,
       // S.session.model remains authoritative; the helper only resolves a
@@ -1272,63 +1287,7 @@ async function send(){
       explicit_model_pick:_explicitPick||undefined,
       attachments:uploaded.length?uploaded:undefined
     })});
-
-    if(startData.title) applySessionTitleUpdate(activeSid, startData.title, {provisionalText:displayText.slice(0,64), rememberProvisional:true});
-
-    if(startData.effective_model && S.session){
-      const _sentModel=_modelState.model;
-      if(_explicitPick && _sentModel && startData.effective_model!==_sentModel && typeof showToast==='function'){
-        showToast('Model '+_sentModel+' changed to '+startData.effective_model+' — profile provider mismatch', 5000);
-      }
-      S.session.model=startData.effective_model;
-      S.session.model_provider=startData.effective_model_provider||S.session.model_provider||null;
-      localStorage.setItem('hermes-webui-model', startData.effective_model);
-      if(typeof _writePersistedModelState==='function') _writePersistedModelState(startData.effective_model,S.session.model_provider||null);
-      if($('modelSelect')) _applyModelToDropdown(startData.effective_model, $('modelSelect'),S.session.model_provider||null);
-      if(typeof syncTopbar==='function') syncTopbar();
-    }else if(startData.effective_model_provider && S.session){
-      S.session.model_provider=startData.effective_model_provider;
-      if(typeof _writePersistedModelState==='function') _writePersistedModelState(S.session.model||'',S.session.model_provider||null);
-      if($('modelSelect')&&typeof _applyModelToDropdown==='function') _applyModelToDropdown(S.session.model||'', $('modelSelect'), S.session.model_provider||null);
-      if(typeof syncModelChip==='function') syncModelChip();
-      if(typeof syncTopbar==='function') syncTopbar();
-    }
-    streamId=startData.stream_id;
-    S.activeStreamId = streamId;
-    if(typeof appendThinking==='function') appendThinking('',{pending:true});
-    // setBusy(true) already ran with activeStreamId=null; refresh now that we
-    // have a stream id so the primary button can switch to Stop (see getComposerPrimaryAction).
-    if(typeof updateSendBtn==='function') updateSendBtn();
-    if(S.session&&typeof startData.pending_started_at==='number'){
-      S.session.pending_started_at=startData.pending_started_at;
-    }
-    if(S.session&&S.session.session_id===activeSid){
-      S.session.active_stream_id = streamId;
-    }
-    if(S.session&&S.session.session_id===activeSid&&typeof showLiveRunStatus==='function'){
-      const _startedAt=typeof startData.pending_started_at==='number'
-        ? startData.pending_started_at
-        : (S.session.pending_started_at||Date.now()/1000);
-      showLiveRunStatus(activeSid,{startedAt:_startedAt});
-    }
-    if(typeof upsertActiveSessionForLocalTurn==='function'){
-      // Third optimistic pass: stream_id is now known, so the row can reconcile
-      // against real active-stream metadata before the background refresh lands.
-      upsertActiveSessionForLocalTurn({title:S.session&&S.session.title||displayText.slice(0,64),messageCount:S.messages.length,timestampMs:Date.now()});
-    }
-    if(!INFLIGHT[activeSid]){
-      INFLIGHT[activeSid]={messages:optimisticMessages,uploaded:uploadedNames,toolCalls:[]};
-    }
-    const currentInflight=INFLIGHT[activeSid];
-    markInflight(activeSid, streamId);
-    if(typeof saveInflightState==='function'){
-      saveInflightState(activeSid,{streamId,messages:currentInflight.messages||optimisticMessages,uploaded:uploadedNames,toolCalls:currentInflight.toolCalls||[]});
-    }
-    // Refresh session list so background streaming indicators appear immediately for the
-    // session that was just started and any others that may already be running.
-    if(typeof renderSessionList === 'function') {
-      void renderSessionList();
-    }
+    postStartData = startData;
   }catch(e){
     const errMsg=String((e&&e.message)||'');
     // If /api/chat/start returns 404, the session was deleted server-side
@@ -1392,6 +1351,70 @@ async function send(){
     return;
   }
 
+  const startData = postStartData || {};
+  streamId = postStartData ? postStartData.stream_id : null;
+  S.activeStreamId = streamId;
+  // setBusy(true) already ran with activeStreamId=null; refresh now that we
+  // have a stream id so the primary button can switch to Stop (see
+  // getComposerPrimaryAction).
+  if(typeof updateSendBtn==='function') updateSendBtn();
+  _runOptionalPostStartUiStep('post-start ui/bookkeeping', ()=>{
+    const _modelState=modelStateForPostStart || _chatPayloadModelState();
+    const _explicitPick=explicitPickForPostStart;
+    if(startData&&startData.title) applySessionTitleUpdate(activeSid, startData.title, {provisionalText:displayText.slice(0,64), rememberProvisional:true});
+
+    if(startData&&startData.effective_model && S.session){
+      const _sentModel=_modelState&&_modelState.model;
+      if(_explicitPick && _sentModel && startData.effective_model!==_sentModel && typeof showToast==='function'){
+        showToast('Model '+_sentModel+' changed to '+startData.effective_model+' — profile provider mismatch', 5000);
+      }
+      S.session.model=startData.effective_model;
+      S.session.model_provider=startData.effective_model_provider||S.session.model_provider||null;
+      localStorage.setItem('hermes-webui-model', startData.effective_model);
+      if(typeof _writePersistedModelState==='function') _writePersistedModelState(startData.effective_model,S.session.model_provider||null);
+      if($('modelSelect')) _applyModelToDropdown(startData.effective_model, $('modelSelect'),S.session.model_provider||null);
+      if(typeof syncTopbar==='function') syncTopbar();
+    }else if(startData&&startData.effective_model_provider && S.session){
+      S.session.model_provider=startData.effective_model_provider;
+      if(typeof _writePersistedModelState==='function') _writePersistedModelState(S.session.model||'',S.session.model_provider||null);
+      if($('modelSelect')&&typeof _applyModelToDropdown==='function') _applyModelToDropdown(S.session.model||'', $('modelSelect'), S.session.model_provider||null);
+      if(typeof syncModelChip==='function') syncModelChip();
+      if(typeof syncTopbar==='function') syncTopbar();
+    }
+
+    if(typeof appendThinking==='function') appendThinking('',{pending:true});
+    if(S.session&&typeof startData.pending_started_at==='number'){
+      S.session.pending_started_at=startData.pending_started_at;
+    }
+    if(S.session&&S.session.session_id===activeSid){
+      S.session.active_stream_id = streamId;
+    }
+    if(S.session&&S.session.session_id===activeSid&&typeof showLiveRunStatus==='function'){
+      const _startedAt=typeof startData?.pending_started_at==='number'
+        ? startData.pending_started_at
+        : (S.session.pending_started_at||Date.now()/1000);
+      showLiveRunStatus(activeSid,{startedAt:_startedAt});
+    }
+    if(typeof upsertActiveSessionForLocalTurn==='function'){
+      // Third optimistic pass: stream_id is now known, so the row can reconcile
+      // against real active-stream metadata before the background refresh lands.
+      upsertActiveSessionForLocalTurn({title:S.session&&S.session.title||displayText.slice(0,64),messageCount:S.messages.length,timestampMs:Date.now()});
+    }
+    if(!INFLIGHT[activeSid]){
+      INFLIGHT[activeSid]={messages:optimisticMessages,uploaded:uploadedNames,toolCalls:[]};
+    }
+    const currentInflight=INFLIGHT[activeSid];
+    markInflight(activeSid, streamId);
+    if(typeof saveInflightState==='function'){
+      saveInflightState(activeSid,{streamId,messages:currentInflight.messages||optimisticMessages,uploaded:uploadedNames,toolCalls:currentInflight.toolCalls||[]});
+    }
+    // Refresh session list so background streaming indicators appear immediately for the
+    // session that was just started and any others that may already be running.
+    if(typeof renderSessionList === 'function') {
+      void renderSessionList();
+    }
+  });
+
   // Open SSE stream and render tokens live
   attachLiveStream(activeSid, streamId, uploadedNames);
 
diff --git a/tests/test_1062_busy_input_modes.py b/tests/test_1062_busy_input_modes.py
index 0a91ebb7..af80096d 100644
--- a/tests/test_1062_busy_input_modes.py
+++ b/tests/test_1062_busy_input_modes.py
@@ -229,27 +229,41 @@ class TestBusySendButton:
         )
 
     def test_send_refreshes_primary_button_after_chat_start_stream_id(self):
-        """send() must call updateSendBtn in the chat/start try block after assigning streamId.
+        """send() must call updateSendBtn after streamId is assigned and before attachLiveStream.
 
-        setBusy(true) already ran updateSendBtn while activeStreamId was still null, so the
-        Stop affordance did not appear until something else (e.g. typing) called
-        updateSendBtn again.
+        setBusy(true) runs before the API request with activeStreamId still null.  The
+        Stop affordance must be refreshed as soon as we have streamId so it cannot be
+        skipped by optional post-start UI failures.
         """
         send_start = MESSAGES_JS.find("async function send(")
         assert send_start >= 0, "send() not found in messages.js"
         send_end = MESSAGES_JS.find("const LIVE_STREAMS={}", send_start)
         assert send_end > send_start, "could not find end of send() body"
         send_body = MESSAGES_JS[send_start:send_end]
+        api_idx = send_body.find("const startData=await api('/api/chat/start'")
+        assert api_idx >= 0, "send() should issue /api/chat/start"
+        catch_idx = send_body.find("}catch(e){", api_idx)
+        assert catch_idx >= 0, "send() should have API error catch after /api/chat/start"
         assign = "S.activeStreamId = streamId;"
         apos = send_body.find(assign)
         assert apos >= 0, "send() must assign S.activeStreamId from startData"
-        after_assign = send_body[apos:]
-        end_try = after_assign.find("  }catch(e){")
-        assert end_try > 0, "send() outer try/catch not found after stream id assign"
-        try_after_assign = after_assign[:end_try]
-        assert "updateSendBtn" in try_after_assign, (
-            "send() must call updateSendBtn() in the chat/start try block after assigning "
-            "streamId so the primary button switches to Stop without waiting for composer input"
+        assert apos > catch_idx, (
+            "send() must assign S.activeStreamId only after /api/chat/start succeeds"
+        )
+        update_idx = send_body.find("updateSendBtn();", apos)
+        assert update_idx >= 0, "send() must call updateSendBtn() after assigning streamId"
+        assert update_idx > apos, (
+            "send() should call updateSendBtn() after S.activeStreamId is assigned"
+        )
+        attach_idx = send_body.find("attachLiveStream(activeSid, streamId, uploadedNames);")
+        assert attach_idx > update_idx, (
+            "send() should refresh primary button before opening SSE stream attach"
+        )
+        optional_idx = send_body.find("_runOptionalPostStartUiStep('post-start ui/bookkeeping'", apos)
+        assert optional_idx >= 0, "send() should run optional post-start UI/bookkeeping"
+        assert optional_idx > update_idx, (
+            "send() must call updateSendBtn() before entering optional post-start helper so optional failures "
+            "cannot skip it."
         )
 
 
diff --git a/tests/test_inflight_send_start_race.py b/tests/test_inflight_send_start_race.py
index 4b0c7ba5..521e384a 100644
--- a/tests/test_inflight_send_start_race.py
+++ b/tests/test_inflight_send_start_race.py
@@ -106,6 +106,27 @@ def test_pre_start_optimistic_block_cannot_prevent_chat_start():
     )
 
 
+def test_post_start_bookkeeping_errors_cannot_block_live_attach():
+    """Any optional post-start UI/bookkeeping failure should be recoverable once stream_id exists."""
+    body = _function_body(MESSAGES_JS, "send")
+    helper_body = _function_body(MESSAGES_JS, "_runOptionalPostStartUiStep")
+    assert "optional post-start UI step failed" in helper_body, (
+        "post-start optional helper failures should stay in warning logs, not user-facing error bubbles"
+    )
+
+    chat_start_idx = body.index("const startData=await api('/api/chat/start'")
+    catch_idx = body.index("}catch(e){", chat_start_idx)
+    optional_idx = body.index("_runOptionalPostStartUiStep('post-start ui/bookkeeping'", catch_idx)
+    stream_id_idx = body.index("streamId = postStartData ? postStartData.stream_id : null;", catch_idx)
+    attach_idx = body.index("attachLiveStream(activeSid, streamId, uploadedNames);")
+    assert catch_idx < stream_id_idx < optional_idx < attach_idx, (
+        "stream-id setup, post-start UI/bookkeeping, and attach must run after successful API catch"
+    )
+    assert "S.messages.push({role:'assistant',content:`**Error:**" not in body[optional_idx : attach_idx], (
+        "post-start optional failures should not append assistant error messages before stream attach"
+    )
+
+
 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")